CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) [Abstract]
Revenues (gross billings of $2.731 billion $2.354 billion, $5.619 billion and $4.830 billion, less worksite employee payroll cost of $2.258 billion, $1.942 billion, $4.610 billion and $3.960 billion, respectively)	$ 472,903	$ 412,418	$ 1,009,284	$ 870,080
Direct costs:
Payroll taxes, benefits and workers' compensation costs	389,062	341,061	834,484	726,038
Gross profit	83,841	71,357	174,800	144,042
Operating expenses:
Salaries, wages and payroll taxes	38,467	34,505	78,064	73,692
Stock-based compensation	2,556	2,410	4,346	4,178
General and administrative expenses	17,023	14,634	38,916	32,128
Commissions	3,255	2,818	6,351	5,605
Advertising	7,539	4,698	13,045	8,575
Depreciation and amortization	3,601	3,723	7,549	7,534
Total Operating Expenses	72,441	62,788	148,271	131,712
Operating income	11,400	8,569	26,529	12,330
Other income (expense):
Interest income	304	255	588	458
Income before income tax expense	11,704	8,824	27,117	12,788
Income tax expense	4,963	3,706	11,590	5,371
Net income	6,741	5,118	15,527	7,417
Less net income allocated to participating securities	(199)	(51)	(463)	(214)
Net income allocated to common shares	$ 6,542	$ 5,067	$ 15,064	$ 7,203
Basic net income per share of common stock (in dollars per share)	$ 0.25	$ 0.20	$ 0.59	$ 0.29
Diluted net income per share of common stock (in dollars per share)	$ 0.25	$ 0.20	$ 0.59	$ 0.28