Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Operating Revenues - Electric from sales to affiliate		$ 0	$ 1	$ 2	$ 3
Operating Expenses - Other taxes from affiliate		2	3	6	7
DES & DESS [Member]
Related Party Transaction [Line Items]
Purchases from affiliate	[1]	53	72	168	202
Solar Affiliates [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		5	4	12	10
DECG [Member]
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		0	5	0	14
Gas purchased for resale		$ 0	$ 11	$ 0	$ 33

[1] Includes capitalized expenditures of $8 million and $23 million for the three months ended September 30, 2021 and 2020, respectively, and $23 million and $48 million for the nine months ended September 30, 2021 and 2020, respectively.