Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Sep. 30, 2020 USD ($)
Other Commitments [Line Items]
2020	$ 261
2021	414
2022	333
2023	299
2024	225
Thereafter	2,239
Total future minimum payments	3,771
Purchased Power [Member]
Other Commitments [Line Items]
2020	15
2021	3
2022	0
2023	0
2024	0
Thereafter	0
Total future minimum payments	18
Transportation [Member]
Other Commitments [Line Items]
2020	56
2021	234
2022	236
2023	211
2024	206
Thereafter	2,127
Total future minimum payments	3,070
Capital Projects [Member]
Other Commitments [Line Items]
2020	119
2021	101
2022	76
2023	68
2024	0
Thereafter	0
Total future minimum payments	364
Fuel and Gas Supply [Member]
Other Commitments [Line Items]
2020	66
2021	58
2022	3
2023	1
2024	0
Thereafter	0
Total future minimum payments	128
Long-term Service Agreements [Member]
Other Commitments [Line Items]
2020	3
2021	11
2022	12
2023	16
2024	16
Thereafter	62
Total future minimum payments	120
Operating Leases [Member]
Other Commitments [Line Items]
2020	1
2021	3
2022	3
2023	3
2024	3
Thereafter	50
Total future minimum payments	63
Demand Side Management [Member]
Other Commitments [Line Items]
2020	1
2021	4
2022	3
2023	0
2024	0
Thereafter	0
Total future minimum payments	$ 8

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Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

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Amount of contractual obligation to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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Amount of contractual obligation to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of contractual obligation to be paid in remainder of current fiscal year.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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