Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 357,984	$ 355,452	$ 358,624	$ 346,293	$ 342,586	$ 341,053	$ 340,309	$ 333,992	$ 1,418,353	$ 1,357,940	$ 1,319,706
Gross profit	106,556	104,381	107,483	100,188	102,627	104,375	103,919	97,135	418,608	408,056	408,499
Net income	$ 16,356	$ 16,177	$ 16,272	$ 9,175	$ 6,140	$ 10,099	$ 11,144	$ 5,902	$ 57,980	$ 33,285	$ 32,773
Earnings per share – basic (in dollars per share)	$ 0.66	$ 0.65	$ 0.66	$ 0.37	$ 0.25	$ 0.40	$ 0.44	$ 0.23	$ 2.34	$ 1.32	$ 1.26
Earnings per share – diluted (in dollars per share)	$ 0.65	$ 0.64	$ 0.65	$ 0.37	$ 0.24	$ 0.40	$ 0.44	$ 0.23	$ 2.30	$ 1.30	$ 1.25