Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity [Abstract]
Purchases of common stock, Shares	0	(42)
Stock option exercises, Shares	350	324
Employee stock plan purchases, Shares	12	14
Purchases of common stock, Amount	$ 0	$ (6,171)
Stock option exercises, Amount	19,298	14,851
Employee stock plan purchases, Amount	$ 1,798	$ 1,650