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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2023
Dec. 31, 2022
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [1] $ 15.3 $ 25.6
Non-Current Regulatory Assets 469.1 441.0
Current Regulatory Liabilities [2] 30.3 23.4
Non-Current Regulatory Liabilities 526.4 526.1
Transmission Formula Rates [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [2] 3.7 4.9
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 326.2 332.5
Wholesale and Retail Contra AFUDC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 80.0 80.7
Plant Removal Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 61.6 60.0
Pension and Other Postretirement Plan Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 15.7 17.6
North Dakota Investment Tax Credits [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 16.8 16.9
Other (Member)    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [2] 3.1 0.1
Non-Current Regulatory Liabilities 4.9 4.2
Boswell Units 1 and 2 Net Plant and Equipment    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 6.7 6.7
Fuel Adjustment Clause [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 5.3 0.0
Non-Jurisdictional Land Sales    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 9.2 7.5
Public Utilities Reserve for Interim Rates    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [2],[3] 23.5 18.4
Pension and Other Postretirement Plan Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 225.2 225.9
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 95.3 97.6
Cost Recovery Riders [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 42.2 41.2
Asset Retirement Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 36.1 35.6
Manufactured Gas Plant [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 14.5 15.1
PPACA Income Tax Deferral [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 4.1 4.1
Other (Member)    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 6.5 7.0
Fuel Adjustment Clause (Member)    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [1] 15.3 25.6
Non-Current Regulatory Assets 15.5 14.5
Taconite Harbor Energy Center    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets $ 29.7 [4] $ 0.0
[1] Current regulatory assets are presented within Prepayments and Other on the Consolidated Balance Sheet.
[2] Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet.
[3] See 2022 Minnesota General Rate Case.
[4] In the first quarter of 2023, Minnesota Power retired Taconite Harbor Units 1 and 2. The remaining net book value was reclassified from property, plant and equipment to a regulatory asset on the Consolidated Balance Sheet when the units were retired. Minnesota Power expects to receive recovery of the remaining net book value from customers.