Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
Schedule of Estimated Fair Value of Financial Instruments
Fair Value, by Balance Sheet Grouping [Table Text Block]
Statement of Comprehensive Income [Abstract]
Comprehensive Income (Loss) Note [Text Block]
Comprehensive Income (Loss) Note [Text Block]
Fair Value, by Balance Sheet Grouping [Table]
Fair Value, by Balance Sheet Grouping [Table]
Fair Value, Hierarchy [Axis]
Fair Value, Hierarchy [Axis]
Fair Value Hierarchy [Domain]
Fair Value Hierarchy [Domain]
Measurement Frequency [Axis]
Measurement Frequency [Axis]
Fair Value, Measurement Frequency [Domain]
Fair Value, Measurement Frequency [Domain]
Fair Value, Measurements, Recurring [Member]
Fair Value, Measurements, Recurring [Member]
Measurement Basis [Axis]
Measurement Basis [Axis]
Fair Value Measurement [Domain]
Fair Value Measurement [Domain]
Portion at Fair Value Measurement [Member]
Portion at Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value
Long-term Debt
Long-term debt, fair value
Long-term Debt, Fair Value
Short-term Debt [Abstract]
Short-Term Debt
Short-term Debt [Text Block]
Property, Plant and Equipment [Abstract]
Schedule of Property, Plant and Equipment [Table]
Property, Plant and Equipment [Table]
Property, Plant and Equipment by Type [Axis]
Property, Plant and Equipment, Type [Axis]
Property, Plant and Equipment, Type [Domain]
Property, Plant and Equipment, Type [Domain]
Land and improvements [Member]
Land and Land Improvements [Member]
Building and improvements [Member]
Building and Building Improvements [Member]
Tranmission, distribution and storage[Member]
Independent Transmission and Distribution System [Member]
Generation [Member]
Public Utility Generation [Member]
Public Utility Generation [Member]
Plant Acquisition adjustment [Member]
Plant Acquisition Adjustment [Member]
Plant Acquisition Adjustment [Member]
Other [Member]
Property, Plant and Equipment, Other Types [Member]
Basin Capital Lease [Member]
Basin Capital Lease [Member]
Range [Axis]
Range [Axis]
Range [Domain]
Range [Domain]
Minimum [Member]
Minimum [Member]
Maximum [Member]
Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment [Line Items]
Land, land rights and easements
Land and Land Improvements
Buildings and improvements
Buildings and Improvements, Gross
Transmission, distribution and storage
Public Utilities, Property, Plant and Equipment, Transmission and Distribution
Generation
Public Utilities, Property, Plant and Equipment, Generation or Processing
Plant acquisition adjustment
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments
Other
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment
Construction work in progress
Public Utilities, Property, Plant and Equipment, Construction Work in Progress
Property, plant and equipment, gross
Property, Plant and Equipment, Gross
Less accumulated depreciation
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment
Property, plant, and equipment, net
Property, Plant and Equipment, Net
Estimated Useful Life
Property, Plant and Equipment, Useful Life
Property, plant, and equipment under capiltal leases
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Other Property, Plant, and Equipment, Gross
Increase in PP&E due to hydro acquisition
Property, Plant and Equipment, Gross, Period Increase (Decrease)
Accounting Policies [Abstract]
Materials and supplies
Inventory, Raw Materials and Supplies, Gross
Storage gas and fuel
Energy Related Inventory, Natural Gas in Storage
Total
Inventory, Net
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information
Schedule of Quarterly Financial Information [Table Text Block]
Compensation and Retirement Disclosure [Abstract]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]
Defined Benefit Plans and Other Postretirement Benefit Plans [Axis]
Defined Benefit Plans and Other Postretirement Benefit Plans [Domain]
Defined Benefit Plan and Other Postretirement Benefit Plan [Domain]
Other Postretirement Benefit Plan [Member]
Defined Contribution Pension [Member]
Pension Plan [Member]
Fair Value Hierarchy [Domain]
Fair Value, Inputs, Level 1 [Member]
Fair Value, Inputs, Level 1 [Member]
Fair Value, Inputs, Level 2 [Member]
Fair Value, Inputs, Level 2 [Member]
Fair Value, Inputs, Level 3 [Member]
Fair Value, Inputs, Level 3 [Member]
Estimate of Fair Value Measurement [Member]
Estimate of Fair Value Measurement [Member]
Defined Benefit Plan, Asset Categories [Axis]
Plan Asset Categories [Domain]
Equity Securities [Member]
Debt Securities [Member]
Major Types of Debt and Equity Securities [Axis]
Major Types of Debt and Equity Securities [Domain]
US Small Midcap Growth [Member]
US Small Midcap Growth [Member]
US Small Midcap Value [Member]
US Small Midcap Value [Member]
S&P 500 Index [Member]
SAndP500Index [Member]
S And P 500 Index [Member]
US Large Cap Growth [Member]
US Large Cap Growth [Member]
US Large Cap Growth [Member]
US Large Cap Value [Member]
US Large Cap Value [Member]
US Large Cap Value [Member]
US Large Cap Passive [Member]
US Large Cap Passive [Member]
US Large Cap Passive
Non US Core [Member]
Non US Core [Member]
Non US Core [Member]
Emerging Markets [Member]
Emerging Markets [Member]
Emerging Markets [Member]
Passive Bond Market [Member]
Passive Bond Market [Member]
Passive Bond Market [Member]
US Core [Member]
US Core Opportunistic [Member]
US Core Opportunistic [Member]
US Passive [Member]
US Passive [Member]
US Passive [Member]
Long Duration [Member]
Long Duration [Member]
Long Duration [Member]
Long Duration Investment Grade [Member]
Long Duration Investment Grade [Member]
Long Duration Investment Grade [Member]
Long Duration Passive [Member]
Long Duration Passive [Member]
-- None. No documentation exists for this element. --
Opportunistic [Member]
Opportunistic [Member]
Opportunistic [Member]
Non US Passive [Member]
Non US Passive [Member]
Non US Passive [Member]
Active Long Corporate [Member]
Active Long Corporate [Member]
Active Long Corporate [Member]
Participating Group Annuity Contract [Member]
Participating Group Annuity Contract [Member]
Participating Group Annuity Contract
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions
Defined Benefit Plan, Contributions by Employer
Cash and cash equivalents
Cash and Cash Equivalents, Fair Value Disclosure
Available-for-sale Securities
Available-for-sale Securities
Income Tax Disclosure [Abstract]
Federal
Federal Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current
Current Federal Tax Expense (Benefit)
Deferred
Deferred Federal Income Tax Expense (Benefit)
Investment tax credits
Investment Tax Credit
State
State and Local Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current
Current State and Local Tax Expense (Benefit)
Deferred
Deferred State and Local Income Tax Expense (Benefit)
Income tax expense (benefit)
Income Tax Expense (Benefit)
Schedule of Short-term Debt [Table]
Schedule of Short-term Debt [Table]
Long-term Debt, Type [Axis]
Long-term Debt, Type [Axis]
Long-term Debt, Type [Domain]
Long-term Debt, Type [Domain]
Short-term Debt, Type [Axis]
Short-term Debt, Type [Axis]
Short-term Debt, Type [Domain]
Short-term Debt, Type [Domain]
Short-term Debt [Member]
Short-term Debt [Member]
Short-term Debt [Line Items]
Short-term Debt [Line Items]
Commercial Paper, Balance
Commercial Paper
Commercial Paper, Interest Rate
Debt Instrument, Interest Rate, Effective Percentage
Maximum short-term debt outstanding
Short-term Debt, Maximum Amount Outstanding During Period
Average short-term debt outstanding
Short-term Debt, Average Outstanding Amount
Weighted average interest rate
Short-term Debt, Weighted Average Interest Rate
Maximum borrowing capacity under commercial paper program
Short-term Debt, Unsecured, Maximum Aggregate Outstanding Amount Issuable
Maximum aggregate amount outstanding at any time of $250 million to provide an alternative financing source for our short-term liquidity needs.
