CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Revenues (gross billings of $4.314 billion, $3.826 billion, $13.040 billion and $11.469 billion less worksite employee payroll cost of $3.611 billion, $3.200 billion, $10.828 billion and $9.515 billion, respectively)	$ 702,538	$ 626,286	$ 2,212,278	$ 1,953,603
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	584,742	519,543	1,831,207	1,612,781
Gross profit	117,796	106,743	381,071	340,822
Operating expenses:
Salaries, wages and payroll taxes	56,897	51,329	170,910	158,311
Stock-based compensation	4,191	3,710	12,527	10,174
Commissions	5,030	4,516	13,646	12,923
Advertising	3,540	3,193	13,299	13,257
General and administrative expenses	21,318	19,572	66,356	65,002
Impairment charges and other	0	0	0	11,120
Depreciation and amortization	4,047	4,487	12,494	14,362
Total operating expenses	95,023	86,807	289,232	285,149
Operating income	22,773	19,936	91,839	55,673
Other income (expense):
Interest income	335	145	927	336
Interest expense	(628)	(126)	(1,915)	(350)
Income before income tax expense	22,480	19,955	90,851	55,659
Income tax expense	8,415	8,005	34,380	22,608
Net income	14,065	11,950	56,471	33,051
Less distributed and undistributed earnings allocated to participating securities	330	303	1,283	822
Net income allocated to common shares	$ 13,735	$ 11,647	$ 55,188	$ 32,229
Basic net income per share of common stock (in dollars per share)	$ 0.66	$ 0.48	$ 2.64	$ 1.32
Diluted net income per share of common stock (in dollars per share)	$ 0.66	$ 0.48	$ 2.64	$ 1.32