Share Repurchase Plan - Schedule of Share Repurchases Under Share Repurchase Plan - (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended			58 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Equity [Abstract]
Total cost of shares repurchased	$ 120	$ 0	$ 91	$ 5,471
Total number of shares repurchased	2,630	0	1,450	66,740
Average cost per share (including transaction fees)	$ 45.67	$ 0	$ 62.76	$ 81.96