CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenues(1)	$ 1,153,010	$ 1,014,372
Payroll taxes, benefits and workers’ compensation costs	926,293	814,652
Gross profit	226,717	199,720
Salaries, wages and payroll taxes	83,380	87,186
Stock-based compensation	6,040	3,135
Commissions	6,952	6,066
Advertising	5,031	3,565
General and administrative expenses	33,162	29,852
Depreciation and amortization	6,691	5,213
Total operating expenses	141,256	135,017
Operating income	85,461	64,703
Other income (expense):
Interest income	3,245	1,456
Interest expense	(1,681)	(1,070)
Income before income tax expense	87,025	65,089
Income tax expense	10,736	15,098
Net income	76,289	49,991
Less distributed and undistributed earnings allocated to participating securities	1,031	585
Net income allocated to common shares	$ 75,258	$ 49,406
Basic net income per share of common stock (in dollars per share)	$ 1.86	$ 1.20
Diluted net income per share of common stock (in dollars per share)	$ 1.85	$ 1.18
Revenue Composition [Abstract]
Gross billings	$ 6,871,670	$ 5,923,356
Worksite employee payroll cost	5,718,660	4,908,984
Revenues(1)	$ 1,153,010	$ 1,014,372