Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Generation plant Electricity Generation Plant, Non-Nuclear [Member] Energy sales refunded to third party Energy Sales, Amount Refunded Energy Sales, Amount Refunded Trading Arrangement: Trading Arrangement [Axis] Maximum Maximum [Member] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Range [Domain] Statistical Measurement [Domain] Less allowances for depreciation and amortization Accumulated Depreciation Public Utilities, Property, Plant and Equipment, Accumulated Depreciation Short-term Debt, Type [Axis] Short-Term Debt, Type [Axis] Cost Reported Value Measurement [Member] Subsequent Event Subsequent Event [Member] Impaired Long-Lived Assets Held and Used by Type [Axis] Impaired Long-Lived Assets Held and Used by Type [Axis] Investments in and advances to coal mines Investments In And Advances To Coal Mines Total investments in and advances to coal mines. These may include (A) an entity in which the entity has significant influence, but does not have control, (B) subsidiaries that are not required to be consolidated and are accounted for using the equity and or cost method, and (C) an entity in which the reporting entity shares control of the entity with another party or group. Includes long-term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership. Document Fiscal Year Focus Document Fiscal Year Focus Non-utility assets Other Capitalized Property Plant and Equipment [Member] Finance Lease [Abstract] Finance Lease [Abstract] Finance Lease Available borrowing capacity under letter of credit sublimit Line Of Credit Facility, Remaining Borrowing Capacity, Letter Of Credit Sublimit Line Of Credit Facility, Remaining Borrowing Capacity, Letter Of Credit Sublimit Adjustment to Compensation, Amount Adjustment to Compensation Amount Salt River Project Salt River Project [Member] Salt River Project United Power United Power, Inc. [Member] United Power, Inc. Customer [Axis] Customer [Axis] Electric Plant Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract] Schedule of Long-term Debt Schedule of Long-Term Debt Instruments [Table Text Block] Yampa Project - Craig Generating Station Units 1 and 2 Yampa Project Craig Station Units 1 And 2 [Member] Yampa Project, a jointly owned generation facility operated by the registrant. EMPLOYEE BENEFIT PLANS Retirement Benefits [Text Block] Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date National Rural Utilities Cooperative Finance Corporation - patronage capital National Rural Utilities Cooperative Finance Corporation - Patronage Capital [Member] Information pertaining to the National Rural Utilities Cooperative Finance Corporation patronage capital. Less current obligations at end of period Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Current Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Current Schedule of Investment in Electric Plant Public Utility Property, Plant, and Equipment [Table Text Block] Schedule of Other Deferred Charges Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Table Text Block] Amendment Flag Amendment Flag Consolidated Entities [Axis] Consolidated Entities [Axis] Current liabilities Liabilities, Current [Abstract] Project [Axis] Project [Axis] Other Other Regulatory Assets [Member] Other Regulatory Assets Craig Generating Station Unit 2 And 3 Craig Generating Station Unit 2 And 3 [Member] Craig Generating Station Unit 2 And 3 Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Basin Electric Power Cooperative Basin Electric Power Cooperative [Member] Information pertaining to Basin Electric Power Cooperative. Mine assets Productive Land [Member] National Rural Utilities Cooperative Finance Corporation - capital term certificates National Rural Utilities Cooperative Finance Corporation - Capital Term Certificates [Member] Information pertaining to the National Rural Utilities Cooperative Finance Corporation capital term certificates. Accumulated Depreciation Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Asset Retirement Obligation And Accrual For Environmental Loss Contingencies [Line Items] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies [Line Items] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Schedule of Net Unrecognized Actuarial Gains and Losses on Employee Benefit Plan Obligations Schedule of Defined Benefit Plan Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block] Finance lease right-of-use assets Finance Lease, Right-of-Use Asset, before Accumulated Amortization Investments [Domain] Investments [Domain] Liabilities settled Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Liabilities Settled Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Liabilities Settled Leases [Abstract] Leases [Abstract] Other comprehensive income: Other Comprehensive Income (Loss), Net of Tax [Abstract] UNS Electric, Inc. UNS Electric, Inc. [Member] UNS Electric, Inc. Amortization period Regulatory Asset, Amortization Period Capitalization Members' Equity [Abstract] Consolidated Entities [Domain] Consolidated Entities [Domain] Equity distribution to noncontrolling interest Payments to Noncontrolling Interests Coal inventory Energy Related Inventory, Coal Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Entity Address, State or Province Entity Address, State or Province Public Utility, Property, Plant and Equipment [Table] Public Utility, Property, Plant and Equipment [Table] Deferred impairment loss Deferred Impairment Loss [Member] Deferred impairment losses on regulatory assets to be recovered from Members. PEO Total Compensation Amount PEO Total Compensation Amount Qualifying age Defined Benefit Plan, Plan Participant Required Age The required age to qualify for participation and/or receive particular benefits of the plan. Deferred credits and other liabilities Liabilities, Noncurrent [Abstract] Intangible assets, net Intangible Asset, Excluding Goodwill, after Accumulated Amortization Retirement Benefits [Abstract] Retirement Benefits [Abstract] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Arrangements and Non-arrangement Transactions [Domain] Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Total current liabilities Liabilities, Current Schedule of Cash, Cash Equivalents and Restricted Cash and Investments Schedule Of Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents And Investments [Table Text Block] Schedule Of Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents And Investments Regulatory Liability [Axis] Regulatory Liability [Axis] Short-term Debt, Type [Domain] Short-Term Debt, Type [Domain] Net Book Value Public Utilities, Property, Plant And Equipment Excluding Construction In Process, Net Period end amount of total net PPE excluding construction work in process. Schedule of Derivative Assets at Fair Value Schedule of Derivative Assets at Fair Value [Table Text Block] Entity Address, City or Town Entity Address, City or Town Regulatory Asset and Liability [Line Items] Regulatory Asset And Liability [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Retirement of patronage capital Payments For Retirement Of Patronage Capital Amount of cash outflow to distribute patronage capital retirements. Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of Accounting Basis of Accounting, Policy [Policy Text Block] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Amortization of NRECA Retirement Security Plan prepayment Amortization Of Retirement And Security Plan Prepayment Amortization of NRECA Retirement and Security Plan prepayment. Other accounts receivable - nontrade Other Accounts Receivable, Non Trade [Member] Information pertaining to other non-trade accounts receivable. Fair Value, Inputs, Level 2 Fair Value, Inputs, Level 2 [Member] Receivable [Domain] Receivable [Domain] Construction Work In Progress Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress General plant Electric General Plant [Member] Represents information pertaining to general plant. Other income Nonoperating Income (Expense) [Abstract] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] CoBank, ACB Cobank, ACB [Member] Information pertaining to CoBank, ACB. Deferred Credits [Table] Deferred Credits [Table] Schedule of deferred credits. Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Debt service ratio (DSR) requirement Debt Service Ratio The Debt Service Ratio (DSR) requirement per the terms of long-term debt agreements. Other comprehensive income Other Comprehensive Income (Loss), Net of Tax Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Current maturities of long-term debt Less current maturities Long-Term Debt, Current Maturities Revenue Benchmark Revenue Benchmark [Member] Lease liabilities - noncurrent Lease, Liability, Noncurrent Lease, Liability, Noncurrent J.M. Shafer Thermo Cogeneration Partnership [Member] Acquisition of Thermo Cogeneration Partnership, which resulted in recording of goodwill related to the J.M. Shafer Generating Station. Name of Property [Axis] Name of Property [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Environmental obligation Environmental Restoration Costs [Member] Accounting Pronouncement - Not Yet Adopted New Accounting Pronouncements, Policy [Policy Text Block] Noncontrolling interest Noncontrolling Interest [Member] United Power Inc United Power Inc [Member] Information pertaining to United Power, Inc. Amortization of debt issuance costs Amortization of Debt Issuance Costs Defined Contribution Plan Disclosure [Line Items] Defined Contribution Plan Disclosure [Line Items] Amortization of transmission credits Amortization Of Transmission Credits Amortization Of Transmission Credits Total debt adjusted for debt issuance costs, discounts and premiums Long-Term Debt Short-term Debt [Abstract] Short-Term Debt [Abstract] Lease revenue Lease Revenue [Member] Lease Revenue Soft-cap price for short term spot market sales (per megawatt hour) Energy Sales, Soft-Cap Price Threshold Energy Sales, Soft-Cap Price Threshold PROPERTY, PLANT AND EQUIPMENT Property, Plant, and Equipment [Text Block] Restricted cash and investments Restricted cash and investments - noncurrent Restricted Cash and Investments, Noncurrent Schedule of Future Expected Minimum Lease Commitments under Operating Leases Lessee, Operating Lease, Liability, to be Paid, Maturity [Table Text Block] Supplemental disclosure of noncash investing and financing activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Document Period End Date Document Period End Date Changes in operating assets and liabilities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Asset retirement obligation Asset Retirement Obligation Costs [Member] Net cash used in investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Unrecovered plant Unrecovered Plant Regulatory Asset [Member] Unrecovered Plant Regulatory Asset Regulatory assets Regulatory Asset Term of issuance Debt Instrument, Term Schedule of Regulatory Assets and Liabilities Schedule Of Regulatory Assets And Liabilities [Table Text Block] Tabular disclosure of regulatory assets and liabilities. Detailed information about assets and liabilities that result from rate actions of a regulator. Rate actions of a regulator can impose an asset or a liability on a regulated enterprise resulting in a regulatory asset or liability. LEASES Lessor, Operating Leases [Text Block] Finance leases, weighted average remaining lease term Finance Lease, Weighted Average Remaining Lease Term Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Operating lease liabilities - current Operating Lease, Liability, Current Actual return on plan assets Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Postemployment Medical Benefit Plans Postemployment Medical Benefit Plans [Member] Unfunded contributory medical plans that provide postemployment medical benefits to non-bargaining unit employees on long-term disability. Utility Member electric sales Electric Sales Member [Member] Information pertaining to electric sales made to members of the cooperative. Accrued interest Interest Payable, Current Deposits and advances Deposits And Advances, Current Amount of asset related to consideration paid for deposits required by contract and advances to vendors that provide economic benefits within a future period of one year or the normal operating cycle, if longer. Investments in and Advances to Affiliates, Schedule of Investments [Abstract] Investments in and Advances to Affiliates, Schedule of Investments [Abstract] Net finance lease right-of-use assets Finance Lease, Right-of-Use Asset, after Accumulated Amortization Mortgages Mortgages [Member] Schedule of Other Plant Assets (Excluding Utility Assets) Property, Plant, and Equipment [Table Text Block] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Name Outstanding Recovery, Individual Name Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Total due for easement right of way Easement Right Of Way, Payments Due Total amount of payments due for easement right of way per terms of agreement. Document Fiscal Period Focus Document Fiscal Period Focus Number of members Number Of Members Number of members of the reporting entity. Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Impairment during the period Equity Securities without Readily Determinable Fair Value, Impairment Loss, Annual Amount Deferred Costs [Abstract] Deferred Costs [Abstract] Accrued property taxes Increase (Decrease) in Property and Other Taxes Payable Purchases of plant Payments to Acquire Property, Plant, and Equipment Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Accretion expense Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Accretion Expense Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Accretion Expense Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Initial amount calculated for contract termination payment Contract Termination Payment, Initial Award Contract Termination Payment, Initial Award Purchased power Utilities Operating Expense, Purchased Power Electric plant Electric Plant [Abstract] No definition available. Company Selected Measure Name Company Selected Measure Name Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] FAIR VALUE Fair Value Measurement and Measurement Inputs, Recurring and Nonrecurring [Text Block] Operating lease right-of-use assets Operating Lease, Right-Of-Use Asset, Gross Amount before accumulated depreciation of lessee's right to use underlying asset under operating lease. CONTRACT ASSETS AND CONTRACT LIABILITIES Contract With Customer, Assets And Liabilities [Text Block] The entire disclosure of customer