Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		$ 0	$ 35	$ 43	$ 95
Operating Revenues - Electric from sales to affiliate		1	1	3	3
Operating Expenses - Other taxes from affiliate		2	2	5	4
DESS [Member]
Related Party Transaction [Line Items]
Direct and allocated costs from services company affiliate	[1]	67	73	207	208
Canadys Refined Coal [Member]
Related Party Transaction [Line Items]
Purchases of coal from affiliate		38	45	100	123
Sales of coal to affiliate		$ 37	$ 45	$ 99	$ 122

[1] Includes capitalized expenditures of $13 million and $10 million for the three months ended September 30, 2019 and 2018, and $33 million and $29 million for the nine months ended September 30, 2019 and 2018, respectively