Quarterly Financial Data (Unaudited) - Summary of Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues, net	$ 197,875	$ 192,345	$ 189,057	$ 183,033	$ 177,274	$ 171,200	$ 170,478	$ 167,036	$ 762,310	$ 685,988	$ 628,492
Gross profit	68,506	69,156	65,211	63,851	61,490	59,433	58,452	58,657	266,724	238,032	225,575
Net income/(loss)	$ (9,355)	$ 21,077	$ 20,378	$ 16,788	$ 15,488	$ 16,050	$ 16,375	$ 13,820	$ 48,888	$ 61,733	$ 51,565
Earnings/(loss) per share:
Basic (in dollars per share)	$ (0.27)	$ 0.62	$ 0.60	$ 0.50	$ 0.46	$ 0.48	$ 0.49	$ 0.41	$ 1.44	$ 1.84	$ 1.55
Diluted (in dollars per share)	$ (0.27)	$ 0.60	$ 0.58	$ 0.48	$ 0.45	$ 0.46	$ 0.47	$ 0.40	$ 1.39	$ 1.79	$ 1.51
Weighted-average number of shares used in computing earnings per share:
Basic (in shares)	34,086,711	33,838,374	33,819,320	33,845,560	33,638,170	33,624,401	33,621,444	33,380,028	33,897,916	33,566,367	33,298,104
Diluted (in shares)	34,086,711	35,043,987	34,993,226	35,108,882	34,714,308	34,675,485	34,510,400	34,351,657	35,110,210	34,563,319	34,178,340
Stock compensation expense	$ 6,270	$ 5,708	$ 5,107	$ 5,956	$ 5,027	$ 4,484	$ 4,450	$ 5,809	$ 23,041	$ 19,770	$ 16,047
Amortization of intangibles	$ 3,483	$ 3,487	$ 3,507	$ 3,498	$ 3,592	$ 2,848	$ 2,718	$ 2,715	$ 13,975	$ 11,873	$ 10,226
Weighted average common shares considered anti-dilutive and not included in computing diluted earnings per share	1,206,335								151,961	92,538	73,896