CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenues (gross billings of $5.923 billion and $5.016 billion, less worksite employee payroll cost of $4.909 billion and $4.133 billion, respectively)	$ 1,014,372	$ 882,664
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	814,652	723,318
Gross profit	199,720	159,346
Operating expenses:
Salaries, wages and payroll taxes	87,186	62,457
Stock-based compensation	3,135	4,503
Commissions	6,066	4,476
Advertising	3,565	3,972
General and administrative expenses	29,852	26,192
Depreciation and amortization	5,213	4,254
Total operating expenses	135,017	105,854
Operating income	64,703	53,492
Other income (expense):
Interest income	1,456	465
Interest expense	(1,070)	(623)
Income before income tax expense	65,089	53,334
Income tax expense	15,098	17,706
Net income	49,991	35,628
Less distributed and undistributed earnings allocated to participating securities	585	781
Net income allocated to common shares	$ 49,406	$ 34,847
Basic net income per share of common stock (in dollars per share)	$ 1.20	$ 0.85
Diluted net income per share of common stock (in dollars per share)	$ 1.18	$ 0.85