CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]
Revenues (gross billings of $3.588 billion and $3.332 billion, less worksite employee payroll cost of $2.951 billion and $2.270 billion, respectively)	$ 636,999	$ 611,836
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	530,823	503,718
Gross profit	106,176	108,118
Operating expenses:
Salaries, wages and payroll taxes	51,032	48,211
Stock-based compensation	2,400	2,310
Commissions	3,246	3,207
Advertising	4,941	5,250
General and administrative expenses	22,732	21,986
Depreciation and amortization	5,234	5,145
Total operating expenses	89,585	86,109
Operating income	16,591	22,009
Other income (expense):
Interest, net	47	69
Other, net	(26)	9
Income before income tax expense	16,612	22,087
Income tax expense	7,048	8,914
Net income	9,564	13,173
Less distributed and undistributed earnings allocated to participating securities	(282)	(383)
Net income allocated to common shares	$ 9,282	$ 12,790
Basic net income per share of common stock (in dollars per share)	$ 0.37	$ 0.51
Diluted net income per share of common stock (in dollars per share)	$ 0.37	$ 0.51