CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Revenues (gross billings of $3.281 billion, $3.167 billion, $6.869 billion and $6.499 billion less worksite employee payroll cost of $2.716 billion, $2.620 billion, $5.667 billion and $5.340 billion, respectively)	$ 564,621	$ 547,274	$ 1,201,620	$ 1,159,110
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	469,168	449,528	999,991	953,246
Gross profit	95,453	97,746	201,629	205,864
Operating expenses:
Salaries, wages and payroll taxes	47,829	45,689	98,861	93,900
Stock-based compensation	3,245	3,292	5,645	5,602
Commissions	3,717	3,533	6,963	6,740
Advertising	8,356	9,720	13,297	14,970
General and administrative expenses	21,116	20,039	43,848	42,025
Impairment charge	2,485	0	2,485	0
Depreciation and amortization	5,291	5,245	10,525	10,390
Total operating expenses	92,039	87,518	181,624	173,627
Operating income	3,414	10,228	20,005	32,237
Other income (expense):
Interest, net	24	60	71	129
Other, net	12	(2,676)	(14)	(2,667)
Income before income tax expense	3,450	7,612	20,062	29,699
Income tax expense	1,559	4,124	8,607	13,038
Net income	1,891	3,488	11,455	16,661
Less distributed and undistributed earnings allocated to participating securities	(139)	(124)	(333)	(481)
Net income allocated to common shares	$ 1,752	$ 3,364	$ 11,122	$ 16,180
Basic net income per share of common stock (in dollars per share)	$ 0.07	$ 0.14	$ 0.45	$ 0.65
Diluted net income per share of common stock (in dollars per share)	$ 0.07	$ 0.14	$ 0.45	$ 0.65