CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) [Abstract]
Revenues (gross billings of $2.835 billion, $2.444 billion, $8.454 billion and $7.274 billion, less worksite employee payroll cost of $2.363 billion, $2.030 billion, $6.973 billion and $5.990 billion, respectively)	$ 471,821	$ 414,146	$ 1,481,105	$ 1,284,226
Direct costs:
Payroll taxes, benefits and workers' compensation costs	384,792	340,460	1,219,276	1,066,498
Gross profit	87,029	73,686	261,829	217,728
Operating expenses:
Salaries, wages and payroll taxes	39,494	34,866	117,558	108,558
Stock-based compensation	2,109	1,970	6,455	6,148
Commissions	3,399	2,889	9,750	8,494
Advertising	5,235	2,605	18,280	11,180
General and administrative expenses	18,912	15,546	57,828	47,674
Depreciation and amortization	3,786	3,732	11,335	11,266
Total Operating Expenses	72,935	61,608	221,206	193,320
Operating income	14,094	12,078	40,623	24,408
Other income (expense):
Interest, net	245	286	829	744
Other, net	(7,501)	0	(7,497)	0
Income before income tax expense	6,838	12,364	33,955	25,152
Income tax expense	2,739	5,130	14,329	10,501
Net income	4,099	7,234	19,626	14,651
Less net income allocated to participating securities	(120)	(214)	(582)	(428)
Net income allocated to common shares	$ 3,979	$ 7,020	$ 19,044	$ 14,223
Basic net income per share of common stock (in dollars per share)	$ 0.16	$ 0.28	$ 0.75	$ 0.56
Diluted net income per share of common stock (in dollars per share)	$ 0.16	$ 0.28	$ 0.74	$ 0.56