Short term borrowings maximum days outstanding
Short Term Borrowings Maximum Days Outstanding
Short Term Borrowings Maximum Days Outstanding
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligation
Asset Retirement Obligation Disclosure [Text Block]
CELP Arbitration Decision [Abstract]
CELP decision [Abstract]
Schedule of Effect of Fourth Quarter Events [Table]
Schedule of Effect of Fourth Quarter Events [Table]
Effect of Fourth Quarter Event Type [Axis]
Effect of Fourth Quarter Event Type [Axis]
Effect of Fourth Quarter Events, Type [Domain]
Effect of Fourth Quarter Events, Type [Domain]
Effect of Fourth Quarter Events [Line Items]
Effect of Fourth Quarter Events [Line Items]
Goodwill [Abstract]
Goodwill [Abstract]
Schedule of Goodwill
Schedule of Goodwill [Table Text Block]
Geographical [Axis]
Geographical [Axis]
Segment, Geographical [Domain]
Geographical [Domain]
MONTANA
MONTANA
SOUTH DAKOTA
SOUTH DAKOTA
Allowance for funds used during construction, rate
Public Utilities, Allowance for Funds Used During Construction, Rate
Interest costs, capitalized during period
Interest Costs Capitalized
Property, plant and equipment, disclosure of composite depreciation rate for plants in service
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liability at January 1,
Asset Retirement Obligation
Accretion expense
Asset Retirement Obligation, Accretion Expense
Liabilities incurred
Asset Retirement Obligation, Liabilities Incurred
Liabilities settled
Asset Retirement Obligation, Liabilities Settled
Revision to cash flows
Asset Retirement Obligation, Revision of Estimate
Liability at December 31,
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment
Schedule of Segment Reporting Information, by Segment [Table Text Block]
Scenario [Axis]
Scenario [Axis]
Scenario, Unspecified [Domain]
Scenario, Unspecified [Domain]
Scenario, Forecast [Member]
Scenario, Forecast [Member]
Investment Geographic Region [Axis]
Investment Geographic Region [Axis]
Investment Geographic Region [Domain]
Investment Geographic Region [Domain]
Domestic [Member]
Domestic [Member]
Domestic [Member]
Defined Benefit Plan, Asset Categories [Axis]
Plan Asset Categories [Domain]
Equity Securities [Member]
Debt Securities [Member]
Discount rate change [Member]
Discount rate change [Member]
Discount rate change [Member]
Change in mortality rate assumption [Member]
Change in mortality rate assumption [Member]
Change in mortality rate assumption [Member]
Pension Plan [Member]
Collective Bargaining Arrangement [Axis]
Collective Bargaining Arrangement [Domain]
Collective Bargaining Arrangement [Domain]
Nonunion [Member]
Nonunion [Member]
Union [Member]
Union [Member]
Defined Benefit Plan, Actuarial Gain (Loss)
Defined Benefit Plan, Actuarial Gain (Loss)
Defined Benefit Plan, Target Plan Asset Allocations
Defined Benefit Plan, Target Plan Asset Allocations
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate
Expected rate of return on plan assets
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets
Long-term rate of increase in compensation levels
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
Health care cost trend rate assumed for current year
Defined Benefit Plan Health Care Cost Trend Rate Assumed For Current Year
Defined Benefit Plan Health Care Cost Trend Rate Assumed For Current Year
Health care cost trend rate decrease assumed for next fiscal year
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year
Ultimate health care cost trend rate
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate
Year that rate reaches ultimate trend rate
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate
Changes Measurement [Member]
Changes Measurement [Member]
Regulatory Asset [Axis]
Regulatory Asset [Domain]
Regulatory Asset [Domain]
Pension Costs [Member]
Change in Benefit Obligation:
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Obligation beginning of period
Defined Benefit Plan, Benefit Obligation
Service cost
Defined Benefit Plan, Service Cost
Interest cost
Defined Benefit Plan, Interest Cost
Actuarial (gain) loss
Settlements
Defined Benefit Plan, Purchases, Sales, and Settlements
Benefits paid
Defined Benefit Plan, Benefits Paid
Benefit obligation end of period
Change in Fair Value of Plan Assets:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period
Defined Benefit Plan, Fair Value of Plan Assets
Return on plan assets
Defined Benefit Plan, Actual Return on Plan Assets
Benefits paid
Fair value of plan assets at end of period
Funded Status
Defined Benefit Plan, Funded Status of Plan
Amounts recognized in balance sheet consist of:
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liability
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities
Noncurrent liability
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent
Net amount recognized
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Amounts recognized in regulatory assets consist of:
Regulatory Assets and Liabilities, Other Disclosures [Abstract]
Prior service (cost) credit
Defined Benefit Plan Amortization Of Prior Service Cost Credit Regulatory Assets
Defined Benefit Plan Amortization Of Prior Service Cost Credit Regulatory Assets
Net actuarial loss
Defined Benefit Plan Actuarial Net Gains Losses In Regulatory Assets
Defined Benefit Plan Actuarial Net Gains Losses In Regulatory Assets
Amounts recognized in AOCI consist of:
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Prior service cost
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax
Net actuarial gain
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax
Total
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Projected Benefit Obligation
Accumulated benefit obligation
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Accumulated Benefit Obligation
Fair value of plan assets
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Fair Value of Plan Assets
Leases [Abstract]
Schedule of Future Minimum Payments for Operating Leases
Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block]
Unsecured Revolving Line of Credit [Abstract]
Unsecured Revolving Line of Credit [Abstract]
Line of Credit Facility [Table]
Line of Credit Facility [Table]
Unsecured Revolving Line Of Credit [Member]
Unsecured Revolving Line Of Credit [Member]
Unsecured Revolving Line Of Credit [Member]
Borrowings Interest Rate By Type [Axis]
Borrowings Interest Rate By Type [Axis]
Borrowings Interest Rate By Type [Axis]
Borrowings Interest Rate By Type [Domain]
Borrowings Interest Rate By Type [Domain]
Borrowings Interest Rate By Type [Domain]
Eurodollar [Member]
Eurodollar [Member]
Base Rate [Member]
Base Rate [Member]
Lender Name [Axis]
Lender Name [Axis]
Line of Credit Facility, Lender [Domain]
Line of Credit Facility, Lender [Domain]
Credit Facility [Axis]
Credit Facility [Axis]
Credit Facility [Domain]
Credit Facility [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility [Line Items]
Maximum borrowing capacity
Line of Credit Facility, Maximum Borrowing Capacity
Basis spread on variable rate
Debt Instrument, Basis Spread on Variable Rate
Number of institutions participating in the credit facility
Line Of Credit Facility, Number Of Institutions Participating In The Faciltiy
Line Of Credit Facility, Number Of Institutions Participating In The Faciltiy
Maximum number of institutions participating in the credit faciltiy pertaining to maximum contributory percentage
Line Of Credit Facility, Maximum Number Of Institutions Participating In The Faciltiy Pertaining To Maximum Contributory Percentage
Line Of Credit Facility, Maximum Number Of Institutions Participating In The Faciltiy Pertaining To Maximum Contributory Percentage
Line of Credit Facility, maximum percentage of total availability provided by a single lender
Line of Credit Faciiity, Maximum Percentage of Total Availability Provided by a Single Lender
Maximum percentage of total availability provided by a single lender for the line of credit facility.
Commitment fees
Line of Credit Facility, Commitment Fee Amount
Letters of credit outstanding, amount
Letters of Credit Outstanding, Amount
Maximum ratio of indebtedness to net capital threshold percentage
Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage
Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage
Long-term Debt and Capital Lease Obligations [Abstract]
Schedule of Long-term Debt Instruments [Table]
Schedule of Long-term Debt Instruments [Table]
Unsecured Debt
Line of Credit [Member]
Secured Debt
Secured Debt [Member]
Unamortized Discount Premium, Net
Debt Instrument Unamortized Discount Premium Net [Member]
Debt Instrument Unamortized Discount Premium Net [Member]
Debt Instrument [Axis]
Debt Instrument [Axis]
Debt Instrument, Name [Domain]
Debt Instrument, Name [Domain]
Unsecured Revolving Line Of Credit
South Dakota, 6.05%, Due 2018
Secured Debt South Dakota Due 2018 [Member]
Secured Debt South Dakota Due 2018 [Member]
South Dakota, 5.01%, Due 2025
Secured Debt South Dakota Due 2025 [Member]
Secured Debt South Dakota Due 2025 [Member]
South Dakota, 4.15%, Due 2042
Secured Debt South Dakota Due 2042 [Member]
Secured Debt South Dakota Due 2042 [Member]
South Dakota, 4.30%, Due 2052
Secured Debt South Dakota Due 2052 [Member]
Secured Debt South Dakota Due 2052 [Member]
South Dakota, 4.85% Due 2043
Secured Debt South Dakota Due 2043 [Member]
Secured Debt South Dakota Due 2043 [Member]
Secured Debt South Dakota Due 2044 [Member]
Secured Debt South Dakota Due 2044 [Member]
Secured Debt South Dakota Due 2044 [Member]
Montana, 6.04%, Due 2016
Secured Debt Montana Due 2016 [Member]
Secured Debt Montana 2016 [Member]
Montana, 6.34%, Due 2019
Secured Debt Montana Due 2019 [Member]
Secured Debt Montana Due 2019 [Member]
Montana, 5.71%, Due 2039
Secured Debt Montana Due 2039 [Member]
Secured Debt Montana Due 2039 [Member]
Montana, 5.01%, Due 2025
Secured Debt Montana Due 2025 [Member]
Secured Debt Montana Due 2025 [Member]
Montana, 4.15%, Due 2042
Secured Debt Montana Due 2042 [Member]
Secured Debt Montana Due 2042 [Member]
Montana, 4.30%, Due 2052
Secured Debt Montana Due 2052 [Member]
Secured Debt Montana Due 2052 [Member]
Montana 4.85%, Due 2043
Secured Debt Montana Due 2043 [Member]
Secured Debt Montana Due 2043 [Member]
Montana 3.99% Due 2028
Secured Debt Montana Due 2028 [Member]
Secured Debt Montana Due 2028 [Member]
Montana 4.176% Due 2044 [Member]
Secured Debt Montana Due 2044 [Member]
Secured Debt Montana Due 2044 [Member]
Montana, 4.65%, Due 2023
Secured Debt Montana Due 2023 [Member]
Secured Debt Montana Due 2023 [Member]
Discount on Notes and Bonds
New Market Tax Credit Financing [Member]
New Market Tax Credit Financing [Member]
New Market Tax Credit Financing [Member]
Debt Instrument [Line Items]
Debt Instrument [Line Items]
Long-term debt
Less current maturities
Long-term Debt, Current Maturities
Long-term debt, excluding current maturities
Long-term Debt, Excluding Current Maturities
Interest rate, stated percentage
Debt Instrument, Interest Rate, Stated Percentage
Maturity date
Debt Instrument, Maturity Date
Capital Lease
Capital Lease Obligations [Abstract]
Total Capital Leases
Capital Lease Obligations
