CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues(1)	$ 1,185,371	$ 993,366	$ 2,472,206	$ 2,222,849
Payroll taxes, benefits and workers’ compensation costs	985,817	773,117	2,021,207	1,768,578
Gross profit	199,554	220,249	450,999	454,271
Salaries, wages and payroll taxes	94,362	90,710	197,437	177,211
Stock-based compensation	13,781	10,694	25,603	17,246
Commissions	8,251	7,475	15,970	15,935
Advertising	8,975	5,720	14,297	10,553
General and administrative expenses	29,211	24,755	60,847	59,608
Depreciation and amortization	9,751	7,908	17,798	15,510
Total operating expenses	164,331	147,262	331,952	296,063
Operating income	35,223	72,987	119,047	158,208
Other income (expense):
Interest income	1,436	369	1,979	2,248
Interest expense	(1,975)	(2,219)	(3,574)	(4,581)
Income before income tax expense	34,684	71,137	117,452	155,875
Income tax expense	9,530	19,286	30,376	41,932
Net income	25,154	51,851	87,076	113,943
Less distributed and undistributed earnings allocated to participating securities	(37)	(276)	(193)	(724)
Net income allocated to common shares	$ 25,117	$ 51,575	$ 86,883	$ 113,219
Basic net income per share of common stock (in dollars per share)	$ 0.65	$ 1.34	$ 2.26	$ 2.93
Diluted net income per share of common stock (in dollars per share)	$ 0.65	$ 1.33	$ 2.24	$ 2.91
Revenue Composition [Abstract]
Gross billings	$ 7,637,851	$ 6,355,683	$ 15,688,273	$ 13,792,437
Worksite employee payroll cost	6,452,480	5,362,317	13,216,067	11,569,588
Revenues(1)	$ 1,185,371	$ 993,366	$ 2,472,206	$ 2,222,849