Quarterly Financial Data (Unaudited) (Details) - 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]
Revenues(1)	$ 966,756	$ 925,126	$ 922,295	$ 1,014,372	$ 826,494		$ 795,513		$ 795,552		$ 882,664		$ 3,828,549	[1]	$ 3,300,223	[1]	$ 2,941,347	[1]
Gross profit	161,591	166,054	154,544	199,720	142,866		139,966		130,553		159,346		681,909		572,731		491,610
Operating income	32,619	48,133	33,581	64,703	23,712		29,799		22,938		53,492		179,036		129,941		106,306
Net income	$ 24,655	$ 36,207	$ 24,560	$ 49,991	$ 15,554		$ 19,202		$ 14,018		$ 35,628		$ 135,413		$ 84,402		$ 65,991
Basic net income per share of common stock (in dollars per share)	$ 0.59	$ 0.86	$ 0.59	$ 1.20	$ 0.36	[2]	$ 0.46	[2]	$ 0.34	[2]	$ 0.85	[2]	$ 3.24		$ 2.02		$ 1.55
Diluted net income per share (in dollars per share)	$ 0.59	$ 0.86	$ 0.58	$ 1.18	$ 0.36	[2]	$ 0.46	[2]	$ 0.33	[2]	$ 0.85	[2]	$ 3.22		$ 2.01		$ 1.54

[1] (1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31,(in thousands)201820172016 Gross billings$23,830,731$20,173,812$17,932,857Less: WSEE payroll cost20,002,18216,873,58914,991,510Revenues$3,828,549$3,300,223$2,941,347

[2] (1) Adjusted to reflect the two-for-one split of our common stock effected on December 18, 2017 as a stock dividend.