Pension and Other Postretirement Benefit Plans - Contributions (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 15, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value		$ 0.0	$ 13.5	$ 0.0
ESOP Compensation Expense		$ 11.4	$ 11.0	$ 9.2
Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares		0	216,000	0
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value			$ 13.5
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Cash		$ 15.0	1.7	$ 6.3
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value		$ 0.0	13.5	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 2019		$ 0.0
Pension Plan [Member] | Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Cash	$ 10.4
Postretirement Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected Employer Contributions in 2019		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