CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Revenues (gross billings of $3.703 billion, $3.281 billion, $7.643 billion and $6.869 billion less worksite employee payroll cost of $3.075 billion, $2.716 billion, $6.316 billion and $5.667 billion, respectively)	$ 627,838	$ 564,621	$ 1,327,317	$ 1,201,620
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	523,619	469,168	1,093,238	999,991
Gross profit	104,219	95,453	234,079	201,629
Operating expenses:
Salaries, wages and payroll taxes	50,234	47,829	106,982	98,861
Stock-based compensation	4,041	3,245	6,464	5,645
Commissions	4,103	3,717	8,407	6,963
Advertising	7,389	8,356	11,107	13,297
General and administrative expenses	20,332	21,116	44,387	43,848
Impairment charges and other	1,313	2,485	11,120	2,485
Depreciation and amortization	4,590	5,291	9,875	10,525
Total operating expenses	92,002	92,039	198,342	181,624
Operating income	12,217	3,414	35,737	20,005
Other income (expense):
Interest, net	(8)	24	(1)	71
Other, net	(32)	12	(32)	(14)
Income before income tax expense	12,177	3,450	35,704	20,062
Income tax expense	4,863	1,559	14,603	8,607
Net income	7,314	1,891	21,101	11,455
Less distributed and undistributed earnings allocated to participating securities	179	139	521	333
Net income allocated to common shares	$ 7,135	$ 1,752	$ 20,580	$ 11,122
Basic net income per share of common stock (in dollars per share)	$ 0.29	$ 0.07	$ 0.83	$ 0.45
Diluted net income per share of common stock (in dollars per share)	$ 0.29	$ 0.07	$ 0.83	$ 0.45