Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Equity [Abstract]
Purchases of common stock, Shares	(42)	(758)
Stock option exercises, Shares	324	79
Employee stock plan purchases, Shares	14	8
Purchases of common stock, Amount	$ (6,171)	$ (94,497)
Stock option exercises, Amount	14,851	1,781
Employee stock plan purchases, Amount	$ 1,650	$ 1,238