CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenues (gross billings of $14.187 billion, $13.462 billion and $12.992 billion, less worksite employee payroll cost of $11.829 billion, $11.206 billion and $10.833 billion, respectively)	$ 2,357,788	$ 2,256,112	$ 2,158,824
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	1,953,983	1,862,861	1,776,603
Gross profit	403,805	393,251	382,221
Operating expenses:
Salaries, wages and payroll taxes	200,118	181,444	168,807
Stock-based compensation	11,053	11,103	9,814
Commissions	15,285	14,581	14,515
Advertising	22,183	23,795	21,586
General and administrative expenses	82,618	81,699	77,564
Impairment charges	3,687	3,342	4,191
Depreciation and amortization	21,387	21,064	18,250
Total operating expenses	356,331	337,028	314,727
Operating income	47,474	56,223	67,494
Other income (expense):
Interest, net	106	158	609
Other, net	47	(2,649)	187
Income before income tax expense	47,627	53,732	68,290
Income tax expense	19,623	21,700	27,888
Net income	28,004	32,032	40,402
Less distributed and undistributed earnings allocated to participating securities	(2,002)	(916)	(1,224)
Net income allocated to common shares	$ 26,002	$ 31,116	$ 39,178
Basic net income per share of common stock (in dollars per share)	$ 1.05	$ 1.25	$ 1.57
Diluted net income per share of common stock (in dollars per share)	$ 1.05	$ 1.25	$ 1.56