Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2019	$ 23
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, 2023	0
Future Minimum Purchased Power Payments Due, Thereafter	0
Total future minimum purchased power payments due	23
Future Minimum Transportation Payments Due, 2019	100
Future Minimum Transportation Payments Due, 2020	196
Future Minimum Transportation Payments Due, 2021	191
Future Minimum Transportation Payments Due, 2022	184
Future Minimum Transportation Payments Due, 2023	161
Future Minimum Transportation Payments Due, Thereafter	1,647
Total future minimum transportation payments due	2,479
Future Minimum Capital Projects Payments Due, 2019	246
Future Minimum Capital Projects Payments Due, 2020	86
Future Minimum Capital Projects Payments Due, 2021	35
Future Minimum Capital Projects Payments Due, 2022	9
Future Minimum Capital Projects Payments Due, 2023	3
Future Minimum Capital Projects Payments Due, Thereafter	13
Total future minimum capital projects payments due	392
Future Minimum Fuel and Gas Supply Payments Due, 2019	125
Future Minimum Fuel and Gas Supply Payments Due, 2020	33
Future Minimum Fuel and Gas Supply Payments Due, 2021	3
Future Minimum Fuel and Gas Supply Payments Due, 2022	3
Future Minimum Fuel and Gas Supply Payments Due, 2023	1
Future Minimum Fuel and Gas Supply Payments Due, Thereafter	0
Total future minimum fuel and gas supply payments due	165
Future Minimum Long-term Service Agreements Payments Due, 2019	3
Future Minimum Long-term Service Agreements Payments Due, 2020	6
Future Minimum Long-term Service Agreements Payments Due, 2021	7
Future Minimum Long-term Service Agreements Payments Due, 2022	7
Future Minimum Long-term Service Agreements Payments Due, 2023	11
Future Minimum Long-term Service Agreements Payments Due, Thereafter	78
Total future minimum long-term service agreements payments due	112
Future Minimum Operating Leases Payments Due, 2019	1
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, 2023	2
Future Minimum Operating Leases Payments Due, Thereafter	35
Total future minimum operating leases payments due	44
Future Minimum Demand Side Management Payments Due, 2019	2
Future Minimum Demand Side Management Payments Due, 2020	1
Future Minimum Demand Side Management Payments Due, 2021	0
Future Minimum Demand Side Management Payments Due, 2022	0
Future Minimum Demand Side Management Payments Due, 2023	0
Future Minimum Demand Side Management Payments Due, Thereafter	0
Total future minimum demand side management payments due	3
Future Minimum Payments Due, 2019	500
Future Minimum Payments Due, 2020	324
Future Minimum Payments Due, 2021	238
Future Minimum Payments Due, 2022	205
Future Minimum Payments Due, 2023	178
Future Minimum Payments Due, Thereafter	1,773
Total future minimum payments	$ 3,218