Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2023		Dec. 31, 2022
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[1]	$ 15.7		$ 25.6
Non-Current Regulatory Assets		454.5		441.0
Current Regulatory Liabilities	[2]	36.4		23.4
Non-Current Regulatory Liabilities		538.0		526.1
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2]	2.5		4.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		320.7		332.5
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		79.3		80.7
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		63.7		60.0
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		13.8		17.6
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		16.6		16.9
Other (Member)
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2]	2.1		0.1
Non-Current Regulatory Liabilities		6.7		4.2
Boswell Units 1 and 2 Net Plant and Equipment
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		6.7		6.7
Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		17.8		0.0
Non-Jurisdictional Land Sales
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.7		7.5
Public Utilities Reserve for Interim Rates
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2],[3]	31.8		18.4
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		223.6		225.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		93.3		97.6
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		42.4		41.2
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		36.6		35.6
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		14.3		15.1
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.0		4.1
Other (Member)
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.8		7.0
Fuel Adjustment Clause (Member)
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[1]	15.7		25.6
Non-Current Regulatory Assets		5.4		14.5
Taconite Harbor Energy Center
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 27.1	[4]	$ 0.0

[1] Current regulatory assets are presented within Prepayments and Other on the Consolidated Balance Sheet.

[2] Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet.

[3] See 2022 Minnesota General Rate Case.

[4] In the first quarter of 2023, Minnesota Power retired Taconite Harbor Units 1 and 2. The remaining net book value was reclassified from property, plant and equipment to a regulatory asset on the Consolidated Balance Sheet when the units were retired. Minnesota Power expects to receive recovery of the remaining net book value from customers.