REGULATORY MATTERS - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	$ 6.0	$ 9.3
Total Non-Current Regulatory Assets	394.7	425.4
Total Current Regulatory Liabilities	25.3	3.9
Total Non-Current Regulatory Liabilities	561.0	574.0
Income Taxes
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	297.3	310.0
Wholesale and Retail Contra AFUDC
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	76.6	78.0
Plant Removal Obligations
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	71.3	67.0
Non-Jurisdictional Land Sales
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	43.2	30.2
Defined Benefit Pension and Other Postretirement Benefit Plans
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	42.3	48.6
Investment Tax Credits
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	13.4	13.6
Fuel Adjustment Clause
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	12.9	0.0
Total Non-Current Regulatory Liabilities	3.4	15.5
Provision for Interim Rate Refund
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	11.0	0.0
Transmission Formula Rates Refund
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	0.4	1.5
Boswell Units 1 and 2 Net Plant and Equipment
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	6.7	6.7
Other
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	1.0	2.4
Total Non-Current Regulatory Liabilities	6.8	4.4
Defined Benefit Pension and Other Postretirement Benefit Plans
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	215.9	218.6
Income Taxes
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	83.3	88.1
Asset Retirement Obligations
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	38.9	37.7
Cost Recovery Riders
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	19.8	33.8
Taconite Harbor
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	15.8	20.9
Manufactured Gas Plant
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	13.1	13.2
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	5.4	8.7
Total Non-Current Regulatory Assets	0.7	5.0
Other
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	0.6	0.6
Total Non-Current Regulatory Assets	$ 3.4	$ 4.2