REGULATORY MATTERS - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	$ 4.8	$ 1.6
Total Non-Current Regulatory Assets	363.0	371.7
Total Current Regulatory Liabilities	3.3	31.5
Total Non-Current Regulatory Liabilities	586.7	570.5
Provision for Interim Rate Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	23.0
Fuel Adjustment Clause
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	2.2	7.2
Total Non-Current Regulatory Liabilities	5.7	0.6
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	1.1	1.3
Total Non-Current Regulatory Liabilities	4.1	0.5
Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	283.2	292.7
Wholesale and Retail Contra AFUDC
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	79.4	77.6
Plant Removal Obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	78.2	74.2
Non-Jurisdictional Land Sales
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	74.8	59.7
Defined Benefit Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	37.2	40.7
Investment Tax Credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	17.4	17.8
Boswell Units 1 and 2 Net Plant and Equipment
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	6.7	6.7
Fuel Adjustment Clause
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	3.2	0.0
Total Non-Current Regulatory Assets	0.7	4.5
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	1.6	1.6
Total Non-Current Regulatory Assets	1.8	2.9
Defined Benefit Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	197.2	200.3
Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	81.4	84.9
Asset Retirement Obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	43.1	41.0
Taconite Harbor
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	10.5	17.3
Manufactured Gas Plant
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	10.8	11.6
Cost Recovery Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	13.9	5.5
PPACA Income Tax Deferral
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	$ 3.6	$ 3.7