contract balances and changes in contract balances. Ownership interest held by parent (as a percent) Subsidiary, Ownership Percentage, Parent Elk Ridge Elk Ridge Mining And Reclamation LLC [Member] Represents information pertaining to Elk Ridge Mining and Reclamation, LLC. Total debt Long-Term Debt, Gross Advances to operating agents of jointly owned facilities Due From Joint Venture Partners For an unclassified balance sheet, amounts due from the operating agent of the entity in which the reporting entity shares joint control with another party or group. Credit Facility [Domain] Credit Facility [Domain] Operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Formula rate settlement Formula Rate Settlement Regulatory Liability [Member] Formula Rate Settlement Regulatory Liability Other accounts receivable Other accounts receivable - trade: Other Receivables, Net, Current Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Public Utility [Axis] Public Utility [Axis] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Entity Emerging Growth Company Entity Emerging Growth Company Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Deferred income tax expense Deferred Income Tax Charge [Member] Letter of credit sublimit Line Of Credit Facility, Maximum Borrowing Capacity, Letter Of Credit Sublimit Line Of Credit Facility, Maximum Borrowing Capacity, Letter Of Credit Sublimit Underlying Security Market Price Change Underlying Security Market Price Change, Percent Other plant Other plant assets Property, Plant, and Equipment, Other, before Accumulated Depreciation Commercial Paper Commercial Paper [Member] DERIVATIVES Derivative Instruments and Hedging Activities Disclosure [Text Block] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Regulatory Asset [Axis] Regulatory Asset [Axis] Less imputed interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Unrecovered Plants Nucla And Escalante Generating Stations [Member] Information pertaining to the unrecovered Nucla and Escalante Generating Stations. Medical Benefit Plans Medical Benefit Plans [Member] Unfunded contributory medical plans that provide postretirement and postemployment medical benefits, or just postemployment medical benefits, to non-bargaining unit employees. Short-term borrowings Short-Term Debt Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Impairment losses of long-lived assets Long-Lived Asset, Held and Used, Impairment Loss Schedule of Asset Retirement Obligations and Environmental Remediation Obligations Schedule Of Asset Retirement Obligation And Accrual For Environmental Loss Contingencies [Table Text Block] Schedule Of Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Transmission Utilities Operating Expense, Transmission Amount of operating expense for transmission operations by regulated operation. Award Timing Predetermined Award Timing Predetermined [Flag] Escalante Generating Station Escalante Generating Station [Member] Information pertaining to the Escalante Generating Station. Thereafter Lessee, Operating Lease, Liability, to be Paid, after Rolling Year Five Number of plans Defined Benefit Plan, Number Of Plans The number of plans sponsored by entity. Project [Domain] Project [Domain] Net electric plant Net electric plant Public Utilities, Property, Plant and Equipment, Net Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Other Other Plant in Service [Member] Coal inventory Increase (Decrease) in Coal Inventories Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Total lease payments Lessee, Operating Lease, Liability, to be Paid Schedule of Employee Benefit Plan Obligations Schedule of Changes in Projected Benefit Obligations [Table Text Block] All Executive Categories All Executive Categories [Member] Weighted-average discount rate Weighted Average Discount Rate, Percent [Abstract] Weighted Average Discount Rate, Percent Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] La Plata Electric Association La Plata Electric Association [Member] La Plata Electric Association Total long-term debt Long-Term Debt, Fair Value OTHER DEFERRED CREDITS AND OTHER LIABILITIES Other Liabilities Disclosure [Text Block] Transmission revenue Transmission Revenue [Member] Transmission Revenue Other Other Operating Revenue, Including Revenue From Contract With Customer Amount of operating revenue classified as other including revenue from contract with customer. Net operating lease right-of-use assets Operating Lease, Right-of-Use Asset Deferred Membership Withdrawal Income Revenue From Membership Withdrawal Subject To Refund [Member] Revenue from termination fee for early withdrawal from membership resulting in deferral subject to refund to Members through reduced rates in future periods. Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation [Abstract] Concentration Risk Type [Domain] Concentration Risk Type [Domain] OATT deposits Transmission Deposits Transmission Deposits Deferred charges Deferred Costs, Current [Abstract] Operating expenses Operating Income (Loss) [Abstract] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Debt Disclosure [Abstract] Debt Disclosure [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] INVESTMENTS IN OTHER ASSOCIATIONS Investment in Debt and Equity Securities and Other Trading Assets [Text Block] Borrowings under line of credit Long-Term Line of Credit Minimum Minimum [Member] Investment Type [Axis] Investment Type [Axis] Expiration Date Trading Arrangement Expiration Date Net increase (decrease) in cash, cash equivalents and restricted cash and investments Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Excluding Exchange Rate Effect, Including Discontinued Operation Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax [Roll Forward] Accumulated Other Comprehensive Income (Loss), Pension And Other Postretirement Benefit Plans, Net Of Tax [Roll Forward] A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. Schedule of Revenue from Contracts with Non-member Customers and Other Operating Revenue Disaggregation of Revenue [Table Text Block] VARIABLE INTEREST ENTITIES Variable Interest Entity Disclosure [Text Block] Change in Colowyo asset retirement obligation Increase (Decrease) in Asset Retirement Obligations Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Cash and cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Total equity Equity at beginning of period Equity at end of period Equity, Including Portion Attributable to Noncontrolling Interest Operating Leases Lessee, Operating Lease, Liability, to be Paid, Rolling Maturity [Abstract] Customer deposits Customer Deposits, To Be Returned Amounts received from customers that are expected to be returned upon satisfactory contract completion. Class A Members Full Utility [Member] Information pertaining to Class A utility full requirement members. Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Other Other Liabilities Springerville certificates, Series B, 7.14%, due through 2033 Springerville Certificates Series B Due Through2033 [Member] Information pertaining to Springerville certificates. 7.14% rate due through 2033. Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Derivative [Table] Derivative [Table] Exercise Price Award Exercise Price Investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Document Quarterly Report Document Quarterly Report Capital credit allocations from cooperatives and income from coal mines under refund distributions Capital Credit Allocations From Cooperatives And Income From Coal Mines Under (Over) Refund Distribution Capital credit allocations from cooperatives and income from coal mines under (over) refund distributions. Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Plant In Service Public Utilities, Property, Plant And Equipment, Plant In Service Excluding Construction In Progress Period end amount of total gross PPE, excluding construction work in progress. Payments of long-term debt Repayments of Long-Term Debt Regulatory Assets and Liabilities Disclosure [Abstract] Regulatory Assets and Liabilities Disclosure [Abstract] Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Year 5 Lessee, Operating Lease, Liability, to be Paid, Rolling Year Five Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Springerville Unit 3 Holding LLC Springerville Unit3 Holding Llc [Member] Information pertaining to the Springerville Unit 3 Holding LLC which is the owner operator of the Springerville Generating Station Unit 3. All Other Long-Term Debt All Other Long-Term Debt [Member] All Other Long-Term Debt Equity Component [Domain] Equity Component [Domain] PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Electric Plant in Service Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Other Total other deferred charges Deferred Costs Operating expenses Costs and Expenses Adoption Date Trading Arrangement Adoption Date Finance leases, weighted average discount rate (as a percent) Finance Lease, Weighted Average Discount Rate, Percent Contract liabilities Contract with Customer, Liability [Abstract] Provision for rate refunds - FERC Compliance Filing - Rate Schedule 281 Provision For Rate Refunds - FERC Compliance Filing - Rate Schedule 281 [Member] Provision For Rate Refunds - FERC Compliance Filing - Rate Schedule 281 Counterparty Name [Domain] Counterparty Name [Domain] Total other income Nonoperating Income (Expense) Other Utilities Operating Expense, Other Swingline sublimit Line of Credit Facility, Maximum Borrowing Capacity, Swingline Sublimit Line of Credit Facility, Maximum Borrowing Capacity, Swingline Sublimit Changes in deferred charges Increase (Decrease) in Deferred Charges, Investing ASSETS Assets [Abstract] Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Total operating lease liabilities Total Operating Lease, Liability Insider Trading Policies and Procedures [Line Items] Total finance lease liabilities Finance Lease, Liability Other Proceeds from (Payment for) Other Financing Activity Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Transmission plant Electric Transmission [Member] Transmission Right of Way Easements Transmission Right Of Way Easements [Member] Information relating to transmission right of way easements. Business Combination [Domain] Business Combination [Domain] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Lease revenue Operating Lease, Lease Income Total non-member electric sales and other operating revenue Non Member Electric Sales And Other Operating Revenue [Member] Information pertaining to the aggregate of non-member electric sales and other operating revenue. Entity Central Index Key Entity Central Index Key All Adjustments to Compensation All Adjustments to Compensation [Member] Name of Property [Domain] Name of Property [Domain] Other Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Other Item Business Combination [Axis] Business Combination [Axis] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Net cash provided by (used in) operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation LEGAL Legal Matters and Contingencies [Text Block] Income Statement [Abstract] Income Statement [Abstract] Document Transition Report Document Transition Report Ownership [Axis] Ownership [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] 2022 Revolving Credit Agreement Revolving Credit Agreement 2022 [Member] Revolving Credit Agreement 2022 Investment, Name [Axis] Investment, Name [Axis] Deferred Income Tax Assets, Net Deferred Income Tax Assets, Net Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Finance lease liabilities - noncurrent Finance Lease, Liability, Noncurrent Coal mining Operating Expense, Coal Mining Amount of operating expense related to coal mining activities. Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Change in estimate Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Revision Of Estimate Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Revision Of Estimate City Area Code City Area Code Accumulated other comprehensive income (loss) AOCI Attributable to Parent [Member] All Individuals All Individuals [Member] SUBSEQUENT EVENT Subsequent Events [Text Block] Name Forgone Recovery, Individual Name Regulatory Liability [Domain] Regulatory Liability [Domain] Less allowances for depreciation, amortization and depletion Accumulated Depreciation, Depletion And Amortization, Property, Plant, And Equipment, Other Accumulated Depreciation, Depletion And Amortization, Property, Plant, And Equipment, Other Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Marketable securities Marketable Securities, Noncurrent Retirement of patronage capital Partners' Capital Account, Distributions Statement [Line Items] Statement [Line Items] CASH, CASH EQUIVALENTS AND RESTRICTED CASH AND INVESTMENTS Cash, Cash Equivalents, Restricted Cash And Investments Disclosure [Text Block] Cash, Cash Equivalents, Restricted Cash And Investments Disclosure Interest expense, net of amounts capitalized Interest Interest Expense, Net Of Capitalized Interest Amount of the cost of borrowed funds, net of capitalized interest costs, accounted for as interest expense Total assets Assets Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plan [Table] Entity [Domain] Entity [Domain] Environmental reclamation obligations Accrual for Environmental Loss Contingencies Colowyo Mine Colowyo Mine [Member] Information pertaining to the Colowyo Mine. Long-term contracts Long-Term Contract with Customer [Member] Fair value of plan assets at beginning of period Fair value of plan assets at end of period Defined Benefit Plan, Plan Assets, Amount Reportable Legal Entities Reportable Legal Entities [Member] Lease asset additions Lease Obligation Incurred Less: Accumulated amortization Finance Lease, Right-of-Use Asset, Accumulated Amortization Interest Interest Expense, Nonoperating Investments in other associations Investments in other associations Equity Securities without Readily Determinable Fair Value, Amount Contract with Customer, Duration [Domain] Contract with Customer, Duration [Domain] Postretirement medical benefit obligation at beginning of period Total postretirement and postemployment medical obligations at end of period Defined Benefit Plan, Benefit Obligation Transmission easements Easement Right Of Way, Present Value Of Payments Due Present value of easement right of way payments due recorded. PEO PEO [Member] Accrued property taxes Accrual for Taxes Other than Income Taxes, Current Total other assets and investments Other Assets And Investments Sum of the carrying amounts as of the balance sheet date of investments