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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets $ 1.6 $ 9.3
Total Non-Current Regulatory Assets 371.7 425.4
Total Current Regulatory Liabilities 31.5 3.9
Total Non-Current Regulatory Liabilities 570.5 574.0
Provision for Interim Rate Refund    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 23.0 0.0
Fuel Adjustment Clause    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 7.2 0.0
Total Non-Current Regulatory Liabilities 0.6 15.5
Other    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 1.3 3.9
Total Non-Current Regulatory Liabilities 0.5 4.4
Income Taxes    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 292.7 310.0
Wholesale and Retail Contra AFUDC    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 77.6 78.0
Plant Removal Obligations    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 74.2 67.0
Defined Benefit Pension and Other Postretirement Benefit Plans    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 40.7 48.6
Non-Jurisdictional Land Sales    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 59.7 30.2
Investment Tax Credits    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Liabilities 17.8 13.6
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 0.0 8.7
Total Non-Current Regulatory Assets 4.5 5.0
Other    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets 1.6 0.6
Total Non-Current Regulatory Assets 2.9 4.2
Defined Benefit Pension and Other Postretirement Benefit Plans    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 200.3 218.6
Income Taxes    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 84.9 88.1
Asset Retirement Obligations    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 41.0 37.7
Taconite Harbor    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 17.3 20.9
Manufactured Gas Plant    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 11.6 13.2
Cost Recovery Riders    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets 5.5 33.8
PPACA Income Tax Deferral    
Regulatory Assets and Liabilities [Line Items]    
Total Non-Current Regulatory Assets $ 3.7 $ 3.9