Income Taxes (Schedule of Deferred Income Taxes) (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred income taxes:
Total deferred income tax assets	$ 1,258	$ 971
Total deferred income tax liabilities	1,868	1,960
Total net deferred income tax liabilities	610	989
Depreciation method and plant basis differences	1,007	998
Excess deferred income taxes	(231)	(148)
Unrecovered nuclear plant cost	553	584
DESC rate refund	(169)	(1)
Toshiba settlement	(219)	(231)
Nuclear decommissioning	(43)	(9)
Deferred state income taxes	200	296
Federal benefit of deferred state income taxes	(42)	(62)
Deferred fuel, purchased energy and gas costs	7	1
Pension benefits	46	46
Other postretirement benefits	(35)	(35)
Loss and credit carryforwards	(391)	(520)
Other	(73)
Other		70
Deferred Investment Tax Credits-Regulated Operations	19	19
Deferred income taxes and investment tax credits	$ 629	$ 1,008