CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues (gross billings of $5.550 billion, $4.742 billion, $11.473 billion and $9.758 billion, less worksite employee payroll cost of $4.628 billion $3.947 billion, $9.537 billion, and $8.080 billion, respectively)	$ 922,295	$ 795,552	$ 1,936,667	$ 1,678,216
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	767,751	664,999	1,582,403	1,388,317
Gross profit	154,544	130,553	354,264	289,899
Operating expenses:
Salaries, wages and payroll taxes	68,748	61,458	155,934	123,915
Stock-based compensation	5,752	5,303	8,887	9,806
Commissions	6,979	5,664	13,045	10,140
Advertising	6,585	6,175	10,150	10,147
General and administrative expenses	27,419	24,610	57,271	50,802
Depreciation and amortization	5,480	4,405	10,693	8,659
Total operating expenses	120,963	107,615	255,980	213,469
Operating income	33,581	22,938	98,284	76,430
Other income (expense):
Interest income	1,807	678	3,263	1,143
Interest expense	(1,108)	(803)	(2,178)	(1,426)
Income before income tax expense	34,280	22,813	99,369	76,147
Income tax expense	9,720	8,795	24,818	26,501
Net income	24,560	14,018	74,551	49,646
Less distributed and undistributed earnings allocated to participating securities	346	248	1,064	909
Net income allocated to common shares	$ 24,214	$ 13,770	$ 73,487	$ 48,737
Basic net income per share of common stock (in dollars per share)	$ 0.59	$ 0.33	$ 1.78	$ 1.18
Diluted net income per share of common stock (in dollars per share)	$ 0.58	$ 0.33	$ 1.77	$ 1.18