CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues(1)	[1]	$ 3,828,549	$ 3,300,223	$ 2,941,347
Payroll taxes, benefits and workers’ compensation costs		3,146,640	2,727,492	2,449,737
Gross profit		681,909	572,731	491,610
Salaries, wages and payroll taxes		301,027	259,531	229,589
Stock-based compensation		20,425	24,345	16,643
Commissions		28,957	22,773	19,288
Advertising		18,554	16,686	16,447
General and administrative expenses		111,068	101,273	86,693
Depreciation and amortization		22,842	18,182	16,644
Total operating expenses		502,873	442,790	385,304
Operating income		179,036	129,941	106,306
Other income (expense):
Interest Income		7,992	3,413	1,267
Interest Expense		(4,668)	(3,213)	(2,396)
Income before income tax expense		182,360	130,141	105,177
Income tax expense		46,947	45,739	39,186
Net income		135,413	84,402	65,991
Less distributed and undistributed earnings allocated to participating securities		1,875	1,517	1,496
Net income allocated to common shares		$ 133,538	$ 82,885	$ 64,495
Basic net income per share of common stock (in dollars per share)		$ 3.24	$ 2.02	$ 1.55
Diluted net income per share of common stock (in dollars per share)		$ 3.22	$ 2.01	$ 1.54

[1] (1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31,(in thousands)201820172016 Gross billings$23,830,731$20,173,812$17,932,857Less: WSEE payroll cost20,002,18216,873,58914,991,510Revenues$3,828,549$3,300,223$2,941,347