CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Revenues (gross billings of $3.940 billion and $3.588 billion, less worksite employee payroll cost of $3.241 billion and $2.951 billion, respectively)	$ 699,479	$ 636,999
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	569,619	530,823
Gross profit	129,860	106,176
Operating expenses:
Salaries, wages and payroll taxes	56,748	51,032
Stock-based compensation	2,423	2,400
Commissions	4,304	3,246
Advertising	3,718	4,941
General and administrative expenses	24,055	22,732
Impairment charges and other	9,807	0
Depreciation and amortization	5,285	5,234
Total operating expenses	106,340	89,585
Operating income	23,520	16,591
Other income (expense):
Interest, net	7	47
Other, net	0	(26)
Income before income tax expense	23,527	16,612
Income tax expense	9,740	7,048
Net income	13,787	9,564
Net income allocated to common shares	355	282
Net income allocated to common shares	$ 13,432	$ 9,282
Basic net income per share of common stock (in dollars per share)	$ 0.54	$ 0.37
Diluted net income per share of common stock (in dollars per share)	$ 0.54	$ 0.37