Less current maturities
Capital Lease Obligations, Current
Capital lease obligations, noncurrent
Capital Lease Obligations, Noncurrent
Commitments and Contingencies Disclosure [Abstract]
Schedule of Environmental Liabilities and Regulation [Table]
Schedule of Environmental Liabilities and Regulation [Table]
Schedule of Environmental Liabilities and Regulation [Table]
Minimum
Maximum
Environmental Remediation Contingency [Axis]
Environmental Remediation Contingency [Axis]
Environmental Remediation Contingency [Domain]
Environmental Remediation Contingency [Domain]
Environmental remediation obligations [Member]
Environmental remediation obligations [Member]
Environmental remediation obligations
Manufactured Gas Plants [Member]
Manufactured Gas Plants [Member]
Manufactured Gas Plants [Member]
Site Contingecy by Site [Axis]
Site Contingecy by Site Axis [Axis]
Site [Domain]
Site [Domain]
Combined Manufacturing Sites [Member]
Combined Manufacturing Sites [Member]
Combined Manufacturing Sites [Member]
Aberdeen South Dakota Site [Member]
Aberdeen South Dakota Site [Member]
Big Stone Generating Facility [Member]
Big Stone Generating Facility [Member]
Coyote Generating Facility [Member]
Coyote Generating Facility [Member]
Neal 4 Generating Facility [Member]
Neal 4 Generating Facility [Member]
Jointly Owned Utility Plant [Axis]
Jointly Owned Utility Plant [Domain]
Colstrip Unit 4 [Member]
Colstrip Unit 4 [Member]
Colstrip Unit 4 [Member]
Neal 4 Generating Facility [Member]
Coyote Generating Facility [Member]
Big Stone Generating Facility [Member]
Statement [Line Items]
Other Commitment
Other Commitment
Long Term Purchase Commitments Term In Years
Long Term Purchase Committnets Term In Years
Long Term Purchase Committnets Term In Years
Environmental remediation obligation, minimum
Environmental Remediation Obligation Minimum
Environmental Remediation Obligation Minimum
Environmental remediation obligation, maximum
Environmental Remediation Obligation Maximum
Environmental Remediation Obligation Maximum
Accrual for environmental loss contingencies
Accrual for Environmental Loss Contingencies
Environmental remediation obligation, to be incurred during next 5 years
Environmental Remediation Obligation, To be Incurred During Next 5 years
Environmental Remediation Obligation, To be Incurred During Next 5 years
Environmental obligation, estimated capital expenditures
Environmental Obligation, Estimated Capital Expenditures
Estimated capital expenditure projections related to environmental obligations.
Number of years for environmental remediation obligation to be incurred
Number Of Years For Environmental Remediation Obligation To Be Incurred
Number Of Years For Environmental Remediation Obligation To Be Incurred
Jointly owned utility plant, proportionate ownership share
Jointly Owned Utility Plant, Proportionate Ownership Share
NOx emissions per million Btu
NOx Emissions Per Million Btu As Calculated On A 30-day Rolling Basis
NOx Emissions Per Million Btu As Calculated On A 30-day Rolling Basis
Capitalized costs, jointly owned utility plant
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress
Long-term Purchase Commitment [Table]
Long-term Purchase Commitment [Table]
Category of Item Purchased [Axis]
Category of Item Purchased [Axis]
Long-term Purchase Commitment, Category of Item Purchased [Domain]
Long-term Purchase Commitment, Category of Item Purchased [Domain]
Purchased Coal and Natural Gas Supply And Natural Gas Transportation Contracts [Member]
Purchased Coal and Natural Gas Supply And Natural Gas Transportation Contracts [Member]
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Committments Costs Incurred
Long Term Purchase Committments Costs Incurred
Long Term Purchase Committments Costs Incurred
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015
Recorded Unconditional Purchase Obligation Due in Next Twelve Months
2016
Recorded Unconditional Purchase Obligation Due in Second Year
2017
Recorded Unconditional Purchase Obligation Due in Third Year
2018
Recorded Unconditional Purchase Obligation Due in Fourth Year
2019
Recorded Unconditional Purchase Obligation Due in Fifth Year
Thereafter
Recorded Unconditional Purchase Obligation Due after Fifth Year
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
Schedule of Nonvested Share Activity
Schedule of Nonvested Share Activity [Table Text Block]
Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table]
Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table]
Plan Name [Axis]
Plan Name [Axis]
Plan Name [Domain]
Plan Name [Domain]
Award Type [Axis]
Award Type [Axis]
Award Type [Domain]
Equity Award [Domain]
Class of Stock [Axis]
Class of Stock [Axis]
Class of Stock [Domain]
Class of Stock [Domain]
Share-based Compensation Arrangement by Share-based Payment Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of Nonvested Restricted Stock Units Activity
Schedule of Nonvested Restricted Stock Units Activity [Table Text Block]
Executive retirement/retention program [Member]
Executive retirement retention program [Member]
Executive retirement/retention program established in December 2011.
Performance Shares [Member]
Performance Shares [Member]
Restricted Stock [Member]
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning nonvested grants (shares)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number
Granted (shares)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period
Vested (shares)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period
Forfeited (shares)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period
Remaining nonvested grants (shares)
Beginning nonvested (weighted-average grant date fair value)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value
Granted (weighted-average grant date fair value)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value
Vested (weighted-average grant date fair value)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value
Forfeited (weighted-average grant date fair value)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value
Remaining nonvested (weighted-average grant date fair value)
Performance and vesting period
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period
Goodwill
Goodwill Disclosure [Text Block]
Property, Plant and Equipment
Property, Plant and Equipment Disclosure [Text Block]
Schedule of Change in Asset Retirement Obligation
Schedule of Change in Asset Retirement Obligation [Table Text Block]
Regulatory Assets and Liabilities Disclosure [Abstract]
Schedule of Regulatory Assets And Liabilities
Schedule of Regulatory Assets And Liabilities [Table Text Block]
Schedule of Regulatory Assets And Liabilities [Table Text Block]
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative [Table]
Derivative [Table]
Derivative Instrument [Axis]
Derivative Contract [Domain]
Interest rate swap at 3.217% [Member]
Interest rate swap at 3.217% [Member]
Interest rate swap at 3.227% [Member]
Interest rate swap at 3.227% [Member] [Member]
Interest rate swap at 3.227% [Member] [Member]
Interest rate swap at 3.227% [Member] [Member]
Interest Rate Swap [Member]
Commodity Contract [Member]
Derivative, by Nature [Axis]
Derivative, Name [Domain]
Forward Contracts [Member]
Hedging Designation [Axis]
Hedging Designation [Domain]
Cash Flow Hedging [Member]
Designated as Hedging Instrument [Member]
Balance Sheet Location [Axis]
Balance Sheet Location [Domain]
Income Statement Location [Axis]
Income Statement Location [Domain]
Interest Expense [Member]
Derivative [Line Items]
Derivative [Line Items]
Number of forward starting swaps
Number of forward starting swaps
Number of forward starting swaps
Amount of hedged item
Derivative, Amount of Hedged Item
Derivative, fixed interest rate
Derivative, Fixed Interest Rate
Debt issuance for Hydro transaction
Other Long-term Debt
Realized loss on cash flow hedging derivatives
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax
Physical purchase and sale of gas and electricity at fixed prices
Physical Purchase And Sale of Gas And Electricity At Fixed Prices
Physical Purchase And Sale of Gas And Electricity At Fixed Prices
Interest rate contracts, amount of gain reclassified from AOCI into income
Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net
Pre-tax loss on cash flow hedges remaining in AOCI
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)
Pre-tax gain on cash flow hedge from AOCI to be reclassified during next 12 months
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net
No swaps outstanding, interest rate fair value derivatives
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net
None of the forward purchase contracts that do not qualify for NPNS contain credit risk-related contingent features
Derivative, Net Liability Position, Aggregate Fair Value
Regulated Operations [Abstract]
Schedule Of Regulated Operations [Table]
Schedule Of Regulated Operations [Table]
Significant Acquisitions and Disposals by Transaction [Axis]
Significant Acquisitions and Disposals by Transaction [Axis]
Significant Acquisitions and Disposals, Transaction [Domain]
Significant Acquisitions and Disposals, Transaction [Domain]
Hydro-electric Assets [Member]
Hydro-electric Assets [Member]
Hydro-electric Assets [Member]
Jointly Owned Utility Plant [Axis]
Jointly Owned Utility Plant [Domain]
Regulatory Liability [Axis]
Regulatory Liability [Axis]
Regulatory Liability [Domain]
Regulatory Liability [Domain]
Revenue Subject to Refund [Member]
Revenue Subject to Refund [Member]
Regulatory Matters [Axis]
Regulatory Matters [Axis]
Regulatory Matters [Domain]
Regulatory Matters [Domain]
South Dakota Electric Rate Filing [Member]
South Dakota Electric Rate Filing [Member]
South Dakota Electric Rate Filing [Member]
Dave Gates Generating Station [Member]
Dave Gates Generating Station [Member]
Regulatory Reviews of Filings [Member]
Regulatory Reviews of Filings [Member]
Montana Natural Gas Production Assets [Member]
Montana Natural Gas Production Assets [Member]
Montana Natural Gas Production Assets [Member]
Regulatory Asset [Axis]
Regulatory Assets [Line Items]
Regulatory Assets [Line Items]
Requested rate increase
Public Utilities, Requested Rate Increase (Decrease), Amount
Approved return on equity, percentage
Public Utilities, Requested Return on Equity, Percentage
Capital structure,percentage of debt
Public Utilities, Requested Debt Capital Structure, Percentage
Capitalized structure, percentage of equity
Public Utilities, Requested Equity Capital Structure, Percentage
Rate base
Public Utilities Rate Base
Public Utilities Rate Base
Deferred revenue subject to refund
Customer Refund Liability, Current
CU4 incremental market purchases identified for further review
Utilities Operating Expense, Purchased Power
Demand side management lost revenue expected recovery
Demand Side Management Lost Revenue Expected Recovery
Demand side management lost revenues of a regulated entity, relating to the planning, implementing, and monitoring activities which are designed to encourage consumers to modify their level and pattern of energy usage, which are expected to be recoverable through rate adjustments.
