Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Dec. 31, 2016 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2017	$ 10.7
Future Minimum Purchased Power Payments Due, 2018	10.1
Future Minimum Purchased Power Payments Due, 2019	0.0
Future Minimum Purchased Power Payments Due, 2020	0.0
Future Minimum Purchased Power Payments Due, 2021	0.0
Future Minimum Purchased Power Payments Due, Thereafter	0.0
Total future minimum purchased power payments due	20.8
Future Minimum Operating Leases Payments Due, 2017	7.0
Future Minimum Operating Leases Payments Due, 2018	3.5
Future Minimum Operating Leases Payments Due, 2019	2.1
Future Minimum Operating Leases Payments Due, 2020	2.1
Future Minimum Operating Leases Payments Due, 2021	2.2
Future Minimum Operating Leases Payments Due, Thereafter	37.8
Total future minimum operating leases payments due	54.7
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2017	68.8
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2018	11.1
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2019	11.8
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2020	6.8
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2021	6.9
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, Thereafter	24.4
Total future minimum long-term service agreements/capital projects payments due	129.8
Future Minimum Clause Recoverable Commitments Payments Due, 2017	398.5
Future Minimum Clause Recoverable Commitments Payments Due, 2018	231.0
Future Minimum Clause Recoverable Commitments Payments Due, 2019	186.2
Future Minimum Clause Recoverable Commitments Payments Due, 2020	162.9
Future Minimum Clause Recoverable Commitments Payments Due, 2021	132.3
Future Minimum Clause Recoverable Commitments Payments Due, Thereafter	1,156.6
Total future minimum clause recoverable commitments payments due	2,267.5
Future Minimum Payments Due, 2017	485.0
Future Minimum Payments Due, 2018	255.7
Future Minimum Payments Due, 2019	200.1
Future Minimum Payments Due, 2020	171.8
Future Minimum Payments Due, 2021	141.4
Future Minimum Payments Due, Thereafter	1,218.8
Total future minimum payments	$ 2,472.8