Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Mar. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2017	$ 11
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, Thereafter	0
Total future minimum purchased power payments due	21
Future Minimum Operating Leases Payments Due, 2017	6
Future Minimum Operating Leases Payments Due, 2018	3
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, Thereafter	38
Total future minimum operating leases payments due	53
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2017	52
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2018	11
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2019	12
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, Thereafter	24
Total future minimum long-term service agreements/capital projects payments due	113
Future Minimum Clause Recoverable Commitments Payments Due, 2017	515
Future Minimum Clause Recoverable Commitments Payments Due, 2018	264
Future Minimum Clause Recoverable Commitments Payments Due, 2019	186
Future Minimum Clause Recoverable Commitments Payments Due, 2020	163
Future Minimum Clause Recoverable Commitments Payments Due, 2021	132
Future Minimum Clause Recoverable Commitments Payments Due, Thereafter	1,157
Total future minimum clause recoverable commitments payments due	2,417
Future Minimum Payments Due, 2017	584
Future Minimum Payments Due, 2018	288
Future Minimum Payments Due, 2019	200
Future Minimum Payments Due, 2020	172
Future Minimum Payments Due, 2021	141
Future Minimum Payments Due, Thereafter	1,219
Total future minimum payments	$ 2,604