Statement of Cash Flows [Abstract] Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Statement [Line Items] Statement [Line Items] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Increase in net liabilities from price risk management activities Increase (Decrease) in Risk Management Assets and Liabilities Regulatory deferrals-price risk management activities Gain (Loss) from Price Risk Management Activity Deferred income taxes Deferred Income Tax Expense (Benefit) Pension and other postretirement benefits Pension and Other Postretirement Benefit Expense Allowance for equity funds used during construction Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity Other non-cash income and expenses, net Other Noncash Income (Expense) Changes in working capital: Increase (Decrease) in Operating Capital [Abstract] Decrease in accounts receivable and unbilled revenues Increase (Decrease) in Receivables Decrease (increase) in inventories Increase (Decrease) in Inventories Increase in margin deposits, net Increase (Decrease) in Margin Deposits Outstanding Decrease in accounts payable and accrued liabilities Increase (Decrease) in Accounts Payable Other working capital items, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Other, net Increase (Decrease) in Other Operating Assets Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Capital expenditures Payments to Acquire Property, Plant, and Equipment Sales tax refund received related to Tucannon River Wind Farm Proceeds from Income Tax Refunds Sales of Nuclear decommissioning trust securities Proceeds from Sale, Maturity and Collection of Investments Purchases of Nuclear decommissioning trust securities Payments to Acquire Investments Other, net Payments for (Proceeds from) Other Investing Activities Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from issuance of long-term debt Proceeds from Issuance of Long-term Debt Payments on long-term debt Repayments of Long-term Debt Change in short-term debt Repayments of Short-term Debt Dividends paid Payments of Dividends Payments on capital leases Repayments of Long-term Capital Lease Obligations Debt issuance costs Payments of Debt Issuance Costs Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities 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 is as follows: Supplemental Cash Flow Information [Abstract] Cash paid for interest, net of amounts capitalized Interest Paid, Net Non-cash investing and financing activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Accrued capital additions Capital Expenditures Incurred but Not yet Paid Accrued dividends payable Dividends Payable Assets obtained under capital lease Capital Lease Obligations Incurred Price Risk Management Note [Abstract] Price risk management note. Derivative [Table] Derivative [Table] Derivative, by Nature [Axis] Derivative, by Nature [Axis] Derivative, Name [Domain] Derivative, Name [Domain] Derivative [Line Items] Derivative [Line Items] Commodity contracts: Current Assets, Commodity Contracts: [Abstract] Commodity contracts: [Abstract] Electricity Volume Related to Derivative Instruments Held at Balance Sheet Date, Commodity Contracts, Electricity Net volume related to the reporting entity's derivative instruments as of the balance sheet date having arisen from price risk management activities. Natural gas Volume Related to Derivative Instruments at Balance Sheet Date, Commodity Contracts, Natural Gas Net volume associated with the reporting entity's derivative instruments recorded as of the balance sheet date having arisen from price risk management activities related to natural gas. Foreign currency Volume Related to Derivative Instruments Held at Balance Sheet Date, Foreign Currency Net amount associated with the reporting entity's derivative instruments recorded as of the balance sheet date having arisen from price risk management activities related to foreign currency. Price Risk Management [Abstract] Price Risk Management [Abstract] Electricity Realized and Unrealized Gains and Losses, Net, Commodity Contracts, Electricity Realized and unrealized gains and losses, net on commodity contracts related to electricity not designated as hedging instruments included in purchased power and fuel expense in the statement of income. Natural Gas Realized and Unrealized Gains and Losses, Net, Commodity Contracts, Natural Gas Realized and unrealized gains and losses, net on commodity contracts related to natural gas not designated as hedging instruments included in purchased power and fuel expense in