CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenues(1)	$ 1,209,628	$ 1,007,820	$ 3,681,834	$ 3,230,669
Payroll taxes, benefits and workers’ compensation costs	1,011,149	822,787	3,032,356	2,591,365
Gross profit	198,479	185,033	649,478	639,304
Salaries, wages and payroll taxes	89,232	89,429	286,669	266,640
Stock-based compensation	10,362	20,864	35,965	38,110
Commissions	8,724	7,722	24,694	23,657
Advertising	9,507	4,781	23,804	15,334
General and administrative expenses	31,134	25,646	91,981	85,254
Depreciation and amortization	9,917	7,819	27,715	23,329
Total operating expenses	158,876	156,261	490,828	452,324
Operating income	39,603	28,772	158,650	186,980
Other income (expense):
Interest income	251	103	2,230	2,351
Interest expense	(1,963)	(1,731)	(5,537)	(6,312)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total	37,891	27,144	155,343	183,019
Income tax expense	10,595	7,135	40,971	49,067
Net income	27,296	20,009	114,372	133,952
Less distributed and undistributed earnings allocated to participating securities	(39)	(104)	(219)	(792)
Net income allocated to common shares	$ 27,257	$ 19,905	$ 114,153	$ 133,160
Basic net income per share of common stock (in dollars per share)	$ 0.71	$ 0.52	$ 2.97	$ 3.45
Diluted net income per share of common stock (in dollars per share)	$ 0.70	$ 0.51	$ 2.94	$ 3.43
Revenue Composition [Abstract]
Gross billings	$ 7,994,006	$ 6,563,727	$ 23,682,279	$ 20,356,164
Worksite employee payroll cost	6,784,378	5,555,907	20,000,445	17,125,495
Revenues(1)	$ 1,209,628	$ 1,007,820	$ 3,681,834	$ 3,230,669