Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 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		$ 491.9	$ 480.9
Current Regulatory Liabilities	[1]	6.2	7.6
Non-Current Regulatory Liabilities		514.9	524.8
Total Regulatory Liabilities		521.1	532.4
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	2.4	2.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		364.5	375.3
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		85.1	86.6
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		45.5	41.2
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		2.4	4.4
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.4	12.0
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		1.3	1.5
Other
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	0.1	1.0
Non-Current Regulatory Liabilities		3.7	3.8
Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	3.7	3.7
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		252.0	259.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		109.4	113.7
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		63.2	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		6.0	8.8
Boswell Units 1 and 2 Net Plant and Equipment [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		2.2	5.0
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.5	4.5
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.9	3.6
Fuel Adjustment Clause
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 18.3	$ 0.0

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