CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenues (gross billings of $3.826 billion, $3.362 billion, $11.469 billion and $10.231 billion less worksite employee payroll cost of $3.200 billion, $2.802 billion, $9.515 billion and $8.469 billion, respectively)	$ 626,286	$ 560,303	$ 1,953,603	$ 1,761,923
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	519,543	459,486	1,612,781	1,459,477
Gross profit	106,743	100,817	340,822	302,446
Operating expenses:
Salaries, wages and payroll taxes	51,329	49,384	158,311	148,245
Stock-based compensation	3,710	2,701	10,174	8,346
Commissions	4,516	3,790	12,923	10,753
Advertising	3,574	4,885	14,681	18,182
General and administrative expenses	19,191	20,295	63,578	64,143
Impairment charges and other	0	0	11,120	2,485
Depreciation and amortization	4,487	5,302	14,362	15,827
Total operating expenses	86,807	86,357	285,149	267,981
Operating income	19,936	14,460	55,673	34,465
Other income (expense):
Interest, net	3	9	2	80
Other, net	16	34	(16)	20
Income before income tax expense	19,955	14,503	55,659	34,565
Income tax expense	8,005	6,118	22,608	14,725
Net income	11,950	8,385	33,051	19,840
Less distributed and undistributed earnings allocated to participating securities	303	243	822	576
Net income allocated to common shares	$ 11,647	$ 8,142	$ 32,229	$ 19,264
Basic net income per share of common stock (in dollars per share)	$ 0.48	$ 0.33	$ 1.32	$ 0.78
Diluted net income per share of common stock (in dollars per share)	$ 0.48	$ 0.33	$ 1.32	$ 0.78