Commitments and Contingencies, Fuel Contracts (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 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]
2018	$ 6.4
2019	0.6
2020	0.6
2021	0.7
2022	0.7
Thereafter	0.7
Total	9.7	[1]
Natural Gas Supply
Fuel Contracts [Abstract]
2018	9.4
2019	0.4
2020	0.3
2021	0.3
2022	0.2
Thereafter	0.0
Total	10.6	[1]
Natural Gas Storage and Transportation
Fuel Contracts [Abstract]
2018	13.3
2019	12.3
2020	10.1
2021	9.5
2022	8.2
Thereafter	30.5
Total	$ 83.9	[1]
Minimum
Unrecorded Unconditional Purchase Obligation [Line Items]
Fuel Contract Expiration Date	2018
Maximum
Unrecorded Unconditional Purchase Obligation [Line Items]
Fuel Contract Expiration Date	2029

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