Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Mar. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Purchased Power Payments Due, 2019	$ 33
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	33
Future Minimum Transportation Payments Due, 2019	145
Future Minimum Transportation Payments Due, 2020	175
Future Minimum Transportation Payments Due, 2021	141
Future Minimum Transportation Payments Due, 2022	133
Future Minimum Transportation Payments Due, 2023	109
Future Minimum Transportation Payments Due, Thereafter	1,014
Total future minimum transportation payments due	1,717
Future Minimum Capital Projects Payments Due, 2019	238
Future Minimum Capital Projects Payments Due, 2020	85
Future Minimum Capital Projects Payments Due, 2021	35
Future Minimum Capital Projects Payments Due, 2022	7
Future Minimum Capital Projects Payments Due, 2023	2
Future Minimum Capital Projects Payments Due, Thereafter	6
Total future minimum capital projects payments due	373
Future Minimum Fuel and Gas Supply Payments Due, 2019	193
Future Minimum Fuel and Gas Supply Payments Due, 2020	32
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	232
Future Minimum Long-term Service Agreements Payments Due, 2019	5
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	114
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, 2023	2
Future Minimum Operating Leases Payments Due, Thereafter	37
Total future minimum operating leases payments due	47
Future Minimum Demand Side Management Payments Due, 2019	3
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	4
Future Minimum Payments Due, 2019	619
Future Minimum Payments Due, 2020	301
Future Minimum Payments Due, 2021	188
Future Minimum Payments Due, 2022	152
Future Minimum Payments Due, 2023	125
Future Minimum Payments Due, Thereafter	1,135
Total future minimum payments	$ 2,520