Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		$ 43	$ 139	$ 127
Operating Revenues – Electric from sales to affiliate		4	5	5
Operating Revenues – Gas from sales to affiliate		1	1	1
Operating Expenses – Other taxes from affiliate		6	6	5
DESS [Member]
Related Party Transaction [Line Items]
Direct and allocated costs from services company affiliate	[1]	297	283	303
Canadys Refined Coal [Member]
Related Party Transaction [Line Items]
Purchases of coal from affiliate		121	150	162
Sales of coal to affiliate		$ 120	$ 149	$ 161

[1] Includes capitalized expenditures of $53 million, $41 million and $82 million for the years ended December 31, 2019, 2018 and 2017, respectively.