Regulatory Matters - Electric Rates (Details) - USD ($) $ in Millions				4 Months Ended	6 Months Ended
		May 01, 2020	Nov. 01, 2019	Apr. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Regulatory Matters [Line Items]
Public Utilities Reserve for Interim Rates	[1]				$ 11.7		$ 0.0
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					14.9	$ 17.4
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

[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.)