Income Taxes - Change in Liability for Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 53	$ 12
Increase for other tax positions of prior years	3	10
Increase from Acquisitions	3	33
Decrease for payments and settlements	(4)	0
Decrease related to tax positions taken in prior years	(16)	(2)
Balance at end of year	$ 39	$ 53