REGULATORY MATTERS - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	$ 0.6	$ 9.3
Total Non-Current Regulatory Assets	386.1	425.4
Total Current Regulatory Liabilities	27.8	3.9
Total Non-Current Regulatory Liabilities	562.1	574.0
Income Taxes
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	292.8	310.0
Wholesale and Retail Contra AFUDC
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	76.2	78.0
Plant Removal Obligations
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	72.8	67.0
Non-Jurisdictional Land Sales
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	55.0	30.2
Defined Benefit Pension and Other Postretirement Benefit Plans
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	39.2	48.6
Investment Tax Credits
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	13.2	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
Provision for Interim Rate Refund
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	17.0	0.0
Fuel Adjustment Clause
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	10.0	0.0
Total Non-Current Regulatory Liabilities	0.6	15.5
Transmission Formula Rates Refund
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	1.5
Other
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	0.8	2.4
Total Non-Current Regulatory Liabilities	5.6	4.4
Defined Benefit Pension and Other Postretirement Benefit Plans
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	214.5	218.6
Income Taxes
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	81.7	88.1
Asset Retirement Obligations
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	39.9	37.7
Taconite Harbor
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	20.6	20.9
Manufactured Gas Plant
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	12.9	13.2
Cost Recovery Riders
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	8.6	33.8
PPACA Income Tax Deferral
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	3.8	3.9
Fuel Adjustment Clause
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	0.0	8.7
Total Non-Current Regulatory Assets	1.5	5.0
Other
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	0.6	0.6
Total Non-Current Regulatory Assets	$ 2.6	$ 4.2