Net Profit per Share - Computation of Basic and Diluted Net Profit Per Share Attributable to Common Stock (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 231,418	$ 189,682	$ 144,020
Denominator:
Weighted-average common shares outstanding, basic (shares)	80,085	79,856	79,998
Dilutive effect of potential common stock (shares)	1,747	1,628	1,523
Total shares, diluted (shares)	81,832	81,484	81,521
Net income per share, basic (usd per share)	$ 2.89	$ 2.38	$ 1.80
Net income per share, diluted (usd per share)	$ 2.83	$ 2.33	$ 1.77