CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues(1)	$ 1,043,388	$ 925,126	$ 3,239,714	$ 2,861,793
Payroll taxes, benefits and workers’ compensation costs	872,842	759,072	2,668,716	2,341,475
Gross profit	170,546	166,054	570,998	520,318
Salaries, wages and payroll taxes	79,264	70,552	237,340	226,486
Stock-based compensation	6,517	5,769	20,813	14,656
Commissions	8,034	6,818	22,727	19,863
Advertising	4,895	3,846	17,474	13,996
General and administrative expenses	29,773	25,294	92,801	82,565
Depreciation and amortization	7,330	5,642	20,929	16,335
Total operating expenses	135,813	117,921	412,084	373,901
Operating income	34,733	48,133	158,914	146,417
Other income (expense):
Interest income	2,574	2,028	8,621	5,291
Interest expense	(2,122)	(1,174)	(5,442)	(3,352)
Income before income tax expense	35,185	48,987	162,093	148,356
Income tax expense	9,326	12,780	31,389	37,598
Net income	25,859	36,207	130,704	110,758
Less distributed and undistributed earnings allocated to participating securities	(284)	(503)	(1,546)	(1,546)
Net income allocated to common shares	$ 25,575	$ 35,704	$ 129,158	$ 109,212
Basic net income per share of common stock (in dollars per share)	$ 0.64	$ 0.86	$ 3.19	$ 2.64
Diluted net income per share of common stock (in dollars per share)	$ 0.63	$ 0.86	$ 3.18	$ 2.63
Revenue Composition [Abstract]
Gross billings	$ 6,555,865	$ 5,810,779	$ 19,804,549	$ 17,284,477
Worksite employee payroll cost	5,512,477	4,885,653	16,564,835	14,422,684
Revenues(1)	$ 1,043,388	$ 925,126	$ 3,239,714	$ 2,861,793