Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2018	$ 10
Future Minimum Purchased Power Payments Due, 2019	0
Future Minimum Purchased Power Payments Due, 2020	0
Future Minimum Purchased Power Payments Due, 2021	0
Future Minimum Purchased Power Payments Due, 2022	0
Future Minimum Purchased Power Payments Due, Thereafter	0
Total future minimum purchased power payments due	10
Future Minimum Operating Leases Payments Due, 2018	2
Future Minimum Operating Leases Payments Due, 2019	2
Future Minimum Operating Leases Payments Due, 2020	2
Future Minimum Operating Leases Payments Due, 2021	2
Future Minimum Operating Leases Payments Due, 2022	2
Future Minimum Operating Leases Payments Due, Thereafter	36
Total future minimum operating leases payments due	46
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2018	303
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2019	74
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2020	7
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2021	7
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2022	7
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, Thereafter	18
Total future minimum long-term service agreements/capital projects payments due	416
Future Minimum Clause Recoverable Commitments Payments Due, 2018	444
Future Minimum Clause Recoverable Commitments Payments Due, 2019	230
Future Minimum Clause Recoverable Commitments Payments Due, 2020	177
Future Minimum Clause Recoverable Commitments Payments Due, 2021	147
Future Minimum Clause Recoverable Commitments Payments Due, 2022	141
Future Minimum Clause Recoverable Commitments Payments Due, Thereafter	1,029
Total future minimum clause recoverable commitments payments due	2,168
Future Minimum Payments Due, 2018	759
Future Minimum Payments Due, 2019	306
Future Minimum Payments Due, 2020	186
Future Minimum Payments Due, 2021	156
Future Minimum Payments Due, 2022	150
Future Minimum Payments Due, Thereafter	1,083
Total future minimum payments	$ 2,640