Pension and Other Postretirement Benefit Plans - Contributions (Details) - USD ($) shares in Thousands, $ in Millions			12 Months Ended
		Jan. 17, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
ESOP Compensation Expense			$ 12.0	$ 11.5	$ 11.2
Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer		$ 6.5
Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares					0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Cash			$ 0.0	10.3	$ 10.7
Defined Benefit Plan - Amendments			The non-union defined benefit pension plan was frozen in 2018, and does not allow further crediting of service or earnings to the plan. Further, it is closed to new participants. The Minnesota Power union defined benefit pension plan is also closed to new participants, and the SWL&P union defined benefit pension plan was closed to new participants effective February 1, 2022.
Expected Employer Contributions in 2022			$ 0.0
Defined Benefit Plan, Plan Assets, Contributions by Employer	[1]		12.3	19.7
Postretirement Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected Employer Contributions in 2022			0.0
Defined Benefit Plan, Plan Assets, Contributions by Employer			0.3	0.7
Postretirement Health and Life [Member] | VEBA [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Other Postretirement Benefit Plans			0.0	0.0	0.0
Postretirement Health and Life [Member] | Irrevocable Grantor Trust [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Other Postretirement Benefit Plans			$ 0.0	$ 0.0	$ 0.0

[1] Includes Participant Contributions noted above, any contributions made by the Company to pension plan trusts and any direct benefit payments made under certain plans.(c)    Presented here net of amounts related to prepaid pension for SWL&P that are reclassified to Regulatory Assets on the Consolidated Balance Sheet.