Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (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]
Net service revenues	$ 342,586	$ 341,053	$ 340,309	$ 333,992	$ 325,998	$ 336,460	$ 335,047	$ 322,201	$ 1,357,940	$ 1,319,706	$ 1,319,238
Gross profit	102,627	104,375	103,919	97,135	99,648	105,380	106,282	97,189	408,056	408,499	414,114
Net income	$ 6,140	$ 10,099	$ 11,144	$ 5,902	$ 9,239	$ 9,020	$ 10,864	$ 3,650	$ 33,285	$ 32,773	$ 42,824
Earnings per share – basic (in dollars per share)	$ 0.25	$ 0.40	$ 0.44	$ 0.23	$ 0.36	$ 0.35	$ 0.41	$ 0.14	$ 1.32	$ 1.26	$ 1.53
Earnings per share – diluted (in dollars per share)	$ 0.24	$ 0.40	$ 0.44	$ 0.23	$ 0.36	$ 0.34	$ 0.41	$ 0.14	$ 1.30	$ 1.25	$ 1.52