Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 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		$ 424.7		$ 420.5
Current Regulatory Liabilities	[1]	14.2		1.9
Non-Current Regulatory Liabilities		569.6		560.3
Total Regulatory Liabilities		583.8		562.2
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	0.8		1.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		395.5		407.2
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		87.4		79.3
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		39.3		35.5
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		14.1		17.0
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.2		12.3
Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[2]	12.2		0.0
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		5.4		5.4
Other
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.7		0.2
Non-Current Regulatory Liabilities		3.5		3.6
Public Utilities Reserve for Interim Rates
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[2]	11.7	[1]	0.0
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		208.0		212.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		122.1		123.4
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		31.7		32.0
Boswell 1 & 2 [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.8		10.7
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		39.3		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		$ 2.8		$ 3.8

[1] Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet.(b)Provision for Interim Rate Refund represents reserves for interim rates resulting from the MPUC’s approval of Minnesota Power’s petition and proposal to resolve and withdraw its rate case. This amount is expected to be refunded to Minnesota Power regulated retail customers in the third quarter of 2020. (See 2020 Minnesota General Rate Case.)

[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.)