CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenues(1)	$ 1,286,835	$ 1,229,483
Payroll taxes, benefits and workers’ compensation costs	1,035,390	995,461
Gross profit	251,445	234,022
Salaries, wages and payroll taxes	103,075	86,501
Stock-based compensation	11,822	6,552
Commissions	7,719	8,460
Advertising	5,322	4,833
General and administrative expenses	31,636	34,853
Depreciation and amortization	8,047	7,602
Total operating expenses	167,621	148,801
Operating income	83,824	85,221
Other income (expense):
Interest income	543	1,879
Interest expense	(1,599)	(2,362)
Income before income tax expense	82,768	84,738
Income tax expense	20,846	22,646
Net income	61,922	62,092
Less distributed and undistributed earnings allocated to participating securities	(197)	(462)
Net income allocated to common shares	$ 61,725	$ 61,630
Basic net income per share of common stock (in dollars per share)	$ 1.62	$ 1.59
Diluted net income per share of common stock (in dollars per share)	$ 1.59	$ 1.58
Revenue Composition [Abstract]
Gross billings	$ 8,050,422	$ 7,436,754
Worksite employee payroll cost	6,763,587	6,207,271
Revenues(1)	$ 1,286,835	$ 1,229,483