Revenue of acquiree since acquisition date, Actual
Revenue of Acquiree since Acquisition Date, Actual
This element represents the amount of revenue of the acquiree since the acquisition date.
Quarterly Financial Data
Quarterly Financial Information [Text Block]
Operating Loss Carryforwards [Table]
Operating Loss Carryforwards [Table]
Expiration [Axis]
Expiration [Axis]
Expiration [Axis]
Expiration [Domain]
Expiration [Domain]
Year 2025 [Member]
Year 2025 [Member]
Expiration dates for unused Federal NOL carryforwards.
Year 2028 [Member]
Year 2028 [Member]
Year 2029 [Member]
Year 2029 [Member]
Year 2029 [Member]
Year 2031 [Member]
Year 2031 [Member]
Expiration dates for unused Federal NOL carryforwards.
Year 2033 [Member]
Year 2033 [Member]
Expiration dates for unused Federal NOL carryforwards.
Year 2034 [Member] [Member]
Year 2034 [Member] [Member]
Year 2034 [Member]
Year 2015 [Member]
Expiration dates for unused State NOL carryforwards.
Year 2016 [Member]
Year 2016 [Member]
Expiration dates for unused State NOL carryforwards.
Year 2018 [Member]
Year 2018 [Member]
Expiration dates for unused State NOL carryforwards.
Year 2020 [Member]
Year 2020 [Member]
Expiration dates for unused State NOL carryforwards.
Year 2021 [Member]
Year 2021 [Member]
Year 2021 [Member]
Income Tax Authority [Axis]
Income Tax Authority [Axis]
Income Tax Authority [Domain]
Income Tax Authority [Domain]
Domestic Tax Authority [Member]
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards [Line Items]
NOL carryforward
Operating Loss Carryforwards
NOL carryforwards, subject to expiration
Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration
Schedule of Segment Reporting Information, by Segment [Table]
Schedule of Segment Reporting Information, by Segment [Table]
Statement, Business Segments [Axis]
Segments [Axis]
Segment [Domain]
Segments [Domain]
Electric
Electric Domestic Regulated [Member]
Electric Domestic Regulated [Member]
Gas
Gas Domestic Regulated [Member]
Gas Domestic Regulated [Member]
Other Segments [Member]
Other Segments [Member]
Eliminations
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information [Line Items]
Operating revenues
Revenues
Cost of sales
Cost of Domestic Regulated Electric and Cost of Natural Gas Purchases
Cost of electricity sold during the reporting period, operations of which is regulated by a federal or state agency and natural gas purchased and associated transportation and storage costs during the period
Gross margin
Gross Profit
Operating, general and administrative
Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses
MSTI impairment
Impairment of Abandoned Project
The adjustment to expense of previously capitalized costs when the entity obtains information that raises substantial doubt about the economic or operational viability of the project.
Property and other taxes
Taxes, Miscellaneous
Depreciation and depletion
Depreciation, Depletion and Amortization
Operating Income
Operating Income (Loss)
Interest expense
Interest and Debt Expense
Other income
Other Nonoperating Income (Expense)
Income tax (expense) benefit
Net Income
Net Income (Loss) Attributable to Parent
Total assets
Assets
Capital expenditures
Property, Plant and Equipment, Additions
Segment and Related Information
Segment Reporting Disclosure [Text Block]
Regulatory Assets and Liabilities
Schedule of Regulatory Assets and Liabilities [Text Block]
Income taxes, cash paid (received)
Income Taxes Paid, Net
Interest, cash paid (received)
Interest Paid
Capital expenditures included in trade accounts payable
Capital Expenditures Incurred but Not yet Paid
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015
Operating Leases, Future Minimum Payments Due, Next Twelve Months
2016
Operating Leases, Future Minimum Payments, Due in Two Years
2017
Operating Leases, Future Minimum Payments, Due in Three Years
2018
Operating Leases, Future Minimum Payments, Due in Four Years
2019
Operating Leases, Future Minimum Payments, Due in Five Years
Employee Benefit Plans
Pension and Other Postretirement Benefits Disclosure [Text Block]
Earnings Per Share [Abstract]
Average Shares Used In Computing Basic And Diluted Earnings Per Share [Table]
Average Shares Used In Computing Basic And Diluted Earnings Per Share [Table]
Average Shares Used In Computing Basic And Diluted Earnings Per Share [Table]
Antidilutive Securities [Axis]
Antidilutive Securities, Name [Domain]
Basic computation
Weighted Average Number of Shares Outstanding, Basic
Restricted stock and performance share awards (1)
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements
Diluted computation
Weighted Average Number of Shares Outstanding, Diluted
Damages sought,value
Loss Contingency, Damages Sought, Value
Retention amount
Unusual or Infrequent Item, Net of Insurance Proceeds
Operating Leases
Leases of Lessee Disclosure [Text Block]
Major Types of Debt and Equity Securities [Axis]
Major Types of Debt and Equity Securities [Domain]
Common Stock [Member]
Common Stock [Member]
Equity-Based Arrangements, Individual Contracts, Type of Deferred Compensation [Axis]
Equity-Based Arrangements, Individual Contracts, Type of Deferred Compensation [Axis]
Equity Based Arrangements Individual Contracts Type of Deferred Compensation
Equity-Based Arrangements, Individual Contracts, Type of Deferred Compensation [Domain]
Deferred Stock Unit [Member]
Deferred Stock Unit [Member]
Deferred Stock Unit [Member]
Stock Compensation Plan [Member]
Stock Compensation Plan [Member]
Number of shares authorized
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized
Number of shares available for grant
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant
Compensation expense
Allocated Share-based Compensation Expense
Compensation expense tax benefit
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense
Compensation expense not yet recognized for nonvested awards
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized
Nonvested awards, total compensation cost not yet recognized, period for recognition
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition
Shares vested in period, total fair value
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value
Maximum percentage of compensation to be deferred
Share Based Compensation Arrangement by Share-based Payment Award, Maximum Percentage Of Compensation To be Deferred
Share Based Compensation By Share Based Payment Award Maximum Percentage Of Compensation To be Deferred
Maximum number of years for distribution payments
Share Based Compensation Arrangement by Share-based Payment Award, Maximum Number Of Years For Distribution Payments
Share Based Compensation Arrangement Share Based Payment Maximum Number Of Years For Distribution Payments
Deferred stock units issued during period, shares
Deferred Compensation Arrangement with Individual, Shares Issued
Deferred stock units total compensation expense
Deferred Compensation Arrangement with Individual, Compensation Expense
Share-based Compensation, Significant Assumptions
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Percent of shares issued based on company performance
Share Based Compensation Arrangement By Share-based Payment Award, Performance Based Awards Percentage Of Shares Issued Relative To Company Actual Performance
Share Based Compensation Arrangement Performance Based Awards Percentage Of Shares Issued Relative To Company Actual Performance
Risk-free interest rate
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate
Expected life, in years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term
Expected volatility, minimum
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Minimum
Expected volatility, maximum
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Maximum
Dividend yield
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate
Geographical [Domain]
Pension contributions
Pension Contributions
Expected contributions to pension plans
Expected Contributions To Pension Plans
Estimated amount of cash or cash equivalents to be contributed by the entity to fund its pension plans during the next year.
