CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenues(1)	$ 925,126	[1]	$ 795,513	$ 2,861,793	$ 2,473,729
Payroll taxes, benefits and workers’ compensation costs	759,072		655,547	2,341,475	2,043,864
Gross profit	166,054		139,966	520,318	429,865
Salaries, wages and payroll taxes	70,552		65,223	226,486	189,138
Stock-based compensation	5,769		6,584	14,656	16,390
Commissions	6,818		5,675	19,863	15,815
Advertising	3,846		3,476	13,996	13,623
General and administrative expenses	25,294		24,513	82,565	75,315
Depreciation and amortization	5,642		4,696	16,335	13,355
Total operating expenses	117,921		110,167	373,901	323,636
Operating income	48,133		29,799	146,417	106,229
Other income (expense):
Interest income	2,028		1,015	5,291	2,158
Interest expense	(1,174)		(894)	(3,352)	(2,320)
Income before income tax expense	48,987		29,920	148,356	106,067
Income tax expense	12,780		10,718	37,598	37,219
Net income	36,207		19,202	110,758	68,848
Less distributed and undistributed earnings allocated to participating securities	503		337	1,546	1,233
Net income allocated to common shares	$ 35,704		$ 18,865	$ 109,212	$ 67,615
Basic net income per share of common stock (in dollars per share)	$ 0.86		$ 0.46	$ 2.64	$ 1.65
Diluted net income per share of common stock (in dollars per share)	$ 0.86		$ 0.46	$ 2.63	$ 1.64
Revenue Composition [Abstract]
Gross billings	$ 5,810,779		$ 4,898,333	$ 17,284,477	$ 14,655,545
Worksite employee payroll cost	4,885,653		4,102,820	14,422,684	12,181,816
Revenues(1)	$ 925,126	[1]	$ 795,513	$ 2,861,793	$ 2,473,729

[1] ____________________________________(1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Three Months Ended September 30, Nine Months Ended September 30,(in thousands)20182017 20182017 Gross billings$5,810,779$4,898,333 $17,284,477$14,655,545Less: WSEE payroll cost4,885,6534,102,820 14,422,68412,181,816Revenues$925,126$795,513 $2,861,793$2,473,729