Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		$ 0	$ 10	$ 0	$ 43
Operating Revenues - Electric from sales to affiliate		2	1	2	2
Operating Expenses - Other taxes from affiliate		1	1	4	3
DESS [Member]
Related Party Transaction [Line Items]
Direct and allocated costs	[1]	63	82	130	140
Canadys Refined Coal [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		0	34	0	62
Sales of coal to affiliate		0	34	0	62
Solar Affiliates [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		4	3	6	4
DECG [Member]
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		5	6	9	9
Gas purchased for resale		$ 11	$ 11	$ 22	$ 23

[1] Includes capitalized expenditures of $13 million and $11 million for the three months ended June 30, 2020 and 2019, respectively, and $25 million and $20 million for the six months ended June 30, 2020 and 2019, respectively.