CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Revenues (gross billings of $3.068 billion, $2.835 billion, $9.339 billion and $8.454 billion, less worksite employee payroll cost of $2.556 billion, $2.363 billion, $7.712 billion and $6.973 billion, respectively)	$ 511,953	$ 471,821	$ 1,626,386	$ 1,481,105
Direct costs:
Payroll taxes benefits and workers compensation costs	413,533	384,792	1,337,668	1,219,276
Gross profit	98,420	87,029	288,718	261,829
Operating expenses:
Salaries wages and payroll taxes	44,032	39,494	127,402	117,558
Stock-based compensation	2,429	2,109	7,385	6,455
Commissions	3,358	3,399	10,299	9,750
Advertising	3,680	5,235	17,001	18,280
General and administrative expenses	21,122	18,912	61,694	57,828
Depreciation and amortization	4,659	3,786	13,336	11,335
Total operating expenses	79,280	72,935	237,117	221,206
Operating income	19,140	14,094	51,601	40,623
Other income (expense):
Interest, net	142	245	462	829
Other, net	(3)	(7,501)	141	(7,497)
Other income (expense)	139	(7,256)	603	(6,668)
Income before income tax expense	19,279	6,838	52,204	33,955
Income tax expense	7,827	2,739	21,247	14,329
Net income	11,452	4,099	30,957	19,626
Less net income allocated to participating securities	(334)	(120)	(898)	(582)
Net income allocated to common shares	$ 11,118	$ 3,979	$ 30,059	$ 19,044
Basic net income per share of common stock (in dollars per share)	$ 0.45	$ 0.16	$ 1.20	$ 0.75
Diluted net income per share of common stock (in dollars per share)	$ 0.45	$ 0.16	$ 1.20	$ 0.74