Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Trading Arrangement: Trading Arrangement [Axis] Regulated Electric and Natural Gas Segments Regulated Electric and Natural Gas Segments [Member] Regulated Electric and Natural Gas Segments Nebraska NEBRASKA Short-Term Debt, Type [Axis] Short-Term Debt, Type [Axis] Title of 12(b) Security Title of 12(b) Security Document Fiscal Year Focus Document Fiscal Year Focus Other, Lighting, Governmental, Irrigation, and Interdepartmental Other Customers [Member] Other class of external customer or group of external customers. Adjustment to Compensation, Amount Adjustment to Compensation Amount Customer [Axis] Customer [Axis] Segment Reporting [Abstract] Segment Reporting [Abstract] Common Stock, Value, Issued Common Stock, Value, Issued Montana MONTANA Commercial Commercial Customers [Member] Commercial class of external customer or group of external customers. Pension and Other Postretirement Benefits Disclosure [Text Block] Retirement Benefits [Text Block] Restatement Determination Date Restatement Determination Date Other Postretirement Benefits [Member] Other Pension, Postretirement and Supplemental Plans [Member] Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Northwestern Energy Group Inc. Northwestern Energy Group Inc. [Member] Northwestern Energy Group Inc. Noncurrent regulatory liabilities Regulatory Liability, Noncurrent Amendment Flag Amendment Flag Current Liabilities: Liabilities, Current [Abstract] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 Acquired from Puget [Member] Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 Acquired from Puget and Avista [Member] Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 Acquired from Puget and Avista Other comprehensive income (loss), net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Debt Instrument, Maturity Date Debt Instrument, Maturity Date 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 Utilities Disclosure [Text Block] Public Utilities Disclosure [Text Block] Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Portion Attributable to Parent PEO Total Compensation Amount PEO Total Compensation Amount Foreign currency translation adjustment Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Common Stock, Par or Stated Value Per Share Common Stock, Par or Stated Value Per Share Asset Acquisition [Axis] Asset Acquisition [Axis] Average Common Shares Outstanding Basic computation Weighted Average Number of Shares Outstanding, Basic Retirement Benefits [Abstract] Retirement Benefits [Abstract] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Regulatory assets Regulatory Asset, Noncurrent Number of Reportable Segments Number of Reportable Segments Total current liabilities  Liabilities, Current Regulatory Liability [Axis] Regulatory Liability [Axis] Gas Regulated Operating Revenue, Gas Short-Term Debt, Type [Domain] Short-Term Debt, Type [Domain] Revenue from contract with customer Revenue from Contract with Customer, Including Assessed Tax Other comprehensive income, net of tax: Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Entity Address, City or Town Entity Address, City or Town Asset Acquisition, Consideration Transferred Asset Acquisition, Consideration Transferred Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Document [Domain] Document [Domain] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Document Information [Line Items] Document Information [Line Items] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Class of Stock [Axis] Class of Stock [Axis] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Total Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Current portion of long-term debt Long-Term Debt, Current Maturities Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Merger transaction costs Merger transaction costs Merger transaction costs Schedule of Long-Term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Treasury stock at cost Treasury Stock, Value Capital expenditures included in accounts payable Capital Expenditures Incurred but Not yet Paid Segment Reporting [Table Text Block] Segment Reporting [Table Text Block] Other Comprehensive Income (Loss), before Tax [Abstract] Other Comprehensive Income (Loss), before Tax [Abstract] Other Comprehensive Income (Loss), before Tax Other Comprehensive Income (Loss), before Tax Electric Regulated Operating Revenue, Electric, Non-Nuclear Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Document Period End Date Document Period End Date Changes in current assets and liabilities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] South Dakota SOUTH DAKOTA Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment and Tax Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax 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] Preferred Stock, Shares