CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenues (gross billings of $15.806 billion, $14.187 billion and $13.462 billion, less worksite employee payroll cost of $13.202 billion, $11.829 billion and $11.206 billion, respectively)	$ 2,603,614	$ 2,357,788	$ 2,256,112
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	2,165,747	1,953,983	1,862,861
Gross profit	437,867	403,805	393,251
Operating expenses:
Salaries, wages and payroll taxes	211,060	200,118	181,444
Stock-based compensation	13,345	11,053	11,103
Commissions	18,479	15,285	14,581
Advertising	15,980	20,084	21,508
General and administrative expenses	84,259	84,717	83,986
Impairment charges and other	10,480	3,687	3,342
Depreciation and amortization	18,565	21,387	21,064
Total operating expenses	372,168	356,331	337,028
Operating income	65,699	47,474	56,223
Other income (expense):
Interest, net	33	106	158
Other, net	(113)	47	(2,649)
Income before income tax expense	65,619	47,627	53,732
Income tax expense	26,229	19,623	21,700
Net income	39,390	28,004	32,032
Less distributed and undistributed earnings allocated to participating securities	981	2,002	916
Net income allocated to common shares	$ 38,409	$ 26,002	$ 31,116
Basic net income per share of common stock (in dollars per share)	$ 1.58	$ 1.05	$ 1.25
Diluted net income per share of common stock (in dollars per share)	$ 1.58	$ 1.05	$ 1.25