Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		$ 0	$ 33
Operating Revenues - Electric from sales to affiliate		0	1
Operating Expenses - Other taxes from affiliate		3	2
DESS [Member]
Related Party Transaction [Line Items]
Direct and allocated costs from services company affiliate	[1]	67	58
Canadys Refined Coal [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		0	28
Sales of coal to affiliate		0	28
Solar Affiliates [Member]
Related Party Transaction [Line Items]
Purchases from affiliate		2	1
DECG [Member]
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		4	11
Gas purchased for resale		$ 11	$ 4

[1] Includes capitalized expenditures of $12 million and $9 million for the three months ended March 31, 2020 and 2019, respectively.