CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Revenues (gross billings of $3.362 billion, $3.236 billion, $10.231 billion and $9.736 billion, less worksite employee payroll cost of $2.802 billion, $2.696 billion, $8.469 billion and $8.037 billion, respectively)	$ 560,303	$ 539,869	$ 1,761,923	$ 1,698,979
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	459,486	442,460	1,459,477	1,395,706
Gross profit	100,817	97,409	302,446	303,273
Operating expenses:
Salaries, wages and payroll taxes	49,384	43,797	148,245	137,697
Stock-based compensation	2,701	2,749	8,346	8,351
Commissions	3,790	3,609	10,753	10,349
Advertising	4,885	4,273	18,182	19,243
General and administrative expenses	20,295	20,567	64,143	62,592
Goodwill and Intangible Asset Impairment	0	0	2,485	0
Depreciation and amortization	5,302	5,302	15,827	15,692
Total operating expenses	86,357	80,297	267,981	253,924
Operating income	14,460	17,112	34,465	49,349
Other income (expense):
Interest, net	9	26	80	155
Other, net	34	(1)	20	(2,668)
Income before income tax expense	14,503	17,137	34,565	46,836
Income tax expense	6,118	7,055	14,725	20,093
Net income	8,385	10,082	19,840	26,743
Less distributed and undistributed earnings allocated to participating securities	(243)	(289)	(576)	(769)
Net income allocated to common shares	$ 8,142	$ 9,793	$ 19,264	$ 25,974
Basic net income per share of common stock (in dollars per share)	$ 0.33	$ 0.39	$ 0.78	$ 1.05
Diluted net income per share of common stock (in dollars per share)	$ 0.33	$ 0.39	$ 0.78	$ 1.04