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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets $ 13.9 $ 25.6
Total Non-Current Regulatory Assets 443.3 441.0
Total Current Regulatory Liabilities 41.9 23.4
Total Non-Current Regulatory Liabilities 549.3 526.1
Provision for Interim Rate Refund    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 39.4 18.4
Transmission Formula Rates Refund    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 1.2 4.9
Other    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 1.3 0.1
Total Non-Current Regulatory Liabilities 5.2 4.2
Income Taxes    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 315.7 332.5
Wholesale and Retail Contra AFUDC    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 78.6 80.7
Plant Removal Obligations    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 65.9 60.0
Fuel Adjustment Clause    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 28.3 0.0
Non-Jurisdictional Land Sales    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 20.7 7.5
North Dakota Investment Tax Credits    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 16.4 16.9
Defined Benefit Pension and Other Postretirement Benefit Plans    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 11.8 17.6
Boswell Units 1 and 2 Net Plant and Equipment    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 6.7 6.7
Fuel Adjustment Clause    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets 12.1 25.6
Total Non-Current Regulatory Assets 4.1 14.5
Other    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets 1.8 0.0
Total Non-Current Regulatory Assets 4.0 7.0
Defined Benefit Pension and Other Postretirement Benefit Plans    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 223.6 225.9
Income Taxes    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 91.4 97.6
Cost Recovery Riders    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 40.8 41.2
Asset Retirement Obligations    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 37.1 35.6
Taconite Harbor    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 24.2 0.0
Manufactured Gas Plant    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 14.1 15.1
PPACA Income Tax Deferral    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets $ 4.0 $ 4.1