Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2018	$ 17
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	17
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	308
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2019	173
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2020	52
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2021	26
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2022	8
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, Thereafter	0
Total future minimum long-term service agreements/capital projects payments due	567
Future Minimum Clause Recoverable Commitments Payments Due, 2018	248
Future Minimum Clause Recoverable Commitments Payments Due, 2019	230
Future Minimum Clause Recoverable Commitments Payments Due, 2020	178
Future Minimum Clause Recoverable Commitments Payments Due, 2021	144
Future Minimum Clause Recoverable Commitments Payments Due, 2022	136
Future Minimum Clause Recoverable Commitments Payments Due, Thereafter	1,122
Total future minimum clause recoverable commitments payments due	2,058
Future Minimum Payments Due, 2018	575
Future Minimum Payments Due, 2019	405
Future Minimum Payments Due, 2020	232
Future Minimum Payments Due, 2021	172
Future Minimum Payments Due, 2022	146
Future Minimum Payments Due, Thereafter	1,158
Total future minimum payments	$ 2,688