Regulated Operations [Abstract] Schedule Of Regulated Operations [Table] Schedule Of Regulated Operations [Table] Schedule Of Regulated Operations [Table] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Regulatory Matters [Axis] Regulatory Matters [Axis] Regulatory Matters [Domain] Regulatory Matters [Domain] Montana Natural Gas Production Assets [Member] Montana Natural Gas Production Assets [Member] Montana Natural Gas Production Assets [Member] Dave Gates Generating Station [Member] Dave Gates Generating Station [Member] Regulatory Reviews of Filings [Member] Regulatory Reviews of Filings [Member] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Revenue Subject to Refund [Member] Revenue Subject to Refund [Member] Statement [Line Items] Customer refund liability, current Customer Refund Liability, Current CU4 incremental market purchases identified for further review Utilities Operating Expense, Purchased Power Revenue recognized, subject to refund Revenue of Acquiree since Acquisition Date, Actual This element represents the amount of revenue of the acquiree since the acquisition date. Demand side management expected lost revenue 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. Income Tax Disclosure [Abstract] Income Tax Contingency [Table] Income Tax Contingency [Table] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Internal Revenue Service (IRS) [Member] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Unrecognized tax benefits Unrecognized Tax Benefits Interest expense or penalties, uncertain tax positions Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense Accrued interest, uncertain tax positions Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued Unrecognized tax benefits that would impact effective tax rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Significant change in unrecognized tax benefits reasonably possible Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit Earliest year subject to examination Open Tax Year Earnings Per Share [Abstract] Earnings Per Share [Text Block] Earnings Per Share [Text Block] Fair Value Disclosures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value by 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] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Total Net Fair Value [Member] Estimate of Fair Value Measurement [Member] Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] Fair Value, Inputs, Level 1 [Member] Significant Other Observable Inputs(Level 2) [Member] Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs(Level 3) [Member] Fair Value, Inputs, Level 3 [Member] Margin Cash Collateral Offset [Member] Netting [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] 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) Fair value, assets, level 1 to level 2 transfers, amount Fair Value, Assets, Level 1 to Level 2 Transfers, Amount Fair value, assets, level 2 to level 1 transfers, amount Fair Value, Assets, Level 2 to Level 1 Transfers, Amount Fair value, liabilities, level 1 to level 2 transfers, amount Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount Fair value, liabilities, level 2 to level 1 transfers, amount Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount Fair Value, transfers into (out of) level 3 Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers, Net Financing Activities [Abstract] Financing Activities [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] 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. Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Debt Instrument [Line Items] Debt Instrument [Line Items] Common stock, aggregate gross sales price CommonStockAggregateGrossSalesPriceMaximum Maximum aggregate gross sales price under and Equity Distribution Agreement where the entity may offer and sell shares of common stock Stock issued during period, shares, new issues Stock Issued During Period, Shares, Period Increase (Decrease) Common stock average share price during the period Common stock average share price Common stock average share price under Equity Distribution Program during the period. Proceeds received from new issues during the period Stock Issued During Period, Value, New Issues Sales commissons and fees Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Subsegments [Axis] Subsegments [Axis] Subsegments [Domain] Subsegments [Domain] Subsegments Consolidation Items [Axis] Subsegments Consolidation Items [Axis] Subsegments Consolidation Items [Domain] Subsegments Consolidation Items [Domain] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Segment Reporting Disclosure [Text Block] Segment Reporting Disclosure [Text Block] Internal Revenue Service (IRS) [Member] Effective tax rate reconciliation 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 repairs deductions Effective Income Tax Rate Reconciliation, Deduction, Other, Percent Production tax credits Effective Income Tax Rate Reconciliation, Deduction, Qualified Production Activity, Percent Prior year permanent return to accrual adjustments Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, 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 Other, net Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Effective