and other assets, excluding property, plant and equipment, that are expected to be realized in cash, sold or consumed after one year or beyond the normal operating cycle, if longer. Measurement Basis [Axis] Measurement Basis [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Total deferred charges Prepaid Expense, Regulatory Assets, And Deferred Charges, Current Sum of the carrying amounts as of the balance sheet date of prepaid expenses, regulatory assets, and deferred costs capitalized at the end of the reporting period. Year 4 Lessee, Operating Lease, Liability, to be Paid, Rolling Year Four Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Other Limited Partners Other Limited Partners [Member] Information pertaining to the other limited partners in the partnership. Contract liabilities recognized Contract with Customer, Liability, Revenue Recognized Subsequent Event Type [Domain] Subsequent Event Type [Domain] 2026 Credit Agreement 2026 Credit Agreement [Member] 2026 Credit Agreement Operating lease liabilities - noncurrent Operating Lease, Liability, Noncurrent Deposits associated with generator interconnection requests Deposits For Generation Interconnection Studies Deposits For Generation Interconnection Studies Patronage capital Retained Earnings [Member] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Springerville Unit 3 Lease Springerville Generating Station Unit3 [Member] Arrangement with Springerville Generating Station Unit 3 to use power generating equipment for a stated period of time. Accounted for as a lease. Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Unrealized gain (loss) on securities available for sale OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Award Timing Disclosures [Line Items] Schedule of Cost and Fair Values of Assets and Liabilities Fair Value, by Balance Sheet Grouping [Table Text Block] Debt issuance costs Payments of Debt Issuance Costs Entity Current Reporting Status Entity Current Reporting Status Net cash provided by financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Accounts receivable Accounts Receivable, after Allowance for Credit Loss [Abstract] Total Shareholder Return Amount Total Shareholder Return Amount Number of electric distribution member systems Number Of Electric Distribution Member Systems Number Of Electric Distribution Member Systems Deferred prepaid lease expense Deferred Prepaid Lease Expense [Member] Deferral of loss on acquisition related to the prepaid lease expense. Amortization of intangible asset Intangible Asset, Finite-Lived, Amortization Expense Annual Depreciation Rate (as a percent) Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service General partner interest (as a percent) Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest Ownership [Domain] Ownership [Domain] Deferred credits and other liabilities Deferred Credits [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Lease right-of-use assets Lease, Right-Of-Use Asset Lease, Right-Of-Use Asset Financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Operating revenues Revenue from (refunds to) contract with customer Revenue from Contract with Customer, Excluding Assessed Tax Total property, plant and equipment Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization Year 2 Lessee, Operating Lease, Liability, to be Paid, Rolling Year Two Provision for rate refunds Provision For Rate Refunds [Member] Provision For Rate Refunds Available borrowing capacity Line of Credit Facility, Remaining Borrowing Capacity Interest cost Defined Benefit Plan, Interest Cost Obligations at beginning of period Total obligations at end of period Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Derivative Asset Derivative Asset Equity to capitalization ratio (ECR) requirement (as a percent) Equity To Capitalization Ratio The Equity to Capitalization Ratio (ECR) requirement per the terms of long-term debt agreements. Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Underlying Securities Award Underlying Securities Amount Net comprehensive income attributable to noncontrolling interest Net comprehensive income attributable to noncontrolling interest Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Estimated Fair Value Estimate of Fair Value Measurement [Member] REVENUE Revenue from Contract with Customer [Text Block] Accrued interest Increase (Decrease) in Interest Payable, Net Interest rate swaps, realized gain Interest Rate Swaps Realized Gain [Member] Rate action of regulator resulting in capitalization or accrual of realized gain on interest rate swap contracts. Equity Securities without Readily Determinable Fair Value [Line Items] Equity Securities without Readily Determinable Fair Value [Line Items] Other Other Cost Method Investments [Member] Information pertaining to other cost method investments. Operating leases, weighted average discount rate (as a percent) Operating Lease, Weighted Average Discount Rate, Percent Asset retirement and environmental remediation obligations Long-term obligations at end of period Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Noncurrent Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Noncurrent Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Schedule of Lease Components Lease, Cost [Table Text Block] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Operating margins Operating Income (Loss) Poudre Valley Electric Association Poudre Valley Electric Association [Member] Poudre Valley Electric Association Current Fiscal Year End Date Current Fiscal Year End Date Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Contract with Customer, Duration [Axis] Contract with Customer, Duration [Axis] Derivative [Line Items] Derivative [Line Items] Total undiscounted lease payments for energy storage agreements Lessee, Operating Lease, Liability, To Be Paid, Energy Storage Agreements Lessee, Operating Lease, Liability, To Be Paid, Energy Storage Agreements Other Other Revenue [Member] Information pertaining to revenue classified as other. Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Entity File Number Entity File Number Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Service cost Defined Benefit Plan, Service Cost Change in short-term borrowings, net Proceeds from (Repayments of) Short-Term Debt Rate stabilization revenue Deferred Revenue Recognized The amount of previously reported deferred or unearned revenue that was recognized as revenue during the period. For cash flows, this element primarily pertains to amortization of deferred credits on long-term arrangements. As a noncash item, it is deducted from net income when calculating cash provided by or used in operations using the indirect method. Equity distribution to noncontrolling interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Other accounts receivable - trade Other Accounts Receivable, Trade [Member] Information pertaining to other trade accounts receivable. Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Measure: Measure [Axis] Schedule of Consolidated Variable Interest Entities Schedule of Variable Interest Entities [Table Text Block] Long-term debt, components Long-Term Debt, Current and Noncurrent [Abstract] OTHER DEFERRED CHARGES Other Assets Disclosure [Text Block] Short-term contracts Short-Term Contract with Customer [Member] Financial liabilities - reclamation Accrued Environmental Loss Contingencies, Noncurrent Annual amortization expense of regulatory asset Amortization Of Regulatory Asset, Annual Amount The expense charged against earnings annually to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs. Equity Components [Axis] Equity Components [Axis] Product and Service [Domain] Product and Service [Domain] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] MBPP - Laramie River Station Missouri Basin Power Project [Member] Missouri Basin Power Project Member, a jointly owned generation facility operated by Basin Electric Power Cooperative. Entity Interactive Data Current Entity Interactive Data Current Finance lease liabilities - current Finance Lease, Liability, Current Net margin attributable to noncontrolling interest Net Income (Loss) Attributable to Noncontrolling Interest Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Deferred income tax liability Deferred Income Tax Liabilities, Net Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Tri-State Share (as a percent) Jointly Owned Utility Plant, Proportionate Ownership Share Impaired Long-Lived Assets Held and Used, Asset Name [Domain] Impaired Long-Lived Assets Held and Used, Asset Name [Domain] Entity Small Business Entity Small Business Comprehensive income (loss) attributable to the Association Comprehensive Income (Loss), Net of Tax, Attributable to Parent Total electric plant Public Utilities, Property, Plant and Equipment, Plant in Service Less debt discounts Debt Instrument, Unamortized Discount Termination Date Trading Arrangement Termination Date Proceeds from settlement of interest rate swaps Proceeds From Settlement Of Interest Rate Swaps Proceeds from the settlement of interest rate swaps. Property, Plant, and Equipment [Line Items] Property, Plant, and Equipment [Line Items] Utility Plant [Domain] Utility Plant [Domain] Non-Utility Members Non Utility [Member] Information pertaining to Non-Utility members. Schedule of Jointly Owned Facilities Schedule of Jointly Owned Utility Plants [Table Text Block] Subsequent Events [Abstract] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Document Type Document Type Materials and supplies Materials, Supplies, and Other Accrued expenses Accrued Liabilities, Current Preliminary surveys and investigations Preliminary Surveys, Plans, And Investigations Expenditures for preliminary surveys, plans and investigations for the purpose of determining the feasibility of contemplated generation and transmission projects which will be amortized over the life of the project. First Mortgage Obligations, Series 2017A , Tranche 1, 3.34%, due through 2029 First Mortgage Obligation Series2017a Tranche1 Due Through2029 [Member] Information pertaining to First Mortgage Obligation, Series 2017A, Tranche 1, 3.34%, due 2029. Fair Value as of Grant Date Award Grant Date Fair Value Asset Retirement Obligation And Accrual For Environmental Loss Contingencies [Table] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies [Table] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Arrangement Duration Trading Arrangement Duration Subsequent Event [Line Items] Subsequent Event [Line Items] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Proceeds from issuance of long-term debt Proceeds from Issuance of Long-Term Debt Local Phone Number Local Phone Number Contract liabilities (unearned revenue) - noncurrent Contract with Customer, Liability, Noncurrent Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Renewable Revolving Credit Agreement Renewable Revolving Credit Agreement [Member] Renewable Revolving Credit Agreement Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Schedule of Other Deferred Credits and Other Liabilities Other Noncurrent Liabilities [Table Text Block] Operating revenues Revenues Restricted cash and investments Restricted cash and investments - current Restricted Cash and Investments, Current Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Accounts payable and accrued expenses Increase (Decrease) in Accounts Payable and Accrued Liabilities Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Total capitalization Capitalization, Long-Term Debt, Noncurrent And Equity This element represents the total consolidated (as applicable) capitalization of the entity which is comprised of its long-term debt, net of current maturities and equity instruments. Executive Category: Executive Category [Axis] Receivable Type [Axis] Receivable Type [Axis] Collective-Bargaining Arrangement [Domain] Collective-Bargaining Arrangement [Domain] Counterparty Name [Axis] Counterparty Name [Axis] EQUITY AND LIABILITIES Liabilities and Equity [Abstract] Other Commitments [Table] Other Commitments [Table] Operating Lease [Abstract] Operating Lease [Abstract] Operating Lease All Award Types Award Type [Domain] Collective-Bargaining Arrangement [Axis] Collective-Bargaining Arrangement [Axis] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Schedule of Variable Interest Entities [Table] Variable Interest Entity [Table] Fair Value, Inputs, Level 3 Fair Value, Inputs, Level 3 [Member] Other noncurrent assets Regulated Entity, Other Assets, Noncurrent Materials and supplies Increase (Decrease) in Materials and Supplies Supplemental cash flow information: Supplemental Cash Flow Information [Abstract] Ownership interest held by noncontrolling interest (as a percent) Subsidiary, Ownership Percentage, Noncontrolling Owner PEO Name PEO Name Deferred Credits and Other Liabilities [Abstract] Deferred Credits and Other Liabilities [Abstract] Contract liabilities (unearned revenue) Contract with Customer, Liability Schedule of Short-term Debt [Table] Short-Term Debt [Table] Property, Plant and Equipment, Type [Axis] Long-Lived Asset, Class [Axis] Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Entity Tax Identification Number Legal Entity [Axis] Legal Entity [Axis] Other Commitments [Line Items] Other Commitments [Line Items] Jointly Owned Utility Plant Interests [Line Items] Jointly Owned Utility Plant Interests [Line Items] Short-term Borrowings - Other Short-Term Borrowings, Other [Member] Short-Term Borrowings, Other Change in Contract with Customer, Asset and Liability [Abstract] Change in Contract with Customer, Asset and Liability [Abstract] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Non-NEOs Non-NEOs [Member] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] PRESENTATION OF FINANCIAL INFORMATION Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Comprehensive income (loss) including noncontrolling interest Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Debt Instrument [Axis] Debt Instrument [Axis] Subsequent Event [Table] Subsequent Event [Table] Interest expense Interest Expense, Operating and Nonoperating [Abstract] In service Public Utilities, Property, Plant And Equipment, Plant In Service, Excluding Construction Work In Progress Period end amount of total gross PPE, excluding construction work in progress. Cash, cash equivalents and restricted cash and investments – beginning Cash, cash equivalents and restricted cash and investments – ending Cash, cash equivalents and restricted cash and investments Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents And Investments Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents And Investments Investment, Name [Domain] Investment, Name [Domain] Amount received for contract termination payment Contract Termination Payment, Amount Received Contract Termination Payment, Amount Received Customer [Domain] Customer [Domain] LEASES Lessee, Operating Leases [Text Block] Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Total current assets Assets, Current Statistical Measurement [Axis] Statistical Measurement [Axis] Insider Trading Arrangements [Line Items] Changes in Member advances Proceeds From (Payments Applied) Member Advances Net increase (decrease) in advances from members which are received in advance of future billings. Long-term debt Long-Term Debt, Excluding Current Maturities Discount rate of environmental reclamation obligations (as a percent) Accrual for Environmental Loss Contingencies, Discount Rate Sale of electric plant Proceeds From Sale Of Electric Plant Assets Proceeds From Sale Of Electric Plant Assets Award Type Award Type [Axis] Funds in excess of soft-cap Energy Sales, Funds in Excess of Soft-Cap Energy Sales, Funds in Excess of Soft-Cap Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Accounts receivable—Utility Members Accounts Receivable, after Allowance for Credit Loss, Current Patronage capital equity Retained Earnings (Accumulated Deficit) Schedule of Short-Term Borrowings Schedule of Short-Term Debt [Table Text Block] Product and Service [Axis] Product and Service [Axis] Net other plant Property, Plant, and Equipment, Other, after Accumulated Depreciation Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Year 3 Lessee, Operating Lease, Liability, to be Paid, Rolling Year Three Short-Term Debt [Line Items] Short-Term Debt [Line Items] Environmental Loss Contingency, Statement Of Financial Position, Extensible Enumeration, Not Disclosed Flag Environmental Loss Contingency, Statement Of Financial Position, Extensible Enumeration, Not Disclosed Flag Environmental Loss Contingency, Statement Of Financial Position, Extensible Enumeration, Not Disclosed Flag Fair Value Measurement [Domain] Fair Value Measurement [Domain] Fuel Utilities Operating Expense, Fuel Used Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover [Abstract] Cover [Abstract] Electric Plant Electric Plant [Member] Represents information pertaining to electric plant consisting of generation plant, transmission plant, general plant and other plant in service. Postretirement Medical Benefit Plans Postretirement Medical Benefit Plans [Member] Unfunded contributory medical plans that provide postretirement medical benefits to non-bargaining unit employees. Core Electric Cooperative Core Electric Cooperative [Member] Core Electric Cooperative Threshold of unrecognized amount of actuarial gains and losses as a proportion of projected benefit obligation (as a percent) Defined Benefit Plan, Threshold, Unrecognized Amount Of Actuarial Gains And Losses As Percentage Of Accumulated Postretirement Benefit Obligation The threshold percentage, represented by unrecognized amount of actuarial gains and losses as a percentage of accumulated postretirement benefit obligation, above which the amounts are reclassified out of AOCI. Non-bargaining unit employees Non Bargaining Unit Employees [Member] Information pertaining to non-bargaining unit employees. Weighted average interest rate (as a percent) Short-Term Debt, Weighted Average Interest Rate, at Point in Time Concentration risk (as a percent) Concentration Risk, Percentage Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Property, plant and equipment Public Utilities, Property, Plant and Equipment, Net [Abstract] Schedule of Investments in Other Associations Equity Securities without Readily Determinable Fair Value [Table Text Block] Equity Awards Adjustments Equity Awards Adjustments [Member] Variable Interest Entity or Potential VIE, Information Unavailability, Disclosures [Abstract] Variable Interest Entity or Potential VIE, Information Unavailability, Disclosures [Abstract] Net asset at end of period Defined Benefit Plan, Funded (Unfunded) Status of Plan Total equity and liabilities Liabilities and Equity Regulatory Asset [Domain] Regulatory Asset [Domain] EBT Trust Investments EBT Trust Investments [Member] EBT Trust Investments Amounts included in accumulated other comprehensive income at beginning of period Amounts included in accumulated other comprehensive income at end of period Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax Interest charged during construction Capitalized Interest Costs, Construction Projects Amount of the capitalized cost of borrowed funds on construction projects. Employee Stock Option Share-Based Payment Arrangement, Option [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Disclosure [Abstract] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Disclosure Current asset retirement and environmental remediation obligations Asset Retirement Obligation, Current Noncontrolling interest Equity, Attributable to Noncontrolling Interest Construction work in progress Public Utilities, Property, Plant and Equipment, Construction Work in Progress Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Production Utilities Operating Expense, Production Amount of operating expense for energy production of regulated operation. Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Schedule of Regulatory Assets and Liabilities [Table] Schedule Of Regulatory Assets And Liabilities [Table] A table of assets and liabilities that are created when regulatory agencies permit public utilities to defer certain costs that are included in rate-setting to the balance sheet. Tabular List, Table Tabular List [Table Text Block] Amortization of prior service credit into other income Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Other income Other Nonoperating Income (Expense) LONG-TERM DEBT Long-Term Debt [Text Block] Accretion of asset retirement obligations Asset Retirement Obligation, Accretion Expense Equity Securities without Readily Determinable Fair Value [Table] Equity Securities without Readily Determinable Fair Value [Table] Transmission credit Transmission Credit [Member] Transmission Credit Name Awards Close in Time to MNPI Disclosures, Individual Name Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Undiscounted future cash outflows of environmental reclamation obligations Accrual for Environmental Loss Contingencies, Gross General and administrative General and Administrative Expense Property, Plant and Equipment, Type [Domain] Long-Lived Asset, Class [Domain] Change in plant expenditures included in accounts payable Increase (Decrease) In Capital Expenditures Included In Accounts Payable Increase (decrease) in accounts payable attributable to capital expenditures incurred but not yet paid at end of period. Benefit payments (net of contributions by participants) Benefit payments Defined Benefit Plan, Benefit Obligation, Benefits Paid Weighted-average remaining lease term (in years) Weighted Average Remaining Lease Term [Abstract] Weighted Average Remaining Lease Term Regulatory liabilities Regulatory Liability Property, Plant, and Equipment [Table] Property, Plant, and Equipment [Table] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Total deferred credits and other liabilities Deferred Credits, Regulatory Liabilities, And Other Liabilities Sum of the carrying amounts as of the balance sheet date of deferred credits, regulatory liabilities, and other liabilities that are recognized at the end of the reporting period. Adjustment to Compensation: Adjustment to Compensation [Axis] Rifle Generating Station Rifle Generating Station [Member] Rifle Generating Station Other Total other deferred credits and other liabilities Other Deferred Credits And Other Liabilities Total carrying amount as of the balance sheet date of other deferred credits and other obligations not separately disclosed in the balance sheet. Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Cash paid for interest Interest Paid, Excluding Capitalized Interest, Operating Activity Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Public Utilities, Property, Plant and Equipment [Abstract] Revenue Subject to Refund Revenue Subject to Refund [Member] Credit Facility [Axis] Credit Facility [Axis] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount ACCOUNTING FOR RATE REGULATION Schedule of Regulatory Assets and Liabilities [Text Block] Rate stabilization Rate Stabilization [Member] Rate Stabilization Maximum amount per commercial paper sublimit Short-Term Debt, Maximum Amount Outstanding During Period Name Measure Name New Horizon Mine New Horizon Mine [Member] New Horizon Mine INCOME TAXES Income Tax Disclosure [Text Block] Accounts payable Accounts Payable, Current Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Debt Instrument [Line Items] Debt Instrument [Line Items] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Entity Filer Category Entity Filer Category Name Trading Arrangement, Individual Name Other Other Deferred Costs, Net Cash and cash equivalents Cash and Cash Equivalent Year 1 Lessee, Operating Lease, Liability, to be Paid, Next Rolling 12 Months Mountain Park Electric Mountain Park Electric, Inc. [Member] Mountain Park Electric, Inc. Financial and other support provided (as a percent) Variable Interest Entity, Financial Or Other Support, Percent The percentage of financial or other support the reporting entity has provided (explicitly or implicitly) to the Variable Interest Entity (VIE). Operating lease expense Operating Lease, Expense ASSET RETIREMENT AND ENVIRONMENTAL REMEDIATION OBLIGATIONS Asset Retirement Obligation And Accrual For Environmental Loss Contingencies [Text Block] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Equity interest (as a percent) Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Net regulatory asset Net Regulatory Assets Capital credits from cooperatives Capital Credits From Cooperatives Amount of cooperative earnings allocated to patrons' capital accounts. Company Selected Measure Amount Company Selected Measure Amount SHORT-TERM BORROWINGS Short-Term Debt [Text Block] Depreciation, amortization and depletion Depreciation, Depletion and Amortization Executive Benefit Restoration Plan Executive Benefit Restoration Plan [Member] Executive Benefit Restoration Plan Accumulated postretirement benefit and postemployment obligations Liability, Retirement and Postemployment Benefits Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Accounts and Financing Receivables [Table] Net margins including noncontrolling interest Net margins including noncontrolling interest Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Current assets Assets, Current [Abstract] Operating leases, weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term Asset Retirement Obligation and Accrual for Environmental Loss Contingencies [Roll Forward] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Roll Forward Analysis [Roll Forward] Asset Retirement Obligation And Accrual For Environmental Loss Contingencies, Roll Forward Analysis Non-PEO NEO Non-PEO NEO [Member] Entity Address, Address Line One Entity Address, Address Line One Discounted basis of environmental reclamation obligation Accrual For Environmental Loss Contingencies, Net Of Discount Accrual For Environmental Loss Contingencies, Net Of Discount Income tax expense Income Tax Expense (Benefit) Jointly Owned Utility Plant Interests [Table] Regulated Operation, Jointly Owned Utility Plant [Table] Director and Executive Elective Deferred Compensation Investments Director And Executive Elective Deferred Compensation Investments [Member] Director And Executive Elective Deferred Compensation Investments Adjustments to reconcile net margins to net cash provided by operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Noncash Item [Abstract] Customer Concentration Risk Customer Concentration Risk [Member] Holcomb Expansion Holcomb Station Project [Member] Holcomb Station Project Other assets and investments Regulated Entity, Other Assets, Noncurrent [Abstract] Award Timing Method Award Timing Method [Text Block] Basis of Consolidation Consolidation, Policy [Policy Text Block] Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Sale of non-utility assets Proceeds From Sale Of Non-Utility Assets Proceeds From Sale Of Non-Utility Assets Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Net margins attributable to the Association Net margins attributable to the Association Net Income (Loss) Attributable to Parent Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Less debt issuance costs Unamortized Debt Issuance Expense Proceeds from asset sales Proceeds from Sale of Property, Plant, and Equipment Statement [Table] Statement [Table] Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] Restatement Determination Date: Restatement Determination Date [Axis] Non-member electric sales Electric Sales Non Member [Member] Information pertaining to electric sales made to entities that are not members of the cooperative. Plus debt premiums Debt Instrument, Unamortized Premium Less: Accumulated amortization Accumulated Amortization, Operating Lease Right-Of-Use Asset Amount of accumulated amortization for lessee operating lease right-of-use asset. Interest Investment Income, Interest Public Utility, Property, Plant and Equipment [Line Items] Public Utility, Property, Plant and Equipment [Line Items] Deferred membership withdrawal Increase (Decrease) In Deferred Membership Withdrawal Increase (Decrease) In Deferred Membership Withdrawal Schedule of Contract Assets and Liabilities Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] Accounts receivable Increase (Decrease) in Receivables Utility Member advances Member Advances, Current Advances received from members of the cooperative for prepayments on billings. Deferred debt prepayment transaction costs Loss on Reacquired Debt [Member] Springerville Partnership Springerville Partnership [Member] Information pertaining to the Springerville Partnership. Depreciation, amortization and depletion Depreciation, Depletion And Amortization, Excluding Intangible Assets The aggregate expense recognized in the current period that allocates the cost of tangible assets or depleting assets to periods that benefit from use of the assets. Excludes amortization on intangible assets. Operating revenues Revenues [Abstract] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Derivative Instruments and Hedging Activities Disclosure [Abstract]