Schedule Of Regulatory Assets And Liabilities [Table]
Schedule Of Regulatory Assets And Liabilities [Table]
Schedule Of Regulatory Assets And Liabilities [Table]
Regulatory Assets [Member]
Other Assets [Member]
Regulatory Liabilities [Member]
Other Liabilities [Member]
Pension [Member]
Employee related benefits [Member]
Employee Related Benefits [Member]
Employee related benefits include post retirement and post employment benefits.
Distribution infrastructure projects
Montana Distribution System Infrastructure Project [Member]
Montana Distribution System Infrastructure Project [Member]
Environmental clean-up [Member]
Environmental Restoration Costs [Member]
Supply costs [Member]
Supply Costs [Member]
Supply Costs [Member]
Electric Supply Costs [Member]
Electric Supply Costs [Member]
Electric Supply Costs [Member]
Income taxes [Member]
Deferred Income Tax Charge [Member]
Deferred financing costs [Member]
Deferred Finaincing Costs [Member]
Deferred Finaincing Costs [Member]
State And Local Taxes And Fees [Member]
State And Local Taxes And Fees [Member]
State and local taxes and fees.
Other [Member]
Other Regulatory Assets (Liabilities) [Member]
Removal cost
Asset Retirement Obligation Costs [Member]
Gas storage sales
493190 Other Warehousing and Storage [Member]
Supply costs [Member]
Deferred revenue [Member]
Environmental clean-up [Member]
South Dakota
Nebraska
NEBRASKA
Regulatory Assets And Liabilities [Line Items]
Regulatory Assets And Liabilities [Line Items]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets
Regulatory Assets
Regulatory liabilities
Regulatory Liabilities
Regulatory assets, remaining amortization period
Regulatory Noncurrent Asset, Amortization Period
Regulatory liability, remaining amortization period
Regulatory Noncurrent Liability, Amortization Period
Stock-Based Compensation
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]
Schedule of Defined Benefit Plans Disclosures [Table]
Schedule of Defined Benefit Plans Disclosures [Table]
Net Periodic Costs [Member]
Net Periodic Costs [Member]
Other Pension Plan, Postretirement or Supplemental Plans [Member]
Components of Net Periodic Benefit Cost
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost
Interest cost
Expected return on plan assets
Defined Benefit Plan, Expected Return on Plan Assets
Amortization of prior service cost (credit)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)
Recognized actuarial loss
Net Periodic Benefit Cost (Credit)
Defined Benefit Plan, Net Periodic Benefit Cost
Accumulated gain
Defined Benefit Plan, Amortization of Gains (Losses)
Schedule of Debt and Capital Leases
Schedule of Debt [Table Text Block]
Jointly Owned Utility Plant Interests [Table]
Jointly Owned Utility Plant Interests [Table]
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant Interests [Line Items]
Number of joint ownership interests in electric generating plants
Number Of Joint Ownership Interest Electric Generating Plants
Number Of Joint Ownership Interest Electric Generating Plants
Ownership percentages
Plant in service
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service
Accumulated depreciation
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation
Schedule of Significant Events, Acquisitions and Disposals [Text Block]
Schedule of Significant Events, Acquisitions and Disposals [Text Block]
Schedule of significant events includes material events that would not reasonably be expected to occur in the foreseeable future that are related to ordinary and typical activities of the regulated entity.
Statement of Financial Position [Abstract]
Statement [Table]
Statement [Line Items]
ASSETS
Assets [Abstract]
Current Assets:
Assets, Current [Abstract]
Cash and cash equivalents
Cash and Cash Equivalents, at Carrying Value
Restricted cash
Restricted Cash and Cash Equivalents, Current
Accounts receivable, net
Accounts Receivable, Net, Current
Inventories
Regulatory assets
Regulatory Assets, Current
Deferred income taxes
Deferred Tax Assets, Net of Valuation Allowance, Current
Other
Other Assets, Current
Total current assets
Assets, Current
Property, plant, and equipment, net
Goodwill
Goodwill
Regulatory assets
Regulatory Assets, Noncurrent
Other noncurrent assets
Other Assets, Noncurrent
Total assets
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities and Equity [Abstract]
Current Liabilities:
Liabilities, Current [Abstract]
Current maturities of capital leases
Short-term borrowings
Accounts payable
Accounts Payable, Current
Accrued expenses
Accrued Liabilities, Current
Regulatory liabilities
Regulatory Liability, Current
Total current liabilities
Liabilities, Current
Long-term capital leases
Long-term debt
Deferred income taxes
Deferred Tax Liabilities, Net, Noncurrent
Noncurrent regulatory liabilities
Regulatory Liability, Noncurrent
Other noncurrent liabilities
Other Liabilities, Noncurrent
Total liabilities
Liabilities
Commitments and Contingencies (Note 20)
Commitments and Contingencies
Shareholders' Equity:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract]
Common stock, par value $0.01; authorized 200,000,000 shares; issued and outstanding 50,522,280 and 46,914,811 respectively; Preferred stock, par value $0.01; authorized 50,000,000 shares; none issued
Common Stock, Value, Issued
Treasury stock at cost
Treasury Stock, Value
Paid-in capital
Additional Paid in Capital, Common Stock
Retained earnings
Retained Earnings (Accumulated Deficit)
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss), Net of Tax
Total shareholders' equity
Stockholders' Equity Attributable to Parent
Total liabilities and shareholders' equity
Liabilities and Equity
Bridge loan [Abstract]
Bridge loan [Abstract]
Bridge Loan [Member]
Bridge Loan [Member]
Bridge term loan facility
Bridge Loan
Bridge term loan facility agreement term
Number Of Days For Bridge Loan Term
Number of days until bridge loan terminates.
Statement [Table]
Legal Entity [Axis]
Entity [Domain]
Allowance for doubtful accounts receivable, current
Allowance for Doubtful Accounts Receivable, Current
Unbilled receivables,current
Unbilled Receivables, Current
Number of days or less of maturity to be considered cash equivalent
Number Of Days Or Less Of Maturity To Be Considered Cash Equivalent
Number Of Days Or Less Of Maturity To Be Considered Cash Equivalent
Schedule of Goodwill [Table]
Schedule of Goodwill [Table]
Natural gas
Goodwill [Line Items]
Goodwill [Line Items]
Goodwill, Period Increase (Decrease)
Goodwill, Period Increase (Decrease)
Schedule of Changes in Projected Benefit Obligations
Schedule of Changes in Projected Benefit Obligations [Table Text Block]
Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets
Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets [Table Text Block]
Schedule of Defined Benefit Plans Disclosures
Schedule of Defined Benefit Plans Disclosures [Table Text Block]
Schedule of Estimated Amortization of Regulatory Assets Into Net Periodic Benefit Costs
Schedule Of Estimated Amortization Of Regulatory Assets Into Net Periodic Benefit Costs [Table Text Block]
Schedule Of Estimated Amortization Of Regulatory Assets Into Net Periodic Benefit Costs [Table Text Block]
Schedule of Assumptions Used
Schedule of Assumptions Used [Table Text Block]
Schedule of Pension And Postretirement Benefits Investment Strategy
Schedule Of Pension And Postretirement Benefits Investment Strategy [Table Text Block]
Schedule Of Pension And Postretirement Benefits Investment Strategy [Table Text Block]
Schedule of Allocation of Plan Assets
Schedule of Allocation of Plan Assets [Table Text Block]
Schedule of Fair Value of Plan Assets by Category
Schedule Of Fair Value Of Plan Assets By Category [Table Text Block]
Schedule Of Fair Value Of Plan Assets By Category [Table Text Block]
Schedule of Pension Contributions
Schedule of pension contributions [Table Text Block]
Schedule of pension contributions made by the entity during the period
Schedule of Expected Benefit Payments
Schedule of Expected Benefit Payments [Table Text Block]
Equity Components [Axis]
Equity Component [Domain]
Common Stock
Paid-in Capital
Additional Paid-in Capital [Member]
Treasury Stock
Treasury Stock [Member]
Retained Earnings
Retained Earnings [Member]
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Member]
Balance
Shares, Balance
Shares, Issued
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income
Other comprehensive income:
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Foreign currency translation adjustment
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax
Reclassification of net gains on derivative instruments from OCI to net income, net of tax
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax
Realized loss on cash flow hedging activities
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax
Postretirement medical liability adjustment
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax
Stock based compensation, value
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures
Stock based compensation, shares
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures
Issuance of shares, value
Stock Issued During Period, Value, New Issues
Issuance of shares, shares
Stock Issued During Period, Shares, New Issues
