CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Revenues (gross billings of $11.700 billion, $10.169 billion and $9.856 billion, less worksite employee payroll cost of $9.724 billion, $8.449 billion and $8.203 billion, respectively)	$ 1,976,219	$ 1,719,752	$ 1,653,096
Direct costs:
Payroll taxes, benefits and workers' compensation costs	1,624,444	1,421,216	1,365,129
Gross profit	351,775	298,536	287,967
Operating expenses:
Salaries, wages and payroll taxes	155,233	146,901	144,086
Stock-based compensation	8,601	8,126	10,064
Commissions	13,451	11,881	11,800
Advertising	26,613	16,447	16,011
General and administrative expenses	75,345	63,214	62,381
Depreciation and amortization	15,218	14,907	16,592
Total operating expenses	294,461	261,476	260,934
Operating income	57,314	37,060	27,033
Other income (expense):
Interest, net	969	991	1,616
Other, net	(7,508)	(30)	0
Income before income tax expense	50,775	38,021	28,649
Income tax expense	20,305	15,581	12,075
Net income	30,470	22,440	16,574
Less net income allocated to participating securities	(908)	(657)	(462)
Net income allocated to common shares	$ 29,562	$ 21,783	$ 16,112
Basic net income per share of common stock	$ 1.16	$ 0.86	$ 0.65
Diluted net income per share of common stock	$ 1.16	$ 0.86	$ 0.65