Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
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		$ 423.8	$ 420.5
Current Regulatory Liabilities	[1]	1.5	1.9
Non-Current Regulatory Liabilities		566.4	560.3
Total Regulatory Liabilities		567.9	562.2
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.3	1.7
Provision for Tax Reform Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	0.2	0.2
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		397.1	407.2
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		84.2	79.3
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		37.4	35.5
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		15.6	17.0
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.3	12.3
Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[2]	8.4	0.0
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		7.4	5.4
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		4.0	3.6
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		211.1	212.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		121.0	123.4
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		31.8	32.0
Boswell 1 & 2 [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		9.3	10.7
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		34.1	24.7
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		8.3	8.2
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.7	4.8
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 3.5	$ 3.8

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

[2] Fuel adjustment clause regulatory liability represents the amount expected to be refunded to customers for the over-collection of fuel adjustment clause recoveries. (See Fuel Adjustment Clause Reform.)