Issuance of shares, value, treasury stock reissued
Stock Issued During Period, Value, Treasury Stock Reissued
Issuance of shares, shares, treasury stock reissued
Stock Issued During Period, Shares, Treasury Stock Reissued
Dividends on common stock
Dividends, Common Stock
Dividends per share
Common Stock, Dividends, Per Share, Declared
Balance
Shares, Balance
Debt Disclosure [Abstract]
Secured Debt South Dakota Due 2044 [Member]
Debt Instrument, Issuance Date
Debt Instrument, Issuance Date
Debt instrument, face amount
Debt Instrument, Face Amount
Maturity date
Components of AOCI [Abstract]
Components of AOCI [Abstract]
Reclassification out of Accumulated Other Comprehensive Income [Table]
Reclassification out of Accumulated Other Comprehensive Income [Table]
Equity Components [Axis]
Equity Component [Domain]
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Defined Benefit Plans Adjustment [Member]
Foreign Currency Gain (Loss) [Member]
Accumulated Translation Adjustment [Member]
Other Comprehensive Income (Loss) [Member]
Other Comprehensive Income (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Axis]
Reclassification out of Accumulated Other Comprehensive Income [Axis]
Reclassification out of Accumulated Other Comprehensive Income [Domain]
Reclassification out of Accumulated Other Comprehensive Income [Domain]
Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance
Other comprehensive income before reclassifications
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax
Amounts reclassified from accumulated other comprehensive income
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax
Net current-period other comprehensive (loss) income
Other Comprehensive Income (Loss), Net of Tax
Ending balance
Use of Estimates
Use of Estimates, Policy [Policy Text Block]
Revenue Recognition
Revenue Recognition, Policy [Policy Text Block]
Cash Equivalents
Cash and Cash Equivalents, Policy [Policy Text Block]
Restricted cash
Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block]
Accounts Receivable, Net
Receivables, Policy [Policy Text Block]
Regulation of Utility Operations
Public Utilities, Policy [Policy Text Block]
Derivative Financial Instruments
Derivatives, Policy [Policy Text Block]
Property, Plant and Equipment
Property, Plant and Equipment, Policy [Policy Text Block]
Income Taxes
Income Tax, Policy [Policy Text Block]
Environmental Costs
Environmental Costs, Policy [Policy Text Block]
Business Combinations
Business Combinations Policy [Policy Text Block]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Table]
Fair Value Measurements, Recurring and Nonrecurring [Table]
Fair Value by Measurement Frequency [Axis]
Quoted Prices In Active Markets for Identical Assets or Liabilities, Level 1 [Member]
Significant Other Observable Inputs, Level 2 [Member]
Significant Unobservable Inputs, Level 3 [Member]
Margin Cash Collateral Offset
Total Net Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 1 to level 2 asset transfers, amount
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount
Level 2 to level 1 assets, transfers, amount
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount
Level 1 to level 2 liabilities transfers, amount
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount
Level 2 to level 1 liabilities, transfers, amount
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount
Restricted cash
Restricted Cash, Fair Value Disclosure
Restricted Cash, Fair Value Disclosure
Rabbi trust investments
Rabbi Trust Investment, Fair Value Disclosure
Rabbi Trust Investment, Fair Value Disclosure
Total
Fair Value, Net Asset (Liability)
Income Tax Contingency [Table]
Income Tax Contingency [Table]
Income Tax Contingency [Line Items]
Income Tax Contingency [Line Items]
Unrecognized tax benefit more likely than not percentage threshold
Unrecognized Tax Benefit More likely Than Not Percentage Threshold
Unrecognized Tax Benefit More likely Than Not Percentage Threshold
Unrecognized tax benefits that would impact effective tax rate
Unrecognized Tax Benefits that Would Impact Effective Tax Rate
Interest and penalty expense
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense
Accrual for interest and penalties
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued
Safe harbor adoption
Income Tax Credits and Adjustments
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits at January 1
Unrecognized Tax Benefits
Gross increases - tax positions in prior period
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions
Gross decreases - tax positions in prior periods
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions
Gross increases - tax positions in current period
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions
Gross decreases - tax positions in current period
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations
Unrecognized Tax Benefits at December 31
Matching employer contributions
Defined benefit plan percentage threshold of differences between actuarial assumptions and actual plan results that are greater than projected benefit or market value
Defined Benefit Plan Percentage Threshold Of Differences Between Actuarial Assumptions And Actual Plan Results That Are Greater Than Projected Benefit Or Market Value
Defined Benefit Plan Percentage Threshold Of Differences Between Actuarial Assumptions And Actual Plan Results That Are Greater Than Projected Benefit Or Market Value
Nature of Operations and Basis of Consolidation [Abstract]
(1) Nature of Operations and Basis of Consolidation [Abstract]
Operations And Basis Of Consolidation [Table]
Operations And Basis Of Consolidation [Table]
Number of customers
Number Of Customers
Number Of Customers
Number of megawatts of qualifying facility
Number Of Megawatts of Qualifying Facility
Number Of Megawatts of Qualifying Facility
Estimated aggregate gross contractual payments through 2024
Estimated Aggregate Gross Contractual Payments
Estimated Aggregate Gross Contractual Payments
Variable interest entity, measure of activity, purchases
Variable Interest Entity, Measure of Activity, Purchases
Major classifications of property, plant and equipment
Property, Plant and Equipment [Table Text Block]
Schedule of jointly owned utility plants
Schedule of Jointly Owned Utility Plants [Table Text Block]
Other long term debt [Abstract]
Other long term debt [Abstract]
New Market Tax Credit [Member]
New Market Tax Credit [Member]
New Market Tax Credit [Member]
Variable Rate [Axis]
Variable Rate [Axis]
Variable Rate [Domain]
Variable Rate [Domain]
Debt Instrument, Redemption, Period [Axis]
Debt Instrument, Redemption, Period [Axis]
Debt Instrument, Redemption, Period [Domain]
Debt Instrument, Redemption, Period [Domain]
Other Long-term Debt
Investments
Investments
Debt Instrument, term
Debt Instrument, Term
Investment in New Market Tax Credit Program
Investment in New Market Tax Credit Program
Investment in a federal tax credit program that provides tax credit incentives to investors for equity investments in Certified Development Entities, which invest in low-income communities
Debt instrument, forgiveness
Debt Instrument, Decrease, Forgiveness
Income Statement [Abstract]
Revenues
Revenues [Abstract]
Electric
Electric Domestic Regulated Revenue
Gas
Gas Domestic Regulated Revenue
Other
Unregulated Operating Revenue
Total Revenues
Operating Expenses
Costs and Expenses [Abstract]
Mountain States Transmission Intertie impairment
Depreciation and depletion
Total Operating Expenses
Costs and Expenses
Interest Expense
Other Income
Income Before Income Taxes
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest
Income Tax Benefit (Expense)
Average Common Shares Outstanding
Basic Earnings per Average Common Share
Earnings Per Share, Basic
Diluted Earnings per Average Common Share
Earnings Per Share, Diluted
Schedule of Weighted Average Number of Shares
Schedule of Weighted Average Number of Shares [Table Text Block]
Statement of Cash Flows [Abstract]
OPERATING ACTIVITIES:
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income
Items not affecting cash:
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization of debt issue costs, discount and deferred hedge gain
Amortization of Debt Issue Costs and Discount and Deferred Hedge Gain
The component of interest expense representing the noncash expenses charged against earnings in the period to allocate debt discount and premium, the costs to issue debt and obtain financing over the related debt instruments, and the amount of expense charged against earnings by an Entity to record deferred gains and losses on hedge instruments.
Amortization of nonvested shares
Share-based Compensation
Equity portion of allowance for funds used during construction
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity
Gain on disposition of assets
Gain (Loss) on Disposition of Other Assets
Deferred income taxes
Deferred Income Tax Expense (Benefit)
Mountain States Transmission Intertie impairment
Noncash Project Abandonment Costs
Gain on CELP arbitration decision
Gain on CELP arbitration decision
The entity recorded a gain of approximately $47.9 million during the fourth quarter of 2012 associated with a favorable arbitration decision related to Colstrip Energy Limited Partnership (CELP).
