CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Income Statement [Abstract]
Revenues(1)	$ 1,577,837	$ 1,286,835
Payroll taxes, benefits and workers’ compensation costs	1,292,063	1,035,390
Gross profit	285,774	251,445
Salaries, wages and payroll taxes	107,439	103,075
Stock-based compensation	9,846	11,822
Commissions	10,310	7,719
Advertising	8,595	5,322
General and administrative expenses	41,005	31,636
Depreciation and amortization	10,184	8,047
Total operating expenses	187,379	167,621
Operating Income (Loss), Total	98,395	83,824
Other income (expense):
Interest income	148	543
Interest expense	(1,925)	(1,599)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total	96,618	82,768
Income tax expense	26,734	20,846
Net income	69,884	61,922
Less distributed and undistributed earnings allocated to participating securities	(47)	(197)
Net income allocated to common shares	$ 69,837	$ 61,725
Basic net income per share of common stock (in dollars per share)	$ 1.82	$ 1.62
Diluted net income per share of common stock (in dollars per share)	$ 1.80	$ 1.59
Revenue Composition [Abstract]
Gross billings	$ 10,357,905	$ 8,050,422
Worksite employee payroll cost	8,780,068	6,763,587
Revenues(1)	$ 1,577,837	$ 1,286,835