CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenues(1)	[1]	$ 4,314,804	$ 3,828,549	$ 3,300,223
Payroll taxes, benefits and workers’ compensation costs		3,581,870	3,146,640	2,727,492
Gross profit		732,934	681,909	572,731
Salaries, wages and payroll taxes		317,124	301,027	259,531
Stock-based compensation		23,993	20,425	24,345
Commissions		31,420	28,957	22,773
Advertising		21,603	18,554	16,686
General and administrative expenses		123,438	111,068	101,273
Depreciation and amortization		28,723	22,842	18,182
Total operating expenses		546,301	502,873	442,790
Operating income		186,633	179,036	129,941
Other income (expense):
Interest income		10,657	7,992	3,413
Interest expense		(7,647)	(4,668)	(3,213)
Income before income tax expense		189,643	182,360	130,141
Income tax expense		38,544	46,947	45,739
Net income		151,099	135,413	84,402
Less distributed and undistributed earnings allocated to participating securities		1,759	1,875	1,517
Net income allocated to common shares		$ 149,340	$ 133,538	$ 82,885
Basic net income per share of common stock (in dollars per share)		$ 3.72	$ 3.24	$ 2.02
Diluted net income per share of common stock (in dollars per share)		$ 3.70	$ 3.22	$ 2.01
Gross billings		$ 27,212,010	$ 23,830,731	$ 20,173,812
Worksite employee payroll cost		$ 22,897,206	$ 20,002,182	$ 16,873,589

[1] ⁽¹⁾  Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows:   Year ended December 31, (in thousands) 2019 2018 2017         Gross billings $ 27,212,010
$ 23,830,731
$ 20,173,812
Less: WSEE payroll cost 22,897,206
20,002,182
16,873,589
Revenues $ 4,314,804
$ 3,828,549
$ 3,300,223