Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($) shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Purchases of common stock, shares	0	(42)
Stock option exercises, shares	1,048	787
Employee stock plan purchases, shares	35	40
Purchases of common stock, amount	$ 0	$ (6,171)
Stock option exercises, amount	70,536	33,568
Employee stock plan purchases, amount	$ 5,978	$ 5,342