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REGULATORY MATTERS - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Schedule of Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets $ 3.9 $ 1.6
Regulatory Assets 364.6 371.7
Total Current Regulatory Liabilities 34.2 31.5
Regulatory Liabilities 580.6 570.5
Provision for Interim Rate Refund    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 27.9 23.0
Fuel Adjustment Clause    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 4.6 7.2
Regulatory Liabilities 6.8 0.6
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Liabilities 1.7 1.3
Regulatory Liabilities 2.3 0.5
Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 287.1 292.7
Wholesale and Retail Contra AFUDC    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 77.9 77.6
Plant Removal Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 75.5 74.2
Non-Jurisdictional Land Sales    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 67.7 59.7
Defined Benefit Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 39.0 40.7
Investment Tax Credits    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 17.6 17.8
Boswell Units 1 and 2 Net Plant and Equipment    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liabilities 6.7 6.7
Fuel Adjustment Clause    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets 2.3 0.0
Regulatory Assets 1.6 4.5
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets 1.6 1.6
Regulatory Assets 1.8 2.9
Defined Benefit Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets 198.7 200.3
Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets 83.0 84.9
Asset Retirement Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets 42.0 41.0
Taconite Harbor    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets 13.9 17.3
Manufactured Gas Plant    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets 11.2 11.6
Cost Recovery Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets 8.7 5.5
PPACA Income Tax Deferral    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets $ 3.7 $ 3.7