Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2018	$ 15
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	15
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	412
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2019	155
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2020	21
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2021	15
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2022	8
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, Thereafter	23
Total future minimum long-term service agreements/capital projects payments due	634
Future Minimum Clause Recoverable Commitments Payments Due, 2018	359
Future Minimum Clause Recoverable Commitments Payments Due, 2019	237
Future Minimum Clause Recoverable Commitments Payments Due, 2020	186
Future Minimum Clause Recoverable Commitments Payments Due, 2021	155
Future Minimum Clause Recoverable Commitments Payments Due, 2022	150
Future Minimum Clause Recoverable Commitments Payments Due, Thereafter	1,124
Total future minimum clause recoverable commitments payments due	2,211
Future Minimum Payments Due, 2018	788
Future Minimum Payments Due, 2019	394
Future Minimum Payments Due, 2020	209
Future Minimum Payments Due, 2021	172
Future Minimum Payments Due, 2022	160
Future Minimum Payments Due, Thereafter	1,183
Total future minimum payments	$ 2,906