CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues (gross billings of $20.174 billion, $17.933 billion, and $15.806 billion, less worksite employee payroll cost of $16.874 billion, $14.992 billion, and $13.202 billion, respectively)	$ 3,300,223	$ 2,941,347	$ 2,603,614
Direct costs:
Payroll taxes, benefits and workers’ compensation costs	2,727,492	2,449,737	2,165,747
Gross profit	572,731	491,610	437,867
Operating expenses:
Salaries, wages and payroll taxes	259,531	229,589	211,060
Stock-based compensation	24,345	16,643	13,345
Commissions	22,773	19,288	18,479
Advertising	16,686	16,447	15,980
General and administrative expenses	101,273	86,693	84,259
Impairment charges and other	0	0	10,480
Depreciation and amortization	18,182	16,644	18,565
Total operating expenses	442,790	385,304	372,168
Operating income	129,941	106,306	65,699
Other income (expense):
Interest Income	3,413	1,267	379
Interest Expense	(3,213)	(2,396)	(459)
Income before income tax expense	130,141	105,177	65,619
Income tax expense	45,739	39,186	26,229
Net income	84,402	65,991	39,390
Less distributed and undistributed earnings allocated to participating securities	1,517	1,496	981
Net income allocated to common shares	$ 82,885	$ 64,495	$ 38,409
Basic net income per share of common stock (in dollars per share)	$ 2.02	$ 1.55	$ 0.79
Diluted net income per share of common stock (in dollars per share)	$ 2.01	$ 1.54	$ 0.79