Affiliated Transactions (Schedule of Affiliated Transactions - Income Statement) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Purchases of fuel used in electric generation from affiliate		$ 10	$ 29	$ 43	$ 60
Operating Revenues - Electric from sales to affiliate		1	1	2	2
Operating Expenses - Other taxes from affiliate		1	1	3	3
DESS [Member]
Related Party Transaction [Line Items]
Direct and allocated costs from services company affiliate	[1]	82	76	140	135
Canadys Refined Coal [Member]
Related Party Transaction [Line Items]
Purchases of coal from affiliate		34	45	62	77
Sales of coal to affiliate		$ 34	$ 45	$ 62	$ 77

[1] Includes capitalized expenditures of $11 million for both the three months ended June 30, 2019 and 2018, and $20 million and $19 million for the six months ended June 30, 2019 and 2018, respectively