CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Revenues (gross billings of $3.332 billion and $3.231 billion, less worksite employee payroll cost of $2.720 billion and $2.636 billion, respectively)	$ 611,836	$ 595,177
Direct costs:
Payroll taxes, benefits and workers' compensation costs	503,718	492,173
Gross profit	108,118	103,004
Operating expenses:
Salaries, wages and payroll taxes	48,211	43,323
Stock-based compensation	2,310	2,155
Commissions	3,207	3,435
Advertising	5,250	4,755
General and administrative expenses	21,986	22,078
Depreciation and amortization	5,145	4,212
Total operating expenses	86,109	79,958
Operating income	22,009	23,046
Other income (expense):
Interest, net	69	164
Other, net	9	124
Income before income tax expense	22,087	23,334
Income tax expense	8,914	9,450
Net income	13,173	13,884
Less distributed and undistributed earnings allocated to participating securities	(383)	(402)
Net income allocated to common shares	$ 12,790	$ 13,482
Basic net income per share of common stock (in dollars per share)	$ 0.51	$ 0.54
Diluted net income per share of common stock (in dollars per share)	$ 0.51	$ 0.54