Outstanding Preferred Stock, Shares Outstanding Debt Instrument, Face Amount Debt Instrument, Face Amount Term Loan Due 2026 Term Loan Due 2026 [Member] Term Loan Due 2026 Operating expenses Costs and Expenses [Abstract] Security Exchange Name Security Exchange Name Term Loan Due 2026-Effective April 11, 2025 Term Loan Due 2026-Effective April 11, 2025 [Member] Term Loan Due 2026-Effective April 11, 2025 Transmission customers Transmission customers [Member] Transmission customers Schedule Of Regulated Operations [Table] Schedule Of Regulated Operations [Table] Schedule Of Regulated Operations [Table] Regulatory amortization Other Energy Sales Revenue Alternative revenue and other. Accumulated other comprehensive loss Accumulated other comprehensive loss Beginning balance Ending balance Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Payment for Pension Benefits Payment for Pension Benefits Comprehensive Income (Loss) Statement of Comprehensive Income [Abstract] Earnings Per Share [Text Block] Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax Earnings Per Share [Abstract] Earnings Per Share [Abstract] Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Schedule of Weighted Average Number of Shares [Table Text Block] Schedule of Weighted Average Number of Shares [Table Text Block] Document Fiscal Period Focus Document Fiscal Period Focus Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Total Liabilities  Liabilities Trading Symbol Trading Symbol Geographical [Domain] Geographical [Domain] Property, plant, and equipment additions 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] Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Company Selected Measure Name Company Selected Measure Name Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) Utility Operating Expense, Fuel, Purchased Supply, and Direct Transmission Expense Utility Operating Expense, Fuel, Purchased Supply, and Direct Transmission Expense (Exclusive of depreciation and depletion) NW Corp Term Loan Due 2027 NW Corp Term Loan Due 2027 [Member] NW Corp Term Loan Due 2027 Equity portion of allowance for funds used during construction Increase (Decrease) in Allowance for Equity Funds Used During Construction Financing Activities Financing Activities [Text Block] Schedule of financing activities for an entity. Information pertaining to short-term and long-term debt instruments or arrangements and equity distribution agreements, including but not limited to identification of terms, features, collateral requirements and other information necessary to a fair presentation. Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Balance, shares Balance, shares Shares, Issued Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Diluted computation Weighted Average Number of Shares Outstanding, Diluted 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] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Regulatory Asset [Axis] Regulatory Asset [Axis] Short-Term Debt Short-Term Debt Other noncurrent assets and liabilities Increase (Decrease) in Other Noncurrent Assets Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Production Tax Credits Cash Transfers Production Tax Credits Cash Transfers Production Tax Credits Cash Transfers Increase (Decrease) in Other Operating Assets and Liabilities, Net Increase (Decrease) in Other Operating Assets and Liabilities, Net Award Timing Predetermined Award Timing Predetermined [Flag] Treasury Stock, Common Treasury Stock, Common [Member] Comprehensive Income (Loss) [Table Text Block] Comprehensive Income (Loss) [Table Text Block] Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Effective Income Tax Rate Reconciliation, Percent Effective Income Tax Rate Reconciliation, Percent Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] South Dakota First Mortgage Bonds Due 2036 South Dakota First Mortgage Bonds Due 2036 [Member] South Dakota First Mortgage Bonds Due 2036 Performance and restricted share awards(1) Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements All Executive Categories All Executive Categories [Member] Regulated Operations [Abstract] Regulated Operations [Abstract] Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Amortization of debt issuance costs, premium, and deferred hedge gain Amortization of debt issue costs discount and deferred hedge gain Amortization of debt issue costs discount and deferred hedge gain Preferred Stock, Par or Stated Value Per Share Preferred Stock, Par or Stated Value Per Share Schedule of Defined Benefit Plans Disclosures [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Interest expense, net Interest expense, net Interest and Debt Expense Industrial Industrial Customers [Member] Industrial class of external customer or group of external customers. Asset Acquisition [Domain] Asset Acquisition [Domain] Scenario [Domain] Scenario [Domain] Issuance of shares, treasury stock Stock Issued During Period, Shares, Treasury Stock Reissued Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Postretirement medical plans Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax Segment Reporting [Text Block] Segment Reporting [Text Block] Dividends on common stock Payments of Ordinary Dividends, Common Stock Expiration Date Trading Arrangement Expiration Date Disaggregation of Revenue [Table Text Block] Disaggregation of Revenue [Table Text Block] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] 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] Regulated Electric Segment Regulated Electric Segment [Member] Regulated Electric Segment Business Acquisition Share Conversion Ratio Merger Exchange Ratio Merger Exchange Ratio Business Combination [Line Items] Business Combination [Line Items] Dividends on common stock Dividends, Common Stock Net periodic benefit cost (credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Regulated Natural Gas Regulated Natural Gas [Member] Regulated Natural Gas Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Common Stock, Shares Authorized Common Stock, Shares Authorized Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Exercise Price Award Exercise Price Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Segment Reporting [Line Items] Segment Reporting [Line Items] Document Quarterly Report Document Quarterly Report 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] Segments [Domain] Segments [Domain] Regulatory liabilities Increase (Decrease) in Regulatory Liabilities Repayments on long-term debt Repayments of Long-Term Debt Scenario [Axis] Scenario [Axis] Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Business Combination [Table] Business Combination [Table] Equity Component [Domain] Equity Component [Domain] PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Colstrip Acquisition and Requests for Cost Recovery [Line Items] Colstrip Acquisition and Requests for Cost Recovery [Line Items] Colstrip Acquisition and Requests for Cost Recovery [Line Items] Business Combination, Asset Acquisition, Transaction between Entities under Common Control, and Joint Venture Formation [Abstract] Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista Corporation [Member] Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Puget and Avista [Member] Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Puget and Avista Total Operating Expenses Costs and Expenses Adoption Date Trading Arrangement Adoption Date Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Total Shareholders' Equity  Balance, beginning of period Balance, end of period Equity, Attributable to Parent 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] Insider Trading Policies and Procedures [Line Items] Other financing activities, net Proceeds from (Payment for) Other Financing Activity Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Deferred income taxes Deferred Income Tax Expense (Benefit) Regulatory assets Regulatory Asset, Current Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Disclosure [Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Entity Central Index Key Entity Central Index Key Components of Net Periodic Benefit Cost (Income) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] All Adjustments to Compensation All Adjustments to Compensation [Member] Equity [Abstract] Equity [Abstract] 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] Cash Provided by Operating Activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Other comprehensive loss before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Income Statement [Abstract] Income Statement [Abstract] Document Transition Report Document Transition Report Long-term finance leases Finance Lease, Liability, Noncurrent Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Total Customer Revenue Total customer revenue [Member] Total Customer Revenue [Member] City Area Code City Area Code Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Document Information [Table] Document Information [Table] All Individuals All Individuals [Member] Name Forgone Recovery, Individual Name Regulatory Liability [Domain] Regulatory Liability [Domain] Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Statement [Line Items] Statement [Line Items] Total Assets  Assets Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plan [Table] Entity [Domain] Entity [Domain] Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Effective Income Tax Rate Reconciliation [Line Items] Effective Income Tax Rate Reconciliation [Line Items] Preferred Stock, Shares Issued Preferred Stock, Shares Issued Cash, Cash Equivalents, and Restricted Cash [Abstract] Cash, Cash Equivalents, and Restricted Cash [Abstract] Cash, Cash