the statement of income. Derivative, Forward Exchange Rate Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments Natural Gas [Member] Natural Gas [Member] 4911 Electric Services [Member] 4911 Electric Services [Member] Liabilities, Total [Member] Liabilities, Total [Member] Derivative Instruments and Hedges, Liabilities Derivative Instruments and Hedges, Liabilities Collateral, Master Netting Arrangements, Letters of Credit Derivative, Collateral, Master Netting Arrangements, Letters of Credit The amount of collateral posted pursuant to derivative instruments with counterparties where a master netting arrangement exists, consisting of letters of credit. Net gain or (loss) recognized in the statement of income offset by regulatory accounting Net gain or loss recognized in the statement of income offset by regulatory accounting Net gain or loss recognized in the statement of income for derivative instruments related to price risk management activities offset by regulatory accounting. Derivative, Net Liability Position, Aggregate Fair Value Derivative, Net Liability Position, Aggregate Fair Value Collateral Posted, Aggregate Fair Value Collateral Already Posted, Aggregate Fair Value Letters of Credit Outstanding, Amount Letters of Credit Outstanding, Amount Restricted Cash and Cash Equivalents, Current Restricted Cash and Cash Equivalents, Current Collateral cash requirement Collateral Aggregate Fair Value The aggregate fair value of assets that would be required to be posted as collateral for derviative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period. Derivative Liability, Fair Value of Collateral Derivative Liability, Fair Value of Collateral Basis of Presentation [Abstract] Basis of Presentation [Abstract] Consolidation, Policy [Policy Text Block] Consolidation, Policy [Policy Text Block] BASIS OF PRESENTATION Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Current assets: Assets, Current [Abstract] Current Assets, Commodity Contracts: Electricity Commodity Contract Asset, Current, Electricity Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to electricity which are expected to be settled within a year or normal operating cycle, if longer. Natural gas Commodity Contract Asset, Current, Natural Gas Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to natural gas which are expected to be settled within a year or normal operating cycle, if longer. Total current derivative assets Current Derivative Assets Fair valules as of the balance sheet date for all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be settled within one year or the normal operating cycle, if longer. Noncurrent assets: Assets, Noncurrent [Abstract] Noncurrent Assets, Commodity Contracts: [Abstract] Noncurrent Assets, Commodity Contracts: [Abstract] Noncurrent Assets, Commodity Contracts: [Abstract] Commodity Contract Asset, Noncurrent, Natural Gas Commodity Contract Asset, Noncurrent, Natural Gas Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to natural gas which are expected to be settled after a year or beyond the normal operating cycle, if longer. Total noncurrent derivative assets Derivative Asset, Noncurrent Total derivative assets not designated as hedging instruments Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value Total derivative assets Derivative Assets, Total Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments. Current liabilities: Liabilities, Current [Abstract] Current Liabilities, Commodity Contracts: [Abstract] Current Liabilities, Commodity Contracts: [Abstract] Electricity Commodity Contract Liability, Current, Electricity Carrying amount as of the balance sheet date of the liability arising from commodity contracts related to electricity which are expected to be settled within a year or the normal operating cycle, if longer. Natural gas Commodity Contract Liability, Current, Natural Gas Carrying amount as of the balance sheet date of the liability arising from the commodity contracts related to natural gas which are expected to be settled within a year or normal operating cycle, if longer. Total current derivative liabilities Current Derivative Liabilities Fair values as of the balance sheet date for all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be settled within one year of the normal operating cycle, if longer. Noncurrent liabilities: Liabilities, Noncurrent [Abstract] Noncurrent Liabilities, Commodity Contracts: [Abstract] Noncurrent Liabilities, Commodity Contracts: [Abstract] Electricity Commodity Contract Liability, Noncurrent, Electricity