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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 12,313	$ 13,277	$ 40,321	$ 38,987
Shares used in net income per common share - basic (in shares)	29,010	29,008	29,101	28,912
Incremental shares of unvested restricted common stock (in shares)	39	66	77	89
Shares used in net income per common share - diluted (in shares)	29,049	29,074	29,178	29,001
Earnings per share:
Basic net income	$ 0.42	$ 0.46	$ 1.39	$ 1.35
Diluted net income	$ 0.42	$ 0.46	$ 1.38	$ 1.34