Equivalents, and Restricted Cash [Abstract] Operating Segments Operating Segments [Member] Long-term Purchase Commitment, Category of Item Purchased [Domain] Long-Term Purchase Commitment, Category of Item Purchased [Domain] Accumulated Other Comprehensive Income [Table Text Block] Accumulated Other Comprehensive Income [Table Text Block] [Table Text Block] for Accumulated Other Comprehensive Income [Table] Issuance of shares Stock Issued During Period, Shares, New Issues PEO PEO [Member] Other noncurrent liabilities Other Liabilities, Noncurrent Goodwill Goodwill Retirement Plan Type [Axis] Retirement Plan Type [Axis] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Retained Earnings [Member] Retained Earnings [Member] Restatement [Domain] Revision of Prior Period [Domain] Operations And Basis Of Consolidation [Table] Operations And Basis Of Consolidation [Table] Operations And Basis Of Consolidation [Table] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Common Stock, Shares, Outstanding Common Stock, Shares, Outstanding Award Timing Disclosures [Line Items] Diluted Earnings per Average Common Share Earnings Per Share, Diluted Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Entity Current Reporting Status Entity Current Reporting Status Cash Provided by Financing Activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Foreign currency translation Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Operating, general, and administrative Utilities Operating Expense, Maintenance and Operations Adjustments to reconcile net income to cash provided by operations: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Property, plant, and equipment, net Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization Interest cost Defined Benefit Plan, Interest Cost Other Comprehensive Income (Loss), Tax [Abstract] Other Comprehensive Income (Loss), Tax [Abstract] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Underlying Securities Award Underlying Securities Amount Revenue from Contract with Customer [Text Block] Revenue from Contract with Customer [Text Block] Issuance of shares, value Stock Issued During Period, Value, New Issues Minimum Expected Future Employer Pension Contributions, Remainder of Fiscal Year Minimum Expected Future Employer Pension Contributions, Remainder of Fiscal Year Minimum Expected Future Employer Pension Contributions, Remainder of Fiscal Year Inventories Increase (Decrease) in Inventories Segment Reporting [Table] Segment Reporting [Table] Prepaid expenses and other Other Assets, Current 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] Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax Operating income Operating Income (Loss) Current Fiscal Year End Date Current Fiscal Year End Date Long-term Debt, Maturity Date Long-Term Debt, Maturity Date Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Northwestern Corporation Northwestern Corporation [Member] Northwestern Corporation Average Shares Used In Computing Basic And Diluted Earnings Per Share [Table] Average Shares Used In Computing Basic And Diluted Earnings Per Share [Table] Average Shares Used In Computing Basic And Diluted Earnings Per Share [Table] Business Combination Business Combination [Text Block] 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] Dividends Declared per Common Share Dividends per share Common Stock, Dividends, Per Share, Declared Entity File Number Entity File Number Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Service cost Defined Benefit Plan, Service Cost Stock based compensation, value Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Total Revenues Total Revenue [Member] Total revenue by major source and customer class. Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista [Member] Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Residential Residential Customers [Member] Residential class of external customer or group of external customers. Common Stock, Shares, Issued Common Stock, Shares, Issued Measure: Measure [Axis] Treasury Stock, Common, Shares Treasury Stock, Common, Shares Asset Retirement Obligations, Noncurrent Asset Retirement Obligations, Noncurrent Comprehensive Income (Loss) Note [Text Block] Comprehensive Income (Loss) Note [Text Block] Equity Components [Axis] Equity Components [Axis] Accounts receivable Increase (Decrease) in Accounts Receivable Product and Service [Domain] Product and Service [Domain] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Entity Interactive Data Current Entity Interactive Data Current Current maturities of finance leases Finance Lease, Liability, Current Deferred income taxes Deferred Income Tax Liabilities, Net Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Jointly Owned Utility Plant, Proportionate Ownership Share Jointly Owned Utility Plant, Proportionate Ownership