Changes in current assets and liabilities:
Increase (Decrease) in Operating Capital [Abstract]
Restricted cash
Increase (Decrease) in Restricted Cash for Operating Activities
Accounts receivable
Increase (Decrease) in Accounts Receivable
Inventories
Increase (Decrease) in Inventories
Other current assets
Increase (Decrease) in Other Current Assets
Accounts payable
Increase (Decrease) in Accounts Payable
Accrued expenses
Increase (Decrease) in Accrued Liabilities
Regulatory assets
Increase (Decrease) in Other Regulatory Assets
Regulatory liabilities
Increase (Decrease) in Regulatory Liabilities
Other noncurrent assets
Increase (Decrease) in Other Noncurrent Assets
Other noncurrent liabilities
Increase (Decrease) in Other Noncurrent Liabilities
Cash provided by operating activities
Net Cash Provided by (Used in) Operating Activities
INVESTING ACTIVITIES:
Net Cash Provided by (Used in) Investing Activities [Abstract]
Property, plant, and equipment additions
Payments to Acquire Property, Plant, and Equipment
Acquisitions
Payments to Acquire Businesses, Net of Cash Acquired
Proceeds from sale of assets
Proceeds from Sale of Property, Plant, and Equipment
Change in restricted cash
Increase (Decrease) in Restricted Cash and Investments
Investment in New Market Tax Credit program
Cash used in investing activities
Net Cash Provided by (Used in) Investing Activities
FINANCING ACTIVITIES:
Net Cash Provided by (Used in) Financing Activities [Abstract]
Dividends on common stock
Payments of Ordinary Dividends, Common Stock
Proceeds from issuance of common stock, net
Proceeds from Issuance of Common Stock
Issuance of long-term debt
Proceeds from Issuance of Long-term Debt
Repayment of long-term debt
Repayments of Long-term Debt
Issuances (repayments) of short-term borrowings, net
Proceeds from (Repayments of) Commercial Paper
Treasury stock activity
Proceeds from (Repurchase of) Equity
Financing costs
Payments of Financing Costs
Cash provided by financing activities
Net Cash Provided by (Used in) Financing Activities
Increase in Cash and Cash Equivalents
Cash and Cash Equivalents, Period Increase (Decrease)
Cash and Cash Equivalents, beginning of period
Cash and Cash Equivalents, end of period
Schedule of Short-term Debt
Schedule of Short-term Debt [Table Text Block]
Document Entity Information [Abstract]
Document Entity Information [Abstract]
Entities [Table]
Entities [Table]
Legal Entity [Axis]
Entity [Domain]
Entity Information [Line Items]
Entity Information [Line Items]
Entity Registrant Name
Entity Registrant Name
Entity Central Index Key
Entity Central Index Key
Current Fiscal Year End Date
Current Fiscal Year End Date
Entity Filer Category
Entity Filer Category
Document Type
Document Type
Document Period End Date
Document Period End Date
Document Fiscal Year Focus
Document Fiscal Year Focus
Document Fiscal Period Focus
Document Fiscal Period Focus
Amendment Flag
Amendment Flag
Entity Common Stock, Shares Outstanding
Entity Common Stock, Shares Outstanding
Entity Well-known Seasoned Issuer
Entity Well-known Seasoned Issuer
Entity Voluntary Filers
Entity Voluntary Filers
Entity Current Reporting Status
Entity Current Reporting Status
Entity Public Float
Entity Public Float
Purchase Price Allocation [Abstract]
Purchase Price Allocation [Abstract]
Purchase price allocation [Table Text Block]
Purchase price allocation [Table Text Block]
Purchase price allocation [Table Text Block]
Balance Sheet Location [Axis]
Balance Sheet Location [Domain]
Regulatory Liabilities [Member]
Montana Distribution System Infrastructure Project [Member]
Natural Gas Supply Costs [Member]
Natural Gas Supply Costs [Member]
Natural gas supply costs [Member]
Montana
Percentage of interest earned on electric and natural gas supply costs
Percentage Of Interest Earned on Electric And Natural Gas Supply Costs
Percentage Of Interest Earned on Electric And Natural Gas Supply Costs
Percentage of estimated increase In local taxes and fees authorized for recovery by MPSC
Percentage Of Estimated Increase In Local Taxes And Fees Authorized For Recovery By MPSC
Percentage Of Estimated Increase In Local Taxes And Fees Authorized For Recovery By MPSC
Other comprehensive (loss) income, net of tax:
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract]
Reclassification of net gains on derivative instruments to net income
Foreign currency translation
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Portion Attributable to Parent
Total Other Comprehensive Income (Loss)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent
Comprehensive Income
Comprehensive Income (Loss), Net of Tax, Attributable to Parent
Earnings Per Share
Earnings Per Share [Text Block]
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Common Stock
Schedule of Stock by Class [Table Text Block]
Income Taxes
Income Tax Disclosure [Text Block]
Maturities of Long-term Debt [Abstract]
2015
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months
2016
Long-term Debt, Maturities, Repayments of Principal in Year Two
2017
Long-term Debt, Maturities, Repayments of Principal in Year Three
2018
Long-term Debt, Maturities, Repayments of Principal in Year Four
2019
Long-term Debt, Maturities, Repayments of Principal in Year Five
Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss)
Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) [Table Text Block]
Schedule Of Income Tax Expense Domestic
Schedule Of Income Tax Expense Domestic [Table Text Block]
Schedule Of Income Tax Expense Domestic [Table Text Block]
Schedule of Effective Income Tax Rate Reconciliation
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
Schedule of Deferred Tax Assets and Liabilities
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]
Summary of Income Tax Contingencies
Summary of Income Tax Contingencies [Table Text Block]
Significant differences from Federal Statutory rate [Abstract]
Significant differences from Federal Statutory rate [Abstract]
Significant differences from Federal Statutory rate [Abstract]
Permanent or flow through adjutsments [Abstract]
Effective Income Tax Rate Reconciliation Permanent Or Flow Through Adjutsments [Abstract]
Effective Income Tax Rate Reconciliation Permanent Or Flow Through Adjutsments [Abstract]
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent
State income, net of federal provisions
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent
Flow-through repair deduction
Effective Income Tax Rate Reconciliation, Deduction, Other, Percent
Release of unrecognized tax benefit
Effective IncomeTax Rate Reconciliation Unrecognized Tax Benefits, Percent
Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to unrecognized tax benefits, net of federal tax expense (benefit).
Prior year permanent return to accrual adjustments
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Percent
Production tax credits
Effective Income Tax Rate Reconciliation, Deduction, Qualified Production Activity, Percent
Plant and depreciation of flow through items
Income Tax Reconciliation Accelerated Tax Depreciation Flow Through Adjustment
Income Tax Reconciliation, Deductions, Accelerated Tax Depreciation, Flow Through Adjustment
Recognition of state net operating loss benefit / valuation allowance release
Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance And State Net Operating Loss Benefit
Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets and recognition of state net operating loss/benefit.
Other, net
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent
Effective income tax rate
Effective Income Tax Rate Reconciliation, Percent
Income (Loss) Before Income Taxes
Income tax calculated at 35% federal statutory rate
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount
State income, net of federal provisions
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount
Flow-through repairs deductions
Effective Income Tax Rate Reconciliation, Deduction, Other, Amount
Release of unrecognized tax benefit
Effective Income Tax Rate Reconciliation Release of Unrecognized Tax Benefits
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to unrecognized tax benefits, net of federal tax expense (benefit).
Prior year permanent return to accrual adjustments
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount
Production tax credits
Effective Income Tax Rate Reconciliation, Deduction, Qualified Production Activity, Amount
Plant and depreciation of flow through items
Income Tax Reconciliation, Deductions, Accelerated Tax Depreciation, Flow Through Adjustment
Income Tax Reconciliation, Accelerated Tax Depreciation, Flow Through Adjustment
Recognition of state net operating loss benefit / valuation allowance release
Effective Income Tax Expense Reconciliation Change In Deferred Tax Assets Valuation Allowance And State Net Operating Loss Benefit
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets and recognition of state net operating loss/ benefit.
Other, net
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount
Total reconciling items
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount
Assets Acquired [Abstract]
Assets Acquired [Abstract]
Assets Acquired [Abstract]
Inventory
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory
Property, plant, and equipment
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment
Other prepayments
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets
Total assets acquired
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets
Liabilities Assumed [Abstract]
Liabilities Assumed [Abstract]
Liabilities Assumed [Abstract]
Other current liabilities
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other
Other deferred credits
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities
Total liabilities assumed
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities
Total purchase price
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net
Risk Management and Hedging Activities
Derivative Instruments and Hedging Activities Disclosure [Text Block]
Deferred Tax Assets, [Abstract]
Deferred Tax Assets, Gross [Abstract]
Pension / postretirement benefits
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions
NOL carryforward
Deferred Tax Assets, Operating Loss Carryforwards
Unbilled revenue
Deferred Tax Assets Unbilled Revenue
Deferred Tax Assets Unbilled Revenue
Compensation accruals
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Other
Customer advances
Deferred Tax Assets Customer Advances
Deferred Tax Assets Customer Advances
AMT credit carryforwards
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax
Environmental liability
Deferred Tax Assets Environmental Liability
Deferred Tax Assets Environmental Liability
Production tax credits
Deferred tax assets production tax credits
Deferred tax assets resulting from qualified production activities
Interest rate hedges
Deferred Tax Assets, Hedging Transactions
QF obligations
Deferred Tax Asset QF obligation
Deferred tax asset associated with our Qualifying Facilities (QF) obligations
Reserves and accruals
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals
Property taxes
Deferred Tax Asset Property Taxes
Deferred Tax Asset Property Taxes
Regulatory liabilities
Deferred tax assets, regulatory liability
Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences derived regulatory liability.
