CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues(1)	$ 993,366	$ 1,043,316	$ 2,222,849	$ 2,196,326
Payroll taxes, benefits and workers’ compensation costs	773,117	869,581	1,768,578	1,795,874
Gross profit	220,249	173,735	454,271	400,452
Salaries, wages and payroll taxes	90,710	74,696	177,211	158,076
Stock-based compensation	10,694	8,256	17,246	14,296
Commissions	7,475	7,741	15,935	14,693
Advertising	5,720	7,548	10,553	12,579
General and administrative expenses	24,755	29,866	59,608	63,028
Depreciation and amortization	7,908	6,908	15,510	13,599
Total operating expenses	147,262	135,015	296,063	276,271
Operating income	72,987	38,720	158,208	124,181
Other income (expense):
Interest income	369	2,802	2,248	6,047
Interest expense	(2,219)	(1,639)	(4,581)	(3,320)
Income before income tax expense	71,137	39,883	155,875	126,908
Income tax expense	19,286	11,327	41,932	22,063
Net income	51,851	28,556	113,943	104,845
Less distributed and undistributed earnings allocated to participating securities	(276)	(309)	(724)	(1,183)
Net income allocated to common shares	$ 51,575	$ 28,247	$ 113,219	$ 103,662
Basic net income per share of common stock (in dollars per share)	$ 1.34	$ 0.69	$ 2.93	$ 2.55
Diluted net income per share of common stock (in dollars per share)	$ 1.33	$ 0.69	$ 2.91	$ 2.54
Revenue Composition [Abstract]
Gross billings	$ 6,355,683	$ 6