Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2022	Dec. 31, 2021
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[1]	$ 38.7	$ 0.0
Non-Current Regulatory Assets		447.1	511.8
Regulatory Assets		485.8	511.8
Current Regulatory Liabilities	[2]	5.2	8.6
Non-Current Regulatory Liabilities		527.4	536.1
Total Regulatory Liabilities		532.6	544.7
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2]	5.1	3.1
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		338.0	353.4
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		81.5	83.7
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		58.8	52.6
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		23.5	28.1
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		13.4	12.2
Other (Member)
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2]	0.1	0.5
Non-Current Regulatory Liabilities		4.9	5.7
Boswell Units 1 and 2 Net Plant and Equipment
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		5.1	0.4
Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2]	0.0	5.0
Regulated Land Sale
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		2.2	0.0
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		217.0	226.4
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		99.3	104.7
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		39.6	63.2
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		34.9	33.1
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		17.7	17.0
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.2	4.3
Other (Member)
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.6	6.7
Fuel Adjustment Clause (Member)
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[1]	38.7	0.0
Non-Current Regulatory Assets		22.9	56.4
Residential Customer Interim Rate Adjustment
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 5.9	$ 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.