CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues(1)	$ 1,007,820	$ 1,043,388	$ 3,230,669	$ 3,239,714
Payroll taxes, benefits and workers’ compensation costs	822,787	872,842	2,591,365	2,668,716
Gross profit	185,033	170,546	639,304	570,998
Salaries, wages and payroll taxes	89,429	79,264	266,640	237,340
Stock-based compensation	20,864	6,517	38,110	20,813
Commissions	7,722	8,034	23,657	22,727
Advertising	4,781	4,895	15,334	17,474
General and administrative expenses	25,646	29,773	85,254	92,801
Depreciation and amortization	7,819	7,330	23,329	20,929
Total operating expenses	156,261	135,813	452,324	412,084
Operating income	28,772	34,733	186,980	158,914
Other income (expense):
Interest income	103	2,574	2,351	8,621
Interest expense	(1,731)	(2,122)	(6,312)	(5,442)
Income before income tax expense	27,144	35,185	183,019	162,093
Income tax expense	7,135	9,326	49,067	31,389
Net income	20,009	25,859	133,952	130,704
Less distributed and undistributed earnings allocated to participating securities	(104)	(284)	(792)	(1,546)
Net income allocated to common shares	$ 19,905	$ 25,575	$ 133,160	$ 129,158
Basic net income per share of common stock (in dollars per share)	$ 0.52	$ 0.64	$ 3.45	$ 3.19
Diluted net income per share of common stock (in dollars per share)	$ 0.51	$ 0.63	$ 3.43	$ 3.18
Revenue Composition [Abstract]
Gross billings	$ 6,563,727	$ 6,555,865	$ 20,356,164	$ 19,804,549
Worksite employee payroll cost	5,555,907	5,512,477	17,125,495	16,564,835
Revenues(1)	$ 1,007,820	$ 1,043,388	$ 3,230,669	$ 3,239,714