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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2022
Dec. 31, 2021
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets Currently Earning a Return With the exception of the regulatory asset for Boswell Units 1 and 2 net plant and equipment, no other regulatory assets are currently earning a return.  
Current Regulatory Assets [1] $ 39.8 $ 0.0
Non-Current Regulatory Assets 464.2 511.8
Regulatory Assets 504.0 511.8
Current Regulatory Liabilities [2] 7.7 8.6
Non-Current Regulatory Liabilities 533.4 536.1
Total Regulatory Liabilities 541.1 544.7
Transmission Formula Rates [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [2] 2.9 3.1
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 344.8 353.4
Wholesale and Retail Contra AFUDC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 83.0 83.7
Plant Removal Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 54.2 52.6
Pension and Other Postretirement Plan Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 26.5 28.1
North Dakota Investment Tax Credits [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 13.7 12.2
Other (Member)    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [2] 0.7 0.5
Non-Current Regulatory Liabilities 9.6 5.7
Boswell Units 1 and 2 Net Plant and Equipment    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 1.6 0.4
Fuel Adjustment Clause [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [2] 4.1 5.0
Pension and Other Postretirement Plan Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 223.2 226.4
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 101.5 104.7
Cost Recovery Riders [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 60.6 63.2
Asset Retirement Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 33.5 33.1
Manufactured Gas Plant [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 17.4 17.0
PPACA Income Tax Deferral [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 4.3 4.3
Other (Member)    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 3.2 6.7
Fuel Adjustment Clause [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [1] 39.8 0.0
Fuel Adjustment Clause (Member)    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 18.5 56.4
Residential Customer Interim Rate Adjustment    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets $ 2.0 $ 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.