Commitments, Guarantees and Contingencies Minimum Annual Payments for Certain Long-Term Commitments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MW
Minimum Annual Payments for Certain Long-Term Commitments [Line Items]
Minimum Lease Payments in 2017	$ 13.7
Minimum Lease Payments in 2018	12.0
Minimum Lease Payments in 2019	10.7
Minimum Lease Payments in 2020	7.5
Minimum Lease Payments in 2021	5.9
Minimum Lease Payments Thereafter	18.3
Coal Supply and Transportation Agreements [Member]
Minimum Annual Payments for Certain Long-Term Commitments [Line Items]
Minimum Annual Payment Obligation in 2017	27.9
Minimum Annual Payment Obligation in 2018	27.0
Minimum Annual Payment Obligation in 2019	1.8
Minimum Annual Payment Obligation in 2020	0.0
Minimum Annual Payment Obligation in 2021	0.0
Minimum Annual Payment Obligation Thereafter	0.0
Power Purchase Agreement [Member]
Minimum Annual Payments for Certain Long-Term Commitments [Line Items]
Minimum Annual Payment Obligation in 2017	98.0	[1]
Minimum Annual Payment Obligation in 2018	102.9	[1]
Minimum Annual Payment Obligation in 2019	105.5	[1]
Minimum Annual Payment Obligation in 2020	113.4	[1]
Minimum Annual Payment Obligation in 2021	143.3	[1]
Minimum Annual Payment Obligation Thereafter	$ 1,803.9	[1]
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires 2040) [Member]
Minimum Annual Payments for Certain Long-Term Commitments [Line Items]
Output Being Purchased (MW) | MW	133

[1] Excludes the agreement with Manitoba Hydro expiring in 2022, as this contract is for surplus energy only, and the 133 MW agreement with Manitoba Hydro commencing in 2020, as our obligation under this contract is subject to construction of additional transmission capacity. Also excludes Oliver Wind I and Oliver Wind II, as Minnesota Power only pays for energy as it is delivered.