income tax rate Effective Income Tax Rate Reconciliation, Percent Organization, Consolidation and Presentation of Financial Statements [Abstract] Operations And Basis Of Consolidation [Table] Operations And Basis Of Consolidation [Table] Operations And Basis Of Consolidation [Table] Legal Entity [Axis] Entity [Domain] 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 for qualifying facilities through 2024 Estimated Aggregate Gross Contractual Payments Estimated Aggregate Gross Contractual Payments Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative [Table] Derivative [Table] Derivative Instrument [Axis] Derivative Contract [Domain] Interest Rate Swap [Member] Derivative, by Nature [Axis] Derivative, by Nature [Axis] Derivative, Name [Domain] Derivative, Name [Domain] Hedging Designation [Axis] Hedging Designation [Domain] 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] Amount of gain reclassified from AOCI Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net Pre-tax gain 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 to be reclassified within twelve months from AOCI to interest expense Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net Interest rate swaps outstanding Interest Rate Cash Flow Hedge Derivative at Fair Value, Net 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 Goodwill [Abstract] Goodwill [Abstract] Goodwill Disclosure [Text Block] Goodwill Disclosure [Text Block] Statement of Comprehensive Income [Abstract] Schedule of Comprehensive Income (Loss) [Table Text Block] Comprehensive Income (Loss) [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Accumulated Other Comprehensive Income [Table Text Block] Accumulated Other Comprehensive Income [Table Text Block] [Table Text Block] for Accumulated Other Comprehensive Income [Table] CONDENSED CONSOLIDATED STATEMENTS OF OTHER COMPREHENSIVE INCOME [Abstract] CONDENSED CONSOLIDATED STATEMENTS OF OTHER COMPREHENSIVE INCOME [Abstract] Statement [Table] Statement [Table] Statement [Line Items] Comprehensive Income (Loss) Net Income Net Income (Loss) Attributable to Parent Other comprehensive (loss) income, net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Reclassification of net gains on derivative instruments Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Foreign currency translation Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Total Other Comprehensive Loss Other Comprehensive Income (Loss), Net of Tax Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) [Table Text Block] Acquisitions [Abstract] Acquisitions [Abstract] Property, Plant and Equipment, Schedule of Significant Acquisitions and Disposals [Table Text Block] Property, Plant and Equipment, Schedule of Significant Acquisitions and Disposals [Table Text Block] Derivative Instruments and Hedging Activities Disclosure [Text Block] Derivative Instruments and Hedging Activities Disclosure [Text Block] CONDENSED CONSOLIDATED BALANCE SHEET PARENTHETICAL [Abstract] CONDENSED CONSOLIDATED BALANCE SHEET PARENTHETICAL [Abstract] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common Stock, Par or Stated Value Per Share Common Stock, Par or Stated Value Per Share Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares, Issued Common Stock, Shares, Issued Common Stock, Shares, Outstanding Common Stock, Shares, Outstanding Preferred Stock, Par or Stated Value Per Share Preferred Stock, Par or Stated Value Per Share Preferred Stock, Shares Authorized Preferred Stock, Shares Authorized Preferred Stock, Shares Issued Preferred Stock, Shares Issued Preferred Stock, Shares Outstanding Preferred Stock, Shares Outstanding Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Compensation and Retirement Disclosure [Abstract] Pension and Other Postretirement Benefits Disclosure [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] 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] Scenario, Forecast [Member] Scenario, Forecast [Member] Site Contingecy by Site [Axis] Site Contingecy by Site [Axis] Site Contingecy by Site Axis [Axis] Site [Domain] Site [Domain] Site [Domain] Neal 4 Generating Facility [Member] Neal 4 Generating Facility [Member] Neal 4 Generating Facility [Member] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Environmental Remediation Site [Axis] Environmental Remediation Site [Axis] Environmental Remediation Site [Domain] Environmental Remediation Site [Domain] 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] Colstrip Unit 4 [Member] Colstrip Unit 4 [Member] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Manufactured Gas Plants [Member] Manufactured Gas Plants [Member] Manufactured Gas Plants [Member] Colstrip Unit 4 [Member] Coyote Generating Facility [Member] Big Stone Generating Facility [Member] 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 next 5 years Environmental Remediation Obligation, To be Incurred During Next 5 years Environmental Remediation Obligation, To be Incurred During Next 5 years Jointly owned utility plant ownership percentage Jointly Owned Utility Plant, Proportionate Ownership Share NOx emissions per million Btu as calculated on a 30 day rolling average basis 