Income Taxes - Summary of Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Unrecognized tax benefits beginning balance	$ 474	$ 463	$ 385
Increases (decreases):
Increases in tax positions for prior years	4	35	36
Decreases in tax positions for prior years	0	(31)	(21)
Increase in tax positions for the current period	40	84	115
Purchase accounting adjustments (See Footnote 1 and Footnote7)	0	(9)	0
Settlements with taxing authorities	0	(2)	(6)
Lapse of statute of limitations	(66)	(66)	(46)
Unrecognized tax benefits ending balance	$ 452	$ 474	$ 463