Pension and Other Postretirement Benefit Plans - Contributions (Details) - USD ($) shares in Thousands, $ in Millions		12 Months Ended
	Jan. 15, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
ESOP Compensation Expense		$ 11.2	$ 10.8	$ 11.4
Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares		0	0	0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Cash		$ 10.7	$ 10.4	$ 15.0
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value		$ 0.0		0.0
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.
Expected Employer Contributions in 2020		$ 0.0
Pension Plan [Member] | Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Cash	$ 10.3
Postretirement Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected Employer Contributions in 2020		0.0
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