Regulatory assets
Deferred Tax Assets, Regulatory Assets and Liabilities
Other, net
Deferred Tax Assets, Other
Deferred Tax Asset
Deferred Tax Assets, Gross
Deferred Tax Liabilities, [Abstract]
Deferred Tax Liabilities, Gross [Abstract]
Excess tax depreciation
Deferred Tax Liabilities Excess Tax Depreciation
Deferred Tax Liabilities Excess Tax Depreciation
Goodwill amortization
Deferred Tax Liabilities, Goodwill
Flow through depreciation
Deferred Tax Liabilities Flow Through Depreciation
Deferred Tax Liabilities Flow Through Depreciation
Regulatory assets
Deferred Tax Liabilities, Regulatory Assets and Liabilities
Deferred Tax Liability
Deferred Tax Liabilities, Net
Deferred Tax Liability, net
Deferred Tax Assets, Net
Schedule of Comprehensive Income (Loss)
Comprehensive Income (Loss) [Table Text Block]
Schedule of Accumulated Comprehensive Income (Loss)
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]
Reclassification out of Accumulated Other Comprehensive Income
Reclassification out of Accumulated Other Comprehensive Income [Table Text Block]
Commitments and Contingencies
Commitments and Contingencies Disclosure [Text Block]
Operating income
Dividends per share
Quarter-end close
Common Stock, Stock Price, Quarter End Close
Common Stock, Stock Price, Quarter End Close
Stock price
Common Stock, Stock Price
Common Stock, Stock Price
Income per average common share, (basic)
Earnings Per Share, Basic [Abstract]
Net income (loss)
Income per average common share (diluted)
Earnings Per Share, Diluted [Abstract]
Net income (loss)
Category of Item Purchased [Axis]
Qualifying Facility Contracts [Member]
Qualifying Facility [Member]
Gross Contractual Obligation [Axis]
Gross Contractual Obligation [Axis]
Gross Contractual Obligation [Domain]
Gross Contractual Obligation [Domain]
Gross Obligation [Member]
Gross Obligation [Member]
Recoverable Amounts [Member]
Recoverable Amounts [Member]
Net Amount [Member]
Net Amount [Member]
Price per MWH of energy required to be purchased per QF agreement
Price Per MWH Of Energy Required To Be Purchase Per QF Agreement
Price Per MWH Of Energy Required To Be Purchase Per QF Agreement
Recorded unconditional purchase obligation portion recoverable through rates
Recorded Unconditional Purchase Obligation Portion Recoverable Through Rates
Recorded Unconditional Purchase Obligation Portion Recoverable Through Rates
Beginning QF liability
Qualifying Facility Liability
Qualifying Facility Liability
Unrecovered amount
Unrecovered Liability Amount
Unrecovered Liability Amount
Interest expense
Interest Expense
Ending QF liability
Contractual obligation related to QF's
Recorded Unconditional Purchase Obligation
Regulatory Matters
Public Utilities Disclosure [Text Block]
Other Comprehensive Loss, Before Tax [Abstract]
Other Comprehensive Income (Loss), before Tax [Abstract]
Foreign currency translation adjustment
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax
Reclassification of net gains on derivative instruments to net income
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax
Realized loss on cash flow hedging derivatives
Pension and postretirement medical liability adjustment
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax
Other comprehensive loss, before tax
Other Comprehensive Income (Loss), before Tax
Other Comprehensive Loss, Tax [Abstract]
Other Comprehensive Income (Loss), Tax [Abstract]
Foreign currency translation adjustment
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax
Reclassification of net gains on derivative instruments to net income
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax
Realized loss on cash flow hedging derivatives
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax
Pension and postretirement medical liability adjustment
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax
Other comprehensive loss, tax
Other Comprehensive Income (Loss), Tax
Other Comprehensive Loss, Net of Tax [Abstract]
Other comprehensive loss, net of tax
Accumulated Other Comprehensive Income, Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Foreign currency translation
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax
Derivative instruments designated as cash flow hedges
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax
Pension and postretirement medical plans
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax
Accumulated other comprehensive income
Nature of Operations and Basis of Consolidation
Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block]
Fair Value Measurements
Fair Value Disclosures [Text Block]
Schedule of Stock by Class [Table]
Equity Distribution Agreement [Member]
Equity Distribution Agreement [Member]
Equity Distribution Agreement, entered into in April 2012, in which the entity may offer and sell shares of common stock from time to time.
Class of Stock [Line Items]
Combined common and preferred stock, shares authorized
Combined Common And Preferred Stock, Shares Authorized
Combined common and preferred stock, shares authorized
Common stock, shares authorized
Common Stock, Shares Authorized
Common stock, par or stated value per share
Common Stock, Par or Stated Value Per Share
Preferred stock, shares authorized
Preferred Stock, Shares Authorized
Preferred stock, par or stated value per share
Preferred Stock, Par or Stated Value Per Share
Common stock reserved for incentive plan awards
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized
Stock issued during period, shares
Shares issued, price per share
Shares Issued, Price Per Share
Common stock aggregate gross sales price, maximum
CommonStockAggregateGrossSalesPriceMaximum
Maximum aggregate gross sales price under and Equity Distribution Agreement where the entity may offer and sell shares of common stock
Net proceeds from sale of stock
Sales commission and other fees
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs
Number of shares issued during the period under the Equity Distribution Agreement
Stock Issued During Period, Shares, Period Increase (Decrease)
Common stock average share price
Common stock average share price
Common stock average share price under Equity Distribution Program during the period.
Shares paid for tax withholding
Shares Paid for Tax Withholding for Share Based Compensation
Significant Accounting Policies
Significant Accounting Policies [Text Block]
Estimated Future Benefit Payments
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]
2015
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months
2016
Defined Benefit Plan, Expected Future Benefit Payments, Year Two
2017
Defined Benefit Plan, Expected Future Benefit Payments, Year Three
2018
Defined Benefit Plan, Expected Future Benefit Payments, Year Four
2019
Defined Benefit Plan, Expected Future Benefit Payments, Year Five
2020-2024
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter
US Small Midcap Growth [Member]
US Small Midcap Value [Member]
US Core Opportunistic [Member]
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Member]
International [Member]
International [Member]
Target asset allocation allowable range of plus or minus
Target allocation of investments by plan
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Actual allocation of investments by plan
Defined Benefit Plan, Actual Plan Asset Allocations
Lease and rental expense
Operating Leases, Rent Expense
Variable Interest Entities
Consolidation, Variable Interest Entity, Policy [Policy Text Block]
Internal Revenue Service (IRS) [Member]
Tax Adjustments [Axis]
Tax Adjustments [Axis]
Tax Adjustments [Domain]
Tax Adjustments [Domain]
Earliest year subject to examination
Open Tax Year
Schedule of Changes In Qualifying Facility Liability
Schedule Of Changes In Qualifying Facility Liability [Table Text Block]
Schedule Of Changes In Qualifying Facility Liability [Table Text Block]
Schedule of Estimated Gross Contractual Obligation Less Amounts Recoverable Through Rates
Schedule Of Estimated Gross Contractual Obligation Less Amounts Recoverable Through Rates [Table Text Block]
Schedule Of Estimated Gross Contractual Obligation Less Amounts Recoverable Through Rates [Table Text Block]
Schedule of Significant Acquisitions and Disposals [Table]
Schedule of Significant Acquisitions and Disposals [Table]
Kerr Project [Member]
Kerr Project [Member]
Kerr Project [Member]
Significant Acquisitions and Disposals [Line Items]
Significant Acquisitions and Disposals [Line Items]
Megawatts of generation
Megawatts of generation
Megawatts of hydro-electric generation capacity.
Purchase price
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds
Owned average load serving requirement
Owned Average Load Serving Requirement
Owned average load serving requirements in Montana.
Approved rate base, Hydro Electric
Public Utilities, Approved Rate Base, Hydro Electric
Public Utilities, Approved Rate Base, Hydro Electric
Approved return on equity, percentage
Public Utilities, Approved Return on Equity, Percentage
Authorized cost of debt
Authorized cost of debt
Cost of debt to be recovered as authorized by a regulatory body
Approved debt capital structure, percentage
Public Utilities, Approved Debt Capital Structure, Percentage
Approved equity capital structure, percentage
Public Utilities, Approved Equity Capital Structure, Percentage
Approved rate increase, amount
Public Utilities, Approved Rate Increase (Decrease), Amount
Transaction costs
Business Acquisition, Transaction Costs
Debt related commitment fees
Debt Related Commitment Fees and Debt Issuance Costs
Estimated reference price less conveyance price
Estimated reference price less conveyance price
Estimated reference price of Kerr Dam less estimated conveyance price
Estimated conveyance price of hydroelectric facility
Estimated conveyance price of hydroelectric facility
Estimated conveyance price of Kerr Dam to CSKT
Statement of Financial Position (Parenthetical) [Abstract]
Statement of Financial Position (Parenthetical) [Abstract]
Common Stock, shares issued
Common Stock, Shares, Issued
Common Stock, shares outstanding
Common Stock, Shares, Outstanding
Preferred Stock, shares issued
Preferred Stock, Shares Issued
Long-term Debt And Capital Leases
Debt and Capital Leases Disclosures [Text Block]
Pension and other employee benefits
Pension and Other Postretirement and Postemployment Benefit Plans, Liabilities, Noncurrent
Future QF obligation, net
Environmental
Accrued Environmental Loss Contingencies, Noncurrent
Customer advances
Customer Advances, Noncurrent
Other
Other Miscellaneous Noncurrent Liability
Other Miscellaneous Noncurrent Liability
Total
Inventories
Schedule of Inventory, Current [Table Text Block]
Other Noncurrent Liabilties
Schedule Of Other Noncurrent Liabilties [Table Text Block]
Schedule Of Other Noncurrent Liabilties [Table Text Block]
Supplemental Cash Flow Information
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]