CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Revenues (gross billings of $4.163 billion, $3.703 billion, $8.727 billion and $7.643 billion less worksite employee payroll cost of $3.456 billion, $3.075 billion, $7.217 billion and $6.316 billion, respectively)	$ 707,332	$ 627,838	$ 1,509,740	$ 1,327,317
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	594,073	523,619	1,246,465	1,093,238
Gross profit	113,259	104,219	263,275	234,079
Operating expenses:
Salaries, wages and payroll taxes	55,998	50,234	114,013	106,982
Stock-based compensation	4,761	4,041	8,336	6,464
Commissions	4,335	4,103	8,616	8,407
Advertising	6,712	6,883	9,759	10,064
General and administrative expenses	21,254	20,838	45,038	45,430
Impairment charges and other	0	1,313	0	11,120
Depreciation and amortization	4,176	4,590	8,447	9,875
Total operating expenses	97,236	92,002	194,209	198,342
Operating income	16,023	12,217	69,066	35,737
Other income (expense):
Interest income	293	84	592	191
Interest expense	(650)	(124)	(1,287)	(224)
Income before income tax expense	15,666	12,177	68,371	35,704
Income tax expense	5,953	4,863	25,965	14,603
Net income	9,713	7,314	42,406	21,101
Less distributed and undistributed earnings allocated to participating securities	229	179	962	521
Net income allocated to common shares	$ 9,484	$ 7,135	$ 41,444	$ 20,580
Basic net income per share of common stock (in dollars per share)	$ 0.45	$ 0.29	$ 1.98	$ 0.83
Diluted net income per share of common stock (in dollars per share)	$ 0.45	$ 0.29	$ 1.98	$ 0.83