CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Statement [Abstract]
Revenues (gross billings of $3.167 billion, $3.039 billion, $6.499 billion and $6.271 billion, less worksite employee payroll cost of $2.620 billion, $2.520 billion, $5.340 billion and $5.156 billion, respectively)	$ 547,274	$ 519,256	$ 1,159,110	$ 1,114,433
Direct costs:
Payroll taxes, benefits and workers' compensation costs	449,528	431,962	953,246	924,135
Gross profit	97,746	87,294	205,864	190,298
Operating expenses:
Salaries, wages and payroll taxes	45,689	40,047	93,900	83,370
Stock-based compensation	3,292	2,801	5,602	4,956
Commissions	3,533	3,506	6,740	6,941
Advertising	9,720	8,566	14,970	13,321
General and administrative expenses	20,039	18,494	42,025	40,572
Depreciation and amortization	5,245	4,465	10,390	8,677
Total operating expenses	87,518	77,879	173,627	157,837
Operating income	10,228	9,415	32,237	32,461
Other income (expense):
Interest, net	60	156	129	320
Other, net	(2,676)	20	(2,667)	144
Income before income tax expense	7,612	9,591	29,699	32,925
Income tax expense	4,124	3,970	13,038	13,420
Net income	3,488	5,621	16,661	19,505
Less distributed and undistributed earnings allocated to participating securities	(124)	(162)	(481)	(564)
Net income allocated to common shares	$ 3,364	$ 5,459	$ 16,180	$ 18,941
Basic net income per share of common stock (in dollars per share)	$ 0.14	$ 0.22	$ 0.65	$ 0.75
Diluted net income per share of common stock (in dollars per share)	$ 0.14	$ 0.22	$ 0.65	$ 0.75