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 Estimated capital expenditures for environmental obligations Environmental Obligation, Estimated Capital Expenditures Estimated capital expenditure projections related to environmental obligations. Joint ownership share of capitalized project costs Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress 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 Income Tax Disclosure [Text Block] Income Tax Disclosure [Text Block] Document and Entity Information Abstract -- None. No documentation exists for this element. -- 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 Schedule of Goodwill [Table] Schedule of Goodwill [Table] Statement, Business Segments [Axis] Electric [Member] Electric Domestic Regulated [Member] Electric Domestic Regulated [Member] Natural gas [Member] Gas Domestic Regulated [Member] Gas Domestic Regulated [Member] Goodwill [Line Items] Goodwill [Line Items] Change in goodwill Goodwill, Period Increase (Decrease) Goodwill Goodwill Consolidation, Variable Interest Entity [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Natural Gas [Member] All Other Segments [Member] Other Segments [Member] Intersegment Elimination [Member] Intersegment Eliminations [Member] 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 Property and other taxes Taxes, Miscellaneous Depreciation and depletion Depreciation, Depletion and Amortization Operating income (loss) Operating Income (Loss) Interest expense Interest and Debt Expense Other income Other Nonoperating Income (Expense) Income tax (expense) benefit Income Tax Expense (Benefit) Net Income Total assets Assets Capital expenditures Property, Plant and Equipment, Additions Debt Disclosure [Text Block] Debt Disclosure [Text Block] Statement of Cash Flows [Abstract] OPERATING ACTIVITIES: Net Cash Provided by (Used in) Operating Activities [Abstract] Items not affecting cash: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and depletion Amortization of debt issue costs, discount and deferred hedge gain Amortization of debt issue costs discount and deferred hedge gain Amortization of debt issue costs discount and deferred hedge gain Amortization of restricted stock 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) 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 Asset acquisition Payments for (Proceeds from) Previous Acquisition Proceeds from sale of assets Proceeds from Sale of Property, Plant, and Equipment Cash used in investing activities Net Cash Provided by (Used in) Investing Activities FINANCING ACTIVITIES: Net Cash Provided by (Used in) Financing Activities [Abstract] Treasury stock activity Proceeds from (Repurchase of) Equity Proceeds from issuance of common stock, net Proceeds from Issuance or Sale of Equity Dividends on common stock Payments of Ordinary Dividends, Common Stock Repayments on long-term debt Repayments of Long-term Debt Issuance (Repayments) of short-term borrowings, net Proceeds from (Repayments of) Commercial Paper Financing costs Payments of Financing Costs Cash used in financing activities Net Cash Provided by (Used in) Financing Activities Increase (Decrease) in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents, beginning of period Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents, end of period Supplemental Cash Flow Information: Supplemental Cash Flow Information [Abstract] Cash paid during the period for: Cash Paid [Abstract] Income taxes Income Taxes Paid, Net Interest Interest Paid Significant non-cash transactions: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Capital expenditures included in accounts payable and accrued expenses Capital Expenditures Incurred but Not yet Paid Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Comprehensive Income (Loss) Note [Text Block] Comprehensive Income (Loss) Note [Text Block] Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Current Assets: Assets, Current [Abstract] Cash and cash equivalents Restricted cash Restricted Cash and Cash Equivalents, Current Accounts receivable, net Accounts Receivable, Net, Current Inventories Inventory, Net 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 Property, Plant and Equipment, Net 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 Capital Lease Obligations, Current Short-term borrowings Commercial Paper Accounts payable Accounts Payable, Current Accrued expenses Accrued Liabilities, Current Regulatory liabilities Regulatory Liability, Current Total current liabilities Liabilities, Current Long-term capital leases Capital Lease Obligations, Noncurrent Long-term debt Long-term Debt, Excluding Current Maturities 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 14) 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 42,752,477 and 39,139,365 shares, 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 Fair Value Disclosures [Text Block] Fair Value Disclosures [Text Block] 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 Other comprehensive loss Other Comprehensive Income (Loss), before Tax Other Comprehensive Income (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 Other comprehensive loss Other Comprehensive Income (Loss), Tax Reclassification of net gains on derivative instruments to net income Accumulated Other Comprehensive Income [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 New Accounting Pronouncement or Change in Accounting Principle, Current Period Disclosures [Abstract] New Accounting Pronouncements [Text Block] New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Accumulated Other Comprehensive Income [Abstract] Accumulated Other Comprehensive Income [Abstract] Accumulated Other Comprehensive Income [Table] Accumulated Other Comprehensive Income [Table] Accumulated Other Comprehensive Income [Table] Gains on Derivative Instruments Designated as Cash Flow Hedges Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] Pension and Postretirement Medical Plans Accumulated Defined Benefit Plans Adjustment [Member] Foreign Currency Translation Foreign Currency Gain (Loss) [Member] Accumulated Other Comprehensive Income [Line Items] Accumulated Other Comprehensive Income [Line Items] Accumulated Other Comprehensive Income [Table] Beginning balance Other comprehensive income before reclassifications Amounts reclassified from accumulated other comprehensive income Net current-period other comprehensive (loss) income Ending balance Schedule of Defined Benefit Plans Disclosures [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] 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] Total Operating Expenses Operating Expenses Operating Income Interest Expense, net Other Income Income Before Income Taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Income Tax Expense Average Common Shares Outstanding Weighted Average Number of Shares Outstanding, Basic Basic Earnings per Average Common Share Earnings Per Share, Basic Diluted Earnings per Average Common Share Earnings Per Share, Diluted Dividends Declared per Common Share Common Stock, Dividends, Per Share, Declared Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] 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 Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Disclosure [Text Block] Public Utilities Disclosure [Text Block] Public Utilities Disclosure [Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Fair Value Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Schedule of Weighted Average Number of Shares [Table Text Block] Schedule of Weighted Average Number of Shares [Table Text Block] 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] Basic computation Dilutive effect of 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 Schedule of Goodwill [Table Text Block] Schedule of Goodwill [Table Text Block] Property, Plant and Equipment Acquisitions [Abstract] Property, Plant and Equipment Acquisitions [Abstract] Schedule of Significant Acquisitions and Disposals [Table] Schedule of Significant Acquisitions and Disposals [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] Kerr Project [Member] Kerr Project [Member] Kerr Project [Member] Significant Acquisitions and Disposals [Line Items] Significant Acquisitions and Disposals [Line Items] Megawatts of hydro-electric generation capacity Megawatts of generation Megawatts of hydro-electric generation capacity. Purchase price Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds 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 hydro facility Estimated conveyance price of hydroelectric facility Estimated conveyance price of Kerr Dam to CSKT Transaction costs Business Acquisition, Transaction Costs Debt related commitment fees and debt issuance costs Debt Related Commitment Fees and Debt Issuance Costs 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. Owned average load serving requirement Owned Average Load Serving Requirement Owned average load serving requirements in Montana. Requested return on equity for base rate Public Utilities, Requested Return On Equity for Base Rate Public Utilities, Requested Return On Equity for Base Rate Capital structure, percentage of debt Public Utilities, Capital Structure, Percentage of Debt Public Utilities, Capital Structure, Percentage of Debt Capitalized structure, percentage of equity Public Utilities, Capitalized Structure, Percentage of Equity Public Utilities, Capitalized Structure, Percentage Of Equity Estimated rate base, Hydro Electric Public Utilities, Estimated Rate Base, Hydro Electric Public Utilities, Estimated Rate Base, Hydro Electric Requested rate base, Hydro Electric Public Utilities, Requested Rate Base, Hydro Electric Public Utilities, Requested Rate Base, Hydro Electric Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined 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] Pension Benefits [Member] Pension Plan, Defined Benefit [Member] Other Postretirement Benefits [Member] Other Pension Plan, Postretirement or Supplemental Plans, Defined Benefit [Member] Components of Net Periodic Benefit Cost (Income) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on plan assets Defined Benefit Plan, Expected Return on Plan Assets Amortization of prior service cost Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Recognized actuarial loss Defined Benefit Plan, Actuarial Gain (Loss) Net Periodic Benefit Cost (Income) Defined Benefit Plan, Net Periodic Benefit Cost