Commitments and Contingencies, Fuel Contracts (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Unrecorded Unconditional Purchase Obligation [Line Items]
Minimum annual tolerance band percentage for future rate recovery or refund of fuel costs (in hundredths)	2.00%
Coal
Fuel Contracts [Abstract]
2017	$ 6.9
2018	2.5
2019	0.8
2020	0.8
2021	0.8
Thereafter	1.7
Total	13.5	[1]
Natural Gas Supply
Fuel Contracts [Abstract]
2017	10.9
2018	0.3
2019	0.3
2020	0.3
2021	0.3
Thereafter	0.4
Total	12.5	[1]
Natural Gas Storage and Transportation
Fuel Contracts [Abstract]
2017	13.2
2018	12.3
2019	11.4
2020	9.1
2021	8.4
Thereafter	36.1
Total	$ 90.5	[1]
Minimum
Unrecorded Unconditional Purchase Obligation [Line Items]
Fuel Contract Expiration Date	2017
Maximum
Unrecorded Unconditional Purchase Obligation [Line Items]
Fuel Contract Expiration Date	2029

[1] Excludes additional amounts allocated to NSP-Wisconsin through intercompany charges.