CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenues (gross billings of $4.742 billion, $4.163 billion, $9.758 billion and $8.727 billion less worksite employee payroll cost of $3.947 billion, $3.456 billion, $8.080 billion and $7.217 billion, respectively)	$ 795,552	$ 707,332	$ 1,678,216	$ 1,509,740
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	664,999	594,073	1,388,317	1,246,465
Gross profit	130,553	113,259	289,899	263,275
Operating expenses:
Salaries, wages and payroll taxes	61,458	55,998	123,915	114,013
Stock-based compensation	5,303	4,761	9,806	8,336
Commissions	5,664	4,335	10,140	8,616
Advertising	6,175	6,712	10,147	9,759
General and administrative expenses	24,610	21,254	50,802	45,038
Depreciation and amortization	4,405	4,176	8,659	8,447
Total operating expenses	107,615	97,236	213,469	194,209
Operating income	22,938	16,023	76,430	69,066
Other income (expense):
Interest income	678	293	1,143	592
Interest expense	(803)	(650)	(1,426)	(1,287)
Income before income tax expense	22,813	15,666	76,147	68,371
Income tax expense	8,795	5,953	26,501	25,965
Net income	14,018	9,713	49,646	42,406
Less distributed and undistributed earnings allocated to participating securities	248	229	909	962
Net income allocated to common shares	$ 13,770	$ 9,484	$ 48,737	$ 41,444
Basic net income per share of common stock (in dollars per share)	$ 0.67	$ 0.45	$ 2.37	$ 1.98
Diluted net income per share of common stock (in dollars per share)	$ 0.66	$ 0.45	$ 2.35	$ 1.98