Net Income per Share - Reconciliation of Basic to Diluted Net Income per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 19,007	$ 15,384	$ 28,008	$ 25,710
Shares used in net income per common share - basic (in shares)	28,979	28,978	29,085	28,873
Incremental shares of unvested restricted common stock (in shares)	110	136	160	169
Shares used in net income per common share - diluted (in shares)	29,089	29,114	29,245	29,042
Earnings per share:
Basic net income	$ 0.66	$ 0.53	$ 0.96	$ 0.89
Diluted net income	$ 0.65	$ 0.53	$ 0.96	$ 0.89