Income Taxes (Details of Income Tax Expense for Continuing Operations Including Noncontrolling Interests) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ 0	$ (16)	$ (410)
State	34	0	(18)
Total current expense (benefit)	34	(16)	(428)
Deferred:
Taxes before operating loss carryforwards, investment tax credits and tax reform	(90)	(216)	262
2017 Tax Reform Act impact	0	(176)	(1)
Tax utilization expense of operating loss carryforwards	102	46	0
State	(57)	(52)	(2)
Total deferred expense (benefit)	(45)	(398)	259
Investment tax credit-amortization	(1)	(2)	(2)
Total income tax expense (benefit)	$ (12)	$ (416)	$ (171)