CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenues(1)	[1]	$ 4,973,070	$ 4,287,004	$ 4,314,804
Payroll taxes, benefits and workers’ compensation costs		4,152,968	3,480,150	3,581,870
Gross profit		820,102	806,854	732,934
Salaries, wages and payroll taxes		379,171	353,273	317,124
Stock-based compensation		40,623	60,145	23,993
Commissions		34,922	32,835	31,420
Advertising		29,097	21,556	21,603
General and administrative expenses		124,413	113,167	123,438
Depreciation and amortization		38,547	31,189	28,723
Total operating expenses		646,773	612,165	546,301
Operating income		173,329	194,689	186,633
Other income (expense):
Interest income		2,447	2,597	10,657
Interest expense		(7,458)	(8,016)	(7,647)
Income before income tax expense		168,318	189,270	189,643
Income tax expense		44,238	51,033	38,544
Net income		124,080	138,237	151,099
Less distributed and undistributed earnings allocated to participating securities		210	782	1,759
Net income allocated to common shares		$ 123,870	$ 137,455	$ 149,340
Basic net income per share of common stock (in dollars per share)		$ 3.22	$ 3.57	$ 3.72
Diluted net income per share of common stock (in dollars per share)		$ 3.18	$ 3.54	$ 3.70
Gross billings		$ 33,318,693	$ 28,168,611	$ 27,212,010
Worksite employee payroll cost		$ 28,345,623	$ 23,881,607	$ 22,897,206

[1] Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31, (in thousands) 202120202019 Gross billings $33,318,693 $28,168,611 $27,212,010  Less: WSEE payroll cost 28,345,623 23,881,607 22,897,206  Revenues $4,973,070 $4,287,004 $4,314,804