Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2021	Dec. 31, 2020
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets Currently Earning a Return		With the exception of the regulatory asset for Boswell Units 1 and 2 net plant and equipment, no other regulatory assets are currently earning a return.
Non-Current Regulatory Assets		$ 502.9	$ 480.9
Current Regulatory Liabilities	[1]	9.0	7.6
Non-Current Regulatory Liabilities		509.5	524.8
Total Regulatory Liabilities		518.5	532.4
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	3.1	2.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		359.3	375.3
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		84.4	86.6
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		47.4	41.2
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		1.4	4.4
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.3	12.0
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		0.0	1.5
Other (Member)
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	3.3	1.0
Non-Current Regulatory Liabilities		4.7	3.8
Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	2.6	3.7
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		247.6	259.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		107.2	113.7
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		61.8	54.0
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		31.4	31.6
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		11.9	8.8
Boswell Units 1 and 2 Net Plant and Equipment [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		3.2	5.0
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.4	4.5
Other (Member)
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.3	3.6
Fuel Adjustment Clause (Member)
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 30.1	$ 0.0

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