Share Entity Small Business Entity Small Business Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Termination Date Trading Arrangement Termination Date Effective Income Tax Rate Reconciliation [Table] Effective Income Tax Rate Reconciliation [Table] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Other noncurrent assets Other Assets, Noncurrent Document Type Document Type Debt Instrument, Repurchased Face Amount Debt Instrument, Repurchased Face Amount Lease Expiration Date Lease Expiration Date Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax Accrued expenses and other Accrued Liabilities, Current Fair Value as of Grant Date Award Grant Date Fair Value Other Tariff and Contract Based Revenue Other Tariff Based Revenue [Member] Other Tariff Based Revenue Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax Arrangement Duration Trading Arrangement Duration Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Issuance of long-term debt Proceeds from Issuance of Long-Term Debt Local Phone Number Local Phone Number Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Colstrip Acquisition and Requests for Cost Recovery [Table] Colstrip Acquisition and Requests for Cost Recovery [Table] Colstrip Acquisition and Requests for Cost Recovery Income before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Commitments and Contingencies (Note 11) Commitments and Contingencies Total Revenues Operating revenues Total Revenues Revenues Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Executive Category: Executive Category [Axis] Other Comprehensive Income (Loss), Tax Other Comprehensive Income (Loss), Tax LIABILITIES AND SHAREHOLDERS' EQUITY Liabilities and Equity [Abstract] South Dakota First Mortgage Bonds Due 2026 South Dakota First Mortgage Bonds Due 2026 [Member] South Dakota First Mortgage Bonds Due 2026 Inventories Inventory, Net All Award Types Award Type [Domain] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Supplemental Cash Flow Information [Abstract] Supplemental Cash Flow Information [Abstract] Amounts reclassified from AOCL Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax PEO Name PEO Name Document Information, Document [Axis] Document Information, Document [Axis] Geographical [Axis] Geographical [Axis] Accrued expenses and other Increase (Decrease) in Accrued Liabilities Restricted cash Restricted Cash Short-Term Debt [Table] Short-Term Debt [Table] 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] Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Beginning Balance Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Ending Balance Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Stock Issued During Period, Value, Treasury Stock Reissued Stock Issued During Period, Value, Treasury Stock Reissued Accounts payable Increase (Decrease) in Accounts Payable 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] Public Utilities, Property, Plant and Equipment, Amount of Disallowed Costs for Recently Completed Plant Public Utilities, Property, Plant and Equipment, Amount of Disallowed Costs for Recently Completed Plant Organization, Consolidation and Presentation of Financial Statements Disclosure Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Debt Instrument [Axis] Debt Instrument [Axis] Stock-based compensation costs Share-Based Payment Arrangement, Noncash Expense Property and other taxes Utilities Operating Expense, Taxes Customer [Domain] Customer [Domain] Basic Earnings per Average Common Share Earnings Per Share, Basic Total current assets  Assets, Current Insider Trading Arrangements [Line Items] Long-term debt Long-Term Debt, Excluding Current Maturities Award Type Award Type [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Accounts receivable, net Accounts Receivable, after Allowance for Credit Loss, Current Retained earnings Retained Earnings (Accumulated Deficit) Product and Service [Axis] Product and Service [Axis] 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] Pension Benefits [Member] Pension Plan [Member] Short-Term Debt [Line Items] Short-Term Debt [Line Items] Common Stock [Member] Common Stock [Member] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover [Abstract] Cover [Abstract] Schedule of Regulated Operations [Line Items] Schedule of Regulated Operations [Line Items] Schedule of Regulated Operations [Line Items] Equity Awards Adjustments Equity Awards Adjustments [Member] Financing Activities [Abstract] Financing Activities [Abstract] Financing Activities Total Liabilities and Shareholders' Equity Liabilities and Equity Regulatory Asset [Domain] Regulatory Asset [Domain] Employee Stock Option Share-Based Payment Arrangement, Option [Member] Asset Retirement Obligation, Current Asset Retirement Obligation, Current Statement, Business Segments [Axis] Segments [Axis] Segments [Axis] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Tabular List, Table Tabular List [Table Text Block] Paid-in capital Additional Paid in Capital Other income, net Other income, net Other Nonoperating Income (Expense) Name Awards Close in Time to MNPI Disclosures, Individual Name Shareholders' Equity: Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Investment in debt & equity securities Payments to Acquire Investments 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] Operating, general, and administrative Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses Administrative and general General and Administrative Expense Preferred Stock, Shares Authorized Preferred Stock, Shares Authorized Gas Domestic Regulated [Member] Gas Domestic Regulated [Member] Gas Domestic Regulated [Member] AOCI, Derivative Qualifying as Hedge, Excluded Component, after Tax AOCI, Derivative Qualifying as Hedge, Excluded Component, after Tax Regulatory Liability Regulatory Liability Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Adjustment to Compensation: Adjustment to Compensation [Axis] Class of Stock [Domain] Class of Stock [Domain] Cash Paid [Abstract] Cash Paid [Abstract] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Interest Paid, Excluding Capitalized Interest, Operating Activity Interest Paid, Excluding Capitalized Interest, Operating Activity Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Revenue Subject to Refund Revenue Subject to Refund [Member] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Name Measure Name Income Tax Disclosure [Text Block] Income Tax Disclosure [Text Block] Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax Accounts payable Accounts Payable, Current Pay vs Performance Disclosure [Line Items] Other Operating Segment Other Operating Segment [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Other current assets Increase (Decrease) in Other Current Assets Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 [Member] Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 [Member] Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 Number of customers Number Of Customers Number Of Customers Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, before Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, before Tax Retirement Plan Type [Domain] Retirement Plan Type [Domain] Additional Paid-in Capital [Member] Additional Paid-in Capital [Member] Entity Filer Category Entity Filer Category Repayment of short-term borrowings Repayments of Short-Term Debt Restatement [Axis] Revision of Prior Period [Axis] Name Trading Arrangement, Individual Name Cash and cash equivalents Cash and cash equivalents Cash and Cash Equivalent Finance Lease, Liability, to be Paid, Fiscal Year Maturity [Abstract] Finance Lease, Liability, to be Paid, Fiscal Year Maturity [Abstract] Company Selected Measure Amount Company Selected Measure Amount Depreciation and depletion Depreciation and depletion Depreciation, Depletion and Amortization Stock based compensation, shares Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture Asset Acquisition, Indemnification Asset, Amount Asset Acquisition, Indemnification Asset, Amount Electric Domestic Regulated [Member] Electric Domestic Regulated [Member] Electric Domestic Regulated [Member] Current Assets: Assets, Current [Abstract] Non-PEO NEO Non-PEO NEO [Member] Entity Address, Address Line One Entity Address, Address Line One CONDENSED CONSOLIDATED STATEMENTS OF OTHER COMPREHENSIVE INCOME [Abstract] CONDENSED CONSOLIDATED STATEMENTS OF OTHER COMPREHENSIVE INCOME [Abstract] CONDENSED CONSOLIDATED STATEMENTS OF OTHER COMPREHENSIVE INCOME [Abstract] Income tax expense Income Tax Expense (Benefit) Income Tax Expense (Benefit) Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage Regulatory assets Increase (Decrease) in Other Regulatory Assets Award Timing Method Award Timing Method [Text Block] Decrease in Cash, Cash Equivalents, and Restricted Cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Net Income Net Income Net Income (Loss) Attributable to Parent Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Location, Statement of Financial Position, Balance [Axis] Location, Statement of Financial Position, Balance [Axis] Statement [Table] Statement [Table] Restatement Determination Date: Restatement Determination Date [Axis] Regulatory liabilities Regulatory Liability, Current Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Recognized actuarial gain Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Regulatory amortization Regulatory Amortization [Member] Regulatory Amortization Line of credit repayments, net Proceeds from (Repayments of) Lines of Credit Revenues Revenues [Abstract] Category of Item Purchased [Axis] Category of Item Purchased [Axis] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block]