Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		$ 0	$ 0	$ 0	$ 43
Operating Revenues - Electric from sales to affiliate		1	1	3	3
Operating Expenses - Other taxes from affiliate		3	2	7	5
DESS [Member]
Related Party Transaction [Line Items]
Direct and allocated costs	[1]	72	67	202	207
Canadys Refined Coal [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		0	38	0	100
Sales of coal to affiliate		0	37	0	99
Solar Affiliates [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		4	3	10	7
DECG [Member]
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		5	6	14	15
Gas purchased for resale		$ 11	$ 10	$ 33	$ 33

[1] Includes capitalized expenditures of $23 million and $13 million for the three months ended September 30, 2020 and 2019, respectively, and $48 million and $33 million for the nine months ended September 30, 2020 and 2019, respectively.