CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) [Abstract]
Revenues (gross billings of $3.231 billion and $2.888 billion, less worksite employee payroll cost of $2.636 billion and $2.352 billion, respectively)	$ 595,177	$ 536,381
Direct costs:
Payroll taxes, benefits and workers' compensation costs	492,173	445,422
Gross profit	103,004	90,959
Operating expenses:
Salaries, wages and payroll taxes	43,323	39,597
Stock-based compensation	2,155	1,790
General and administrative expenses	22,078	21,893
Commissions	3,435	3,096
Advertising	4,755	5,506
Depreciation and amortization	4,212	3,948
Total operating expenses	79,958	75,830
Operating income	23,046	15,129
Other income (expense):
Interest, net	164	280
Other, net	124	4
Income before income tax expense	23,334	15,413
Income tax expense	9,450	6,627
Net income	13,884	8,786
Less net income allocated to participating securities	(402)	(264)
Net income allocated to common shares	$ 13,482	$ 8,522
Basic net income per share of common stock (in dollars per share)	$ 0.54	$ 0.33
Diluted net income per share of common stock (in dollars per share)	$ 0.54	$ 0.33