CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Revenues (gross billings of $3.039 billion, $2.731 billion, $6.271 billion and $5.619 billion, less worksite employee payroll cost of $2.520 billion, $2.258 billion, $5.156 billion and $4.610 billion, respectively)	$ 519,256	$ 472,903	$ 1,114,433	$ 1,009,284
Direct costs:
Payroll taxes, benefits and workers' compensation costs	431,962	389,062	924,135	834,484
Gross profit	87,294	83,841	190,298	174,800
Operating expenses:
Salaries, wages and payroll taxes	40,047	38,467	83,370	78,064
Stock-based compensation	2,801	2,556	4,956	4,346
Commissions	3,506	3,255	6,941	6,351
Advertising	8,566	7,539	13,321	13,045
Depreciation and amortization	4,465	3,601	8,677	7,549
General and administrative expenses	18,494	17,023	40,572	38,916
Total operating expenses	77,879	72,441	157,837	148,271
Operating income	9,415	11,400	32,461	26,529
Other income (expense):
Interest, net	156	304	320	584
Other, net	20	0	144	4
Other income (expense)	176	304	464	588
Income before income tax expense	9,591	11,704	32,925	27,117
Income tax expense	3,970	4,963	13,420	11,590
Net income	5,621	6,741	19,505	15,527
Less net income allocated to participating securities	(162)	(199)	(564)	(463)
Net income allocated to common shares	$ 5,459	$ 6,542	$ 18,941	$ 15,064
Basic net income per share of common stock (in dollars per share)	$ 0.22	$ 0.25	$ 0.75	$ 0.59
Diluted net income per share of common stock (in dollars per share)	$ 0.22	$ 0.25	$ 0.75	$ 0.59