Pension and Other Postretirement Benefit Plans - Contributions (Details) - USD ($) $ in Millions			1 Months Ended	12 Months Ended
	Jan. 17, 2017	Jan. 13, 2017	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions				$ 0.0	$ 0.0	$ 19.5
Defined Contribution RSOP, Employer Contributions				9.2	9.0	9.1
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions				6.3	0.0	0.0
Employer Contributions						19.5
Expected Employer Contributions in 2017				$ 0.0
Defined Benefit Plan, Plan Amendments				In 2006, the non-union defined benefit pension plan was amended to suspend further crediting of service to the plan and to close the plan to new participants. In conjunction with those amendments, contributions were increased to the RSOP. In 2010, the Minnesota Power union defined benefit pension plan was amended to close the plan to new participants beginning February 1, 2011.
Postretirement Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected Employer Contributions in 2017				$ 0.0
Defined Benefit Plan, Plan Amendments				In 2010, our postretirement health plan was amended to close the plan to employees hired after January 31, 2011. The full eligibility requirement was also amended in 2010, to require employees to be at least age 55 with 10 years of participation in the plan. In 2014, our postretirement life plan was amended to close the plan to non-union employees retiring after December 31, 2015.
Subsequent Event [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions		$ 1.7	$ 15.2
Employer Contributions	$ 13.5
VEBA [Member] | Postretirement Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions				$ 0.0	0.0	0.0
Irrevocable Grantor Trust [Member] | Postretirement Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions				$ 0.0	$ 0.0	$ 0.0