Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 325,998	$ 336,460	$ 335,047	$ 322,201	$ 327,699	$ 341,575	$ 337,353	$ 312,611	$ 1,319,706	$ 1,319,238	$ 1,217,331
Gross profit	99,648	105,380	106,282	97,189	103,515	109,821	106,038	94,740	408,499	414,114	374,581
Net income	$ 9,239	$ 9,020	$ 10,864	$ 3,650	$ 11,901	$ 13,545	$ 11,593	$ 5,785	$ 32,773	$ 42,824	$ 90,915
Earnings per share – basic (in dollars per share)	$ 0.36	$ 0.35	$ 0.41	$ 0.14	$ 0.43	$ 0.49	$ 0.41	$ 0.20	$ 1.26	$ 1.53	$ 2.89
Earnings per share – diluted (in dollars per share)	$ 0.36	$ 0.34	$ 0.41	$ 0.14	$ 0.43	$ 0.48	$ 0.41	$ 0.20	$ 1.25	$ 1.52	$ 2.87