Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($) shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Equity [Abstract]
Purchases of common stock, Shares	(42)	(758)
Stock option exercises, Shares	534	241
Employee stock plan purchases, Shares	27	23
Purchases of common stock, Amount	$ (6,171)	$ (94,497)
Stock option exercises, Amount	23,360	5,793
Employee stock plan purchases, Amount	$ 3,427	$ 2,818