Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($) shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Purchases of common stock, Shares	(42)	(758)
Stock option exercises, Shares	787	564
Employee stock plan purchases, Shares	40	34
Purchases of common stock, Amount	$ (6,171)	$ (94,499)
Stock option exercises, Amount	33,568	15,089
Employee stock plan purchases, Amount	$ 5,342	$ 4,429