Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues(1)	$ 1,075,090	$ 1,043,388	$ 1,043,316	$ 1,153,010	$ 966,756	$ 925,126	$ 922,295	$ 1,014,372	$ 4,314,804	[1]	$ 3,828,549	[1]	$ 3,300,223	[1]
Gross profit	161,936	170,546	173,735	226,717	161,591	166,054	154,544	199,720	732,934		681,909		572,731
Operating income	27,719	34,733	38,720	85,461	32,619	48,133	33,581	64,703	186,633		179,036		129,941
Net income	$ 20,395	$ 25,859	$ 28,556	$ 76,289	$ 24,655	$ 36,207	$ 24,560	$ 49,991	$ 151,099		$ 135,413		$ 84,402
Basic net income per share of common stock (in dollars per share)	$ 0.51	$ 0.64	$ 0.69	$ 1.86	$ 0.59	$ 0.86	$ 0.59	$ 1.20	$ 3.72		$ 3.24		$ 2.02
Diluted net income per share (in dollars per share)	$ 0.51	$ 0.63	$ 0.69	$ 1.85	$ 0.59	$ 0.86	$ 0.58	$ 1.18	$ 3.70		$ 3.22		$ 2.01

[1] ⁽¹⁾  Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows:   Year ended December 31, (in thousands) 2019 2018 2017         Gross billings $ 27,212,010
$ 23,830,731
$ 20,173,812
Less: WSEE payroll cost 22,897,206
20,002,182
16,873,589
Revenues $ 4,314,804
$ 3,828,549
$ 3,300,223