Commitments and Contingencies, Fuel Contracts (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 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]
2016	$ 6.7
2017	2.5
2018	2.5
2019	0.8
2020	0.8
Thereafter	2.5
Total	15.8	[1]
Natural Gas Supply
Fuel Contracts [Abstract]
2016	9.7
2017	0.2
2018	0.0
2019	0.0
2020	0.0
Thereafter	0.0
Total	9.9	[1]
Natural Gas Storage and Transportation
Fuel Contracts [Abstract]
2016	13.1
2017	10.4
2018	4.7
2019	3.1
2020	1.9
Thereafter	11.6
Total	$ 44.8	[1]
Minimum
Unrecorded Unconditional Purchase Obligation [Line Items]
Fuel Contract Expiration Date (year)	2016
Maximum
Unrecorded Unconditional Purchase Obligation [Line Items]
Fuel Contract Expiration Date (year)	2029

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