CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Revenues (gross billings of $12.992 billion, $11.700 billion and $10.169 billion, less worksite employee payroll cost of $10.833 billion, $9.724 billion and $8.449 billion, respectively)	$ 2,158,824	$ 1,976,219	$ 1,719,752
Direct costs:
Payroll taxes, benefits and workers' compensation costs	1,776,603	1,624,444	1,421,216
Gross profit	382,221	351,775	298,536
Operating expenses:
Salaries, wages and payroll taxes	168,807	155,233	146,901
Stock-based compensation	9,814	8,601	8,126
Commissions	14,515	13,451	11,881
Advertising	21,586	26,613	16,447
General and administrative expenses	77,564	75,345	63,214
Impairment charge	4,191	0	0
Depreciation and amortization	18,250	15,218	14,907
Total operating expenses	314,727	294,461	261,476
Operating income	67,494	57,314	37,060
Other income (expense):
Interest, net	609	969	991
Other, net	187	(7,508)	(30)
Income before income tax expense	68,290	50,775	38,021
Income tax expense	27,888	20,305	15,581
Net income	40,402	30,470	22,440
Less distributed and undistributed earnings allocated to participating securities	(1,224)	(908)	(657)
Net income allocated to common shares	$ 39,178	$ 29,562	$ 21,783
Basic net income per share of common stock (in dollars per share)	$ 1.57	$ 1.16	$ 0.86
Diluted net income per share of common stock (in dollars per share)	$ 1.56	$ 1.16	$ 0.86