Regulatory Matters - Electric Rates (Details) - USD ($) $ in Millions				4 Months Ended	9 Months Ended		18 Months Ended
		May 01, 2020	Nov. 01, 2019	Apr. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Regulatory Matters [Line Items]
Public Utilities Reserve for Interim Rates	[1]				$ 5.2		$ 0.0
MPUC [Member] | CIP Consolidated Filing [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Revenue from Cost Recovery Riders and Other Regulatory Programs					2.4	$ 2.8
MPUC [Member] | Fuel Adjustment Clause Filing [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Forced Outage Costs							$ 8.0
MPUC [Member] | COVID-19 Related Deferred Accounting [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Reduction to Revenue Due to Idled Customers					30.0
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Revenue from Cost Recovery Riders and Other Regulatory Programs					22.6	$ 24.3
Electric Rates [Member] | MPUC [Member] | 2020 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Annual Additional Revenue Generated from Requested Final Rate Increase			$ 66.0
Requested Rate Increase Percent			10.60%
Interim Rate Increase			$ 36.1
Requested Return on Equity			10.05%
Requested Equity Capital Structure			53.81%
Ongoing Customer Rate Increase		4.10%
Public Utilities, Interim Rate Increase (Decrease), Percentage				5.80%
Public Utilities Reserve for Interim Rates					11.7
Public Utilities, Interim Rates Refunded					$ 6.5

[1] 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. (See Note 2. Regulatory Matters.)