Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Sep. 30, 2019 USD ($)
Other Commitments [Line Items]
2019	$ 329
2020	551
2021	297
2022	236
2023	207
Thereafter	2,345
Total future minimum payments	3,965
Purchased Power [Member]
Other Commitments [Line Items]
2019	6
2020	0
2021	0
2022	0
2023	0
Thereafter	0
Total future minimum payments	6
Transportation [Member]
Other Commitments [Line Items]
2019	50
2020	198
2021	210
2022	212
2023	189
Thereafter	2,206
Total future minimum payments	3,065
Capital Projects [Member]
Other Commitments [Line Items]
2019	186
2020	127
2021	28
2022	8
2023	1
Thereafter	0
Total future minimum payments	350
Fuel and Gas Supply [Member]
Other Commitments [Line Items]
2019	85
2020	217
2021	49
2022	3
2023	1
Thereafter	0
Total future minimum payments	355
Long-term Service Agreements [Member]
Other Commitments [Line Items]
2019	0
2020	6
2021	7
2022	10
2023	13
Thereafter	86
Total future minimum payments	122
Operating Leases [Member]
Other Commitments [Line Items]
2019	1
2020	2
2021	3
2022	3
2023	3
Thereafter	53
Total future minimum payments	65
Demand Side Management [Member]
Other Commitments [Line Items]
2019	1
2020	1
2021	0
2022	0
2023	0
Thereafter	0
Total future minimum payments	$ 2