Income Taxes (Details of Income Tax Expense for Continuing Operations Including Noncontrolling Interests) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ (139)	$ 0	$ (16)
State	3	34	0
Total current expense (benefit)	(136)	34	(16)
Deferred:
Taxes before operating loss carryforwards, investment tax credits and tax reform	158	(90)	(216)
2017 Tax Reform Act impact	0	0	(176)
Tax utilization expense of operating loss carryforwards	33	102	46
State	17	(57)	(52)
Total deferred expense (benefit)	208	(45)	(398)
Investment tax credit-amortization	(1)	(1)	(2)
Total income tax expense (benefit)	$ 71	$ (12)	$ (416)