Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of year	$ 10	$ 50
Additions for tax positions related to the current year	0	1
Reductions for tax positions related to prior years	0	(5)
Settlements for tax positions	(1)	(16)
Lapse of statutes	(1)	(20)
Balance at end of year	$ 8	$ 10