Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Jun. 30, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2017	$ 5
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	15
Future Minimum Operating Leases Payments Due, 2017	3
Future Minimum Operating Leases Payments Due, 2018	4
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	51
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2017	38
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2018	11
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2019	11
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2020	6
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, 2021	7
Future Minimum Long-term Service Agreements/Capital Projects Payments Due, Thereafter	25
Total future minimum long-term service agreements/capital projects payments due	98
Future Minimum Clause Recoverable Commitments Payments Due, 2017	303
Future Minimum Clause Recoverable Commitments Payments Due, 2018	257
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	2,198
Future Minimum Payments Due, 2017	349
Future Minimum Payments Due, 2018	282
Future Minimum Payments Due, 2019	199
Future Minimum Payments Due, 2020	171
Future Minimum Payments Due, 2021	141
Future Minimum Payments Due, Thereafter	1,220
Total future minimum payments	$ 2,362