Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	$ 9.3	$ 25.6
Total Non-Current Regulatory Assets	425.4	441.0
Total Current Regulatory Liabilities	3.9	23.4
Total Non-Current Regulatory Liabilities	574.0	526.1
Provision for Interim Rate Refund
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	18.4
Transmission Formula Rates Refund
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	1.5	4.9
Other
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	2.4	0.1
Total Non-Current Regulatory Liabilities	4.4	4.2
Income Taxes
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	310.0	332.5
Wholesale and Retail Contra AFUDC
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	78.0	80.7
Plant Removal Obligations
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	67.0	60.0
Defined Benefit Pension and Other Postretirement Benefit Plans
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	48.6	17.6
Non-Jurisdictional Land Sales
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	30.2	7.5
Fuel Adjustment Clause
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	15.5	0.0
Investment Tax Credits
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	13.6	16.9
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	8.7	25.6
Total Non-Current Regulatory Assets	5.0	14.5
Other
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	0.6	0.0
Total Non-Current Regulatory Assets	4.2	7.0
Defined Benefit Pension and Other Postretirement Benefit Plans
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	218.6	225.9
Income Taxes
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	88.1	97.6
Asset Retirement Obligations
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	37.7	35.6
Cost Recovery Riders
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	33.8	41.2
Taconite Harbor
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	20.9	0.0
Manufactured Gas Plant
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	13.2	15.1
PPACA Income Tax Deferral
Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	$ 3.9	$ 4.1