Carrying amount as of the balance sheet date of the liability arising from commodity contracts related to electricity which are expected to be settled after a year or beyond the normal operating cycle, if longer. Natural gas Commodity Contract Liability, Noncurrent, Natural Gas Carrying amount as of the balance sheet date of the liability arising from commodity contracts related to natural gas which are expected to be settled after a year or beyond the normal operating cycle, if longer. Total noncurrent derivative liabilities Noncurrent Derivative Liabilities Fair values as of the balance sheet date for all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be settled after one year or normal operating cycle, if longer. Total derivative liabilities not designated as hedging instruments Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value Total derivative liabilities Derivative Liabilities, Total Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. Equity [Abstract] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock [Member] Common Stock [Member] Stock Issued During Period, Shares, New Issues Stock Issued During Period, Shares, New Issues Proceeds from Issuance of Common Stock Proceeds from Issuance of Common Stock Contingencies [Abstract] Contingencies [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Investment in Trojan investment in Trojan The portion of the Company's remaining investment in its Trojan plant that the OPUC allowed the Company to recover. Refund to customers for Trojan Investment including interest refund to customers for Trojan Investment including interest refund to customers for Trojan Investment including interest Class action damages sought Class action lawsuit damages sought Class action lawsuit damages sought by plaintiffs against the Company. Site Contingency, Names of Other Potentially Responsible Parties Site Contingency, Names of Other Potentially Responsible Parties Remediation cost estimate lower range Loss Contingency, Range of Possible Loss, Minimum Remediation cost estimate upper range Loss Contingency, Range of Possible Loss, Maximum Civil Penalty Claim - Per day per violation through January 12, 2009 Civil Penalty Claim - Per day per violation through January 12, 2009 Civil Penalty Claim - Per Day Per Violation through January 12, 2009 Civil Penalty Claim - Per day per violation after January 12, 2009 Civil Penalty Claim - Per day per violation after January 12, 2009 Civil Penalty Claim - Per day per violation after January 12, 2009 Loss Contingency, Actions Taken by Defendant Loss Contingency, Actions Taken by Defendant projects added to plaintiff's suit projects added to plaintiff's suit projects added to plaintiff's suit Loss Contingency, Pending Claims, Number Loss Contingency, Pending Claims, Number Schedule of Stockholders Equity [Table Text Block] Schedule of Stockholders Equity [Table Text Block] Income Statement [Abstract] Revenues, net Electric Revenue Operating expenses: Costs and Expenses [Abstract] Purchased power and fuel Cost of Purchased Power Generation, transmission and distribution Electric Production Expense Administrative and other General and Administrative Expense Taxes other than income taxes Taxes, Miscellaneous Total operating expenses Costs and Expenses Income from operations Operating Income (Loss) Interest expense, net Interest Expense Other income: Other Income and Expenses [Abstract] Allowance for equity funds used during construction Miscellaneous income (expense), net Other Nonoperating Income (Expense) Other income, net Nonoperating Income (Expense) Income before income tax expense Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Income tax expense Income Tax Expense (Benefit) Net income and Comprehensive income Weighted-average shares outstanding (in thousands): Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic Weighted Average Number of Shares Outstanding, Basic Diluted Weighted Average Number of Shares Outstanding, Diluted Earnings per share: Earnings Per Share [Abstract] Basic Earnings Per Share, Basic Diluted Earnings Per Share, Diluted Dividends declared per common share Common Stock, Dividends, Per Share, Declared Guarantees [Abstract] GUARANTEES Guarantees [Text Block] Derivatives, Policy [Policy Text Block] Derivatives, Policy [Policy Text Block] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Electricity [Member] Electricity [Member] Electricity [Member] Net Unrealized Loss [Member] Unrealized Gain Loss On Derivatives [Member] Unrealized Gain Loss On Derivatives [Member] Schedule of Future Expected Realized Gains or Losses [Axis] Schedule of Future Expected Realized Gains or Losses [Axis] Schedule of Future Expected Realized Gains or Losses [Domain] Schedule of Future Expected Realized Gains or Losses [Domain] 2015 Other Commitment, Due in Next Twelve Months 2016 Other Commitment, Due in Second Year 2017 Other Commitment, Due in Third Year 2018 Other Commitment, Due in Fourth Year 2019 Other Commitment, Due in Fifth Year Thereafter Other Commitment, Due after Fifth Year Total Other Commitment Balance Sheet Components [Abstract] Balance Sheet Components [Abstract] Schedule of Regulatory Assets [Table] Schedule of Regulatory Assets [Table] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Current Regulatory Liabilities [Member] Current Regulatory Liabilities [Member] Current Regulatory Liabilities Noncurrent Regulatory Liabilities [Member] Noncurrent Regulatory Liabilities [Member] Noncurrent Regulatory Liabilities Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Current Regulatory Assets [Member] Regulatory Assets, Current [Member] Current Regulatory Assets Noncurrent Regulatory Assets [Member] Noncurrent Regulatory Assets Regulatory Assets and Liabilities [Line Items] Regulatory Assets [Line Items] Price risk management Regulatory assets related to price risk management activities Regulatory assets related to price risk management activities Pension and other postretirement plans Defined Benefit Plan, Assets for Plan Benefits, Noncurrent Deferred income taxes Deferred Tax Assets, Regulatory Assets and Liabilities Debt issuance costs Deferred Finance Costs, Net Other Other Deferred Costs, Net Total regulatory assets Regulatory Assets Asset retirement removal costs Asset retirement removal Asset retirement removal costs Trojan decommissioning activities Trojan decommissioning activities Asset retirement obligation related to Trojan decommissioning activities Asset retirement obligations Asset Retirement Obligations, Noncurrent Other Regulatory liabilities - Other Other regulatory liabilities. Total regulatory liabilities Regulatory Liabilities Fair Value of Financial Instruments [Abstract] Fair Value of Financial Instruments [Abstract] FAIR VALUE OF FINANCIAL INSTRUMENTS Fair Value Disclosures [Text Block] Fair Value, Option, Quantitative Disclosures [Table] Fair Value, Option, Quantitative Disclosures [Table] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Eligible Item or Group for Fair Value Option [Axis] Eligible Item or Group for Fair Value Option [Axis] Fair Value, Option, Eligible Item or Group [Domain] Fair Value, Option, Eligible Item or Group [Domain] Assets [Member] Assets [Member] Liabilities [Member] Derivative Financial Instruments, Liabilities [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Low [Member] Minimum [Member] High [Member] Maximum [Member] Weighted Average [Member] Weighted Average [Member] Commodity Contracts Fair Value, Option, Quantitative Disclosures [Line Items] Electricity physical forward Electricity physical forward purchase Electricity physical forward purchase Natural gas financial swaps Natural gas financial swaps Natural gas financial swaps Total commodity contracts Total commodity contracts Total commodity contracts CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value, Measurements, 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] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets: Assets [Abstract] Nuclear decommissioning trust: (1) Nuclear decommissioning trust: [Abstract] Nuclear decommissioning trust: [Abstract] Debt securities: Debt securities: [Abstract] Debt securities: [Abstract] Domestic government US Government Securities, at Carrying Value Corporate credit Financial Instruments, Owned, Corporate Debt, at Fair Value Money market funds Money Market Funds, at Carrying Value Investments, Fair Value Disclosure Investments, Fair Value Disclosure Non-qualified benefit plan trust: (2) Nonqualified Benefit Plan Trust: [Abstract] Non-qualified benefit plan trust: [Abstract] Domestic Mutual Funds, Equity Equity mutual fund assets held in the Non-qualified benefit plan trust, which are carried at fair value. Debt securities—domestic government Mutual Funds, Debt Debt mutual funds assets in the Non-qualified benefit plan trust, which are carried at fair value. Alternative Investments, Fair Value Disclosure Alternative Investments, Fair Value Disclosure Assets from price risk management activities: (1) (3) Assets from price risk management activities: [Abstract] Assets from price risk management activities: [Abstract] Electricity Commodity Contract Asset, Electricity Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to purchases or sales of electricity. Natural gas Commodity Contract Asset, Natural gas Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to purchases or sales of natural gas. Total Financial Instruments, Owned, at Fair Value Liabilities from price risk management activities: (1) (3) Liabilities from price risk management activities: [Abstract] Liabilities from price risk management activities: [Abstract] Electricity Commodity Contract Liabilities, Electricity Carrying amount as of the balance sheet date of the liabilities arising from commodity contracts related to purchases or sales of electricity. Natural gas Commodity Contract Liabilities, Natural gas Carrying amount as of the balance sheet date of the liabilities arising from commodity contracts related to purchases or sales of natural gas. Total Financial and Nonfinancial Liabilities, Fair Value Disclosure Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Balance as of the beginning of the period Net Liabilities From Price Risk Management Activities Classified As Level 3 Net Liabilities From Price Risk Management Activities Classified as Level 3 Net realized and unrealized losses (gains) Realized and Unrealized Gains and Losses, Net, Commodity Contract Realized and Unrealized Gains and Losses, Net, Commodity Contracts Transfers out of Level 3 to Level 2 Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 Balance as of the end of the period EARNINGS PER SHARE Earnings Per Share [Text Block] Fair Value of Financial Instruments, Policy [Policy Text Block] Fair Value of Financial Instruments, Policy [Policy Text Block] Allocation of Financial Asset to Hierarchy Levels [Policy Text Block] Allocation of Financial Asset to Hierarchy Levels [Policy Text Block] Allocation of Financial Asset to Hierarchy Levels [Policy Text Block] Fair Value Transfer, Policy [Policy Text Block] Fair Value Transfer, Policy [Policy Text Block] Debt, Policy [Policy Text Block] Debt, Policy [Policy Text Block] Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Schedule of Derivative Instruments [Table Text Block] Schedule of Derivative Instruments [Table Text Block] Schedule of Other Derivatives Not Designated as Hedging Instruments, Statements of Financial Performance and Financial Position, Location [Table Text Block] Schedule of Other Derivatives Not Designated as Hedging Instruments, Statements of Financial Performance and Financial Position, Location [Table Text Block] Schedule of Price Risk Derivatives [Table Text Block] Schedule of Price Risk Derivatives [Table Text Block] Schedule of Concentration of Risk, by Counterparty [Table Text Block] Schedule of Concentration of Risk, by Counterparty [Table Text Block] Tabular disclosure of counterparties representing 10% or more of assets and liabilities from price risk management activities recorded in the reporting entity's statement of financial position. Common Stock Including Additional Paid in Capital [Member] AOCI Attributable to Parent [Member] Retained Earnings [Member] Stockholders' Equity, Total [Member] Stockholders' Equity, Total [Member] Common Stock, Shares, Outstanding beginning of period Common Stock, Shares, Outstanding Issuances of shares pursuant to equity-based plans Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures Issuance of shares pursuant to equity-based plans Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options Adjustments Related to Tax Withholding for Share-based Compensation Adjustments Related to Tax Withholding for Share-based Compensation Stockholders' Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Stock-based compensation Share-based Compensation Dividends declared Dividends, Common Stock Net Income Net Income (Loss) Attributable to Parent Common Stock, Shares, Outstanding end of period Stockholders' Equity Document and Entity Information [Abstract] Document and Entity Information [Abstract] Entities [Table] Entities [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Entity Information [Line Items] Entity Information [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date Amendment Flag Amendment Flag Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Trading Symbol Trading Symbol Carty Generating Station [Abstract] Carty Generating Station [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Minimum [Member] Maximum [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Public Utilities, Allowance for Funds Used During Construction, Additions Public Utilities, Allowance for Funds Used During Construction, Additions Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base Allowance for Funds Used During Construction, Capitalized Interest Allowance for Funds Used During Construction, Capitalized Interest Malpractice Loss Contingency, Letters of Credit and Surety Bonds Malpractice Loss Contingency, Letters of Credit and Surety Bonds Construction work-in-progress Public Utilities, Property, Plant and Equipment, Construction Work in Progress Guarantees, Indemnifications and Warranties Policies [Policy Text Block] Guarantees, Indemnifications and Warranties Policies [Policy Text Block] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Pension Plan [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] 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 net actuarial loss Defined Benefit Plan, Amortization of Gains (Losses) Net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost Schedule of Other Current Assets [Table Text Block] Schedule of Other Current Assets [Table Text Block] Schedule of Public Utility Property, Plant, and Equipment [Table Text Block] Public Utility Property, Plant, and Equipment [Table Text Block] Capital Leases in Financial Statements of Lessee Disclosure [Text Block] Schedule of Future Minimum Lease Payments for Capital Leases [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Other Liabilities Disclosure [Text Block] Other Liabilities Disclosure [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Inventory, Policy [Policy Text Block] Inventory, Policy [Policy Text Block] PRICE RISK MANAGEMENT Derivative Instruments and Hedging Activities Disclosure [Text Block] Capital Leases [Abstract] Capital Leases [Abstract] Schedule of Capital Leased Assets [Table] Schedule of Capital Leased Assets [Table] Capital Leased Assets [Line Items] Capital Leased Assets [Line Items] Capital Leases, Future Minimum Payments Due, Next Twelve Months Capital Leases, Future Minimum Payments Due, Next Twelve Months Capital Leases, Future Minimum Payments Due in Two Years Capital Leases, Future Minimum Payments Due in Two Years Capital Leases, Future Minimum Payments Due in Three Years Capital Leases, Future Minimum Payments Due in Three Years Capital Leases, Future Minimum Payments Due in Four Years Capital Leases, Future Minimum Payments Due in Four Years Capital Leases, Future Minimum Payments Due in Five Years Capital Leases, Future Minimum Payments Due in Five Years Capital Leases, Future Minimum Payments Due Thereafter Capital Leases, Future Minimum Payments Due Thereafter Capital Leases, Future Minimum Payments Due Capital Leases, Future Minimum Payments Due Capital Leases, Future Minimum Payments, Interest Included in Payments Capital Leases, Future Minimum Payments, Interest Included in Payments Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments BALANCE SHEET COMPONENTS Details of Certain Balance Sheet Accounts Disclosure [Text Block] Details of Certain Balance Sheet Accounts 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, Option, Quantitative Disclosures [Table Text Block] Fair Value, Option, Quantitative Disclosures [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Equity Stockholders' Equity Note Disclosure [Text Block] Condensed Consolidated Balance Sheets (Unaudited) (Parentheticals) [Abstract] Condensed Consolidated Balance Sheets (Unaudited) (Parentheticals) [Abstract] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Preferred stock, no par value Preferred Stock, No Par Value Preferred stock, shares authorized Preferred Stock, Shares Authorized Preferred stock, issued Preferred Stock, Shares Issued Preferred stock, outstanding Preferred Stock, Shares Outstanding Common stock, no par value Common Stock, No Par Value Common stock, shares authorized Common Stock, Shares Authorized Common stock, shares issued Common Stock, Shares, Issued Common stock, shares outstanding Weighted-average common shares outstanding - basic Dilutive effect of potential common shares Weighted Average Number Diluted Shares Outstanding Adjustment Weighted-average common shares outstanding - diluted Other Current Assets [Table] Other Current Assets [Table] Other Current Assets [Table] Other Current Assets [Line Items] other current assets [Line Items] [Line Items] other current assets [Line Items] [Line Items] Prepaid expenses Prepaid Expense and Other Assets, Current Margin deposits Margin Deposit Assets Assets from price risk management activities Regulatory assets from price risk management activities Regulatory assets from price risk management activities Other Other Assets Other current assets Other Assets, Current contractors [Text Block] contractors [Text Block] Carty Generating Station disclosures[Text Block] Other Current Liabilities [Abstract] Other Current Liabilities [Abstract] Other Current Liabilities [Table] Other Current Liabilities [Table] Other Current Liabilities [Table] Regulatory liabilities—current Regulatory Liability, Current Accrued employee compensation and benefits Employee-related Liabilities, Current Accrued interest payable Interest Payable Accrued dividends payable Dividends Payable, Current Accrued taxes payable Taxes Payable, Current Other Other Current Liabilities - Other Other Current Liabilities - Other Total accrued expenses and other current liabilities Other Liabilities, Current Regulatory deferral of settled derivative instruments Gain (Loss) on Contract Termination Service Area Sq Miles Service Area Sq Miles Approximate square miles of service area. Incorporated Cities Incorporated Cities Number of incorporated cities in service area. Number of Retail Customers Number of Retail Customers Includes residential, commercial and industrial. Service Area Population Service Area Population Service Area Population Percent of State's Population Percent of State's Population Service area population as a percent of the state's population. Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Unamortized Debt Issuance Expense Unamortized Debt Issuance Expense Decoupling Mechanism Deferrals, Net Decoupling Mechanism Deferrals, Net The deferral, which can be either a positive or negaitive amount depending on whether a collection from or refund to customers results, of the amounts recorded in net income for energy useage by customers on a weather adjusted basis that is different from base rates as determined under the Company's decoupling mechanism, expected to be recovered or refunded in future periods. Cash received pursuant to the Residential Exchange Program Increase (Decrease) in Customer Advances and Deposits Schedule of Property, Plant and Equipment [Table] Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Capital Leases, Future Minimum Payments, Interest Included in Payments Interest Portion of Minimum Lease Payments, Sale Leaseback Transactions Capital Leased Assets, Gross Capital Leased Assets, Gross Electric utility plant Public Utilities, Property, Plant and Equipment, Plant in Service Total cost Property, Plant and Equipment, Gross Less: accumulated depreciation and amortization Public Utilities, Property, Plant and Equipment, Accumulated Depreciation Electric utility plant, net Public Utilities, Property, Plant and Equipment, Net Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation Capital Lease Obligations, Current Capital Lease Obligations, Current Capital Lease Obligations, Noncurrent Capital Lease Obligations, Noncurrent Amortization of Leased Asset Amortization of Leased Asset Capital Leases, Income Statement, Interest Expense Capital Leases, Income Statement, Interest Expense Assets from price risk management activities: Counterparty A Counterparty-A Individually significant counterparty of total assets arising from derivative instruments from price risk management activities. Counterparty B Counterparty B Individually significant counterparty of total assets arising from derivative instruments from price risk management activities. Counterparty C Counterparty C Individually significant counterparty of total assets arising from derivative instruments from price risk management activities. Total Concentration of Risk, Derivative Instruments, Assets Represents aggregate of individually significant counterparties of total assets arising from derivative instruments from price risk management activities. Liabilities from price risk management activities: Counterparty D Counterparty D for PRM Liab Counterparty D for PRM Liab Counterparty B Counterparty B for PRM Liabs Individually significant counterparty of total liabilities arising from derivative instruments from price risk management activities. Counterparty E Counterparty E For PRM Liab Counterparty E For PRM Liab Total Concentration of Risk, Derivative Instruments, Liabilities Represents aggregate of individually significant counterparties of total liabilities arising from derivative instruments from price risk management activities. Statement of Financial Position [Abstract] ASSETS Cash and cash equivalents Accounts receivable, net Accounts Receivable, Net, Current Unbilled revenues Unbilled Receivables, Current Inventories Energy Related Inventory Regulatory assets - current Regulatory Assets, Current Other current assets Total current assets Assets, Current Electric utility plant, net Regulatory assets - noncurrent Regulatory Assets, Noncurrent Nuclear decommissioning trust Decommissioning Fund Investments Non-qualified benefit plan trust Assets Held-in-trust, Noncurrent Other noncurrent assets Other Assets, Noncurrent Total assets Assets LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Current liabilities Accounts payable Accounts Payable, Current Liabilities from price risk mangement activities - current Derivative Liability, Current Short-term debt Short-term Debt Current portion of long-term debt Long-term Debt, Current Maturities Accrued expenses and other current liabilities Total current liabilities Liabilities, Current Long-term debt, net of current portion Long-term Debt, Excluding Current Maturities Regulatory liabilities - noncurrent Regulatory Liability, Noncurrent Deferred income taxes Deferred Tax Liabilities, Net, Noncurrent Unfunded status of pension and postretirement plans Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent Liabilities from price risk mangement activities - noncurrent Derivative Liability, Noncurrent Asset retirement obligations Asset Retirement Obligation Non-qualified benefit plan liabilities Other Postretirement Defined Benefit Plan, Liabilities, Noncurrent Other noncurrent liabilities Other Liabilities, Noncurrent Total liabilities Liabilities Commitments and contingencies (see notes) Commitments and Contingencies Equity: Preferred stock, no par value, 30,000,000 shares authorized; none issued and outstanding as of March 31, 2016 and December 31, 2015 Preferred Stock, Value, Issued Common stock, no par value, 160,000,000 shares authorized; 88,899,359 and 88,792,751 shares issued and outstanding as of March 31, 2016 and December 31, 2015, respectively Common Stock, Value, Issued Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Total equity Total liabilities and equity Liabilities and Equity Commitments and Contingencies, Policy [Policy Text Block] Commitments and Contingencies, Policy [Policy Text Block] Unvested performance-based restricted stock units and associated dividend equivalent rights Incremental Common Shares Attributable to Dilutive Effect of Contingently Issuable Shares Additional Fair Value Elements [Abstract] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net Cash Surrender Value, Fair Value Disclosure Cash Surrender Value, Fair Value Disclosure Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Long-term Debt Long-term Debt Long-term Debt, Fair Value Long-term Debt, Fair Value Valuation and Qualifying Accounts Disclosure [Table] Valuation and Qualifying Accounts Disclosure [Table] Extinguishment of Debt [Axis] Extinguishment of Debt [Axis] Extinguishment of Debt, Type [Domain] Extinguishment of Debt, Type [Domain] Notes Payable to Banks [Member] Notes Payable to Banks [Member] Long-term Debt [Member] Long-term Debt [Member] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves [Domain] Valuation Allowances and Reserves [Domain] Valuation and Qualifying Accounts Disclosure [Line Items] Valuation and Qualifying Accounts Disclosure [Line Items] Finite-Lived Intangible Assets, Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Amortization of Intangible Assets Amortization of Intangible Assets Syndicated credit facility scheduled to expire in 2019 Credit Facility expiring in 2019 Credit Facility expiring in 2019 Line of Credit Facility, Current Borrowing Capacity Line of Credit Facility, Current Borrowing Capacity Debt Instrument, Covenant Description Debt Instrument, Covenant Description Ratio of Indebtedness to Net Capital Ratio of Indebtedness to Net Capital Line of Credit Facility, Remaining Borrowing Capacity Line of Credit Facility, Remaining Borrowing Capacity Letters of credit issued Pledged Financial Instruments, Not Separately Reported, Securities for Letter of Credit Facilities Authorized Short-Term Debt Authorized Short-Term Debt FERC-authorized short-term debt through February 6, 2012 Proceeds from Issuance of Long-term Debt Debt Instrument, Face Amount Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Extinguishment of Debt, Amount Capital Leases, Future Minimum Payments, Remainder of Fiscal Year Capital Leases, Future Minimum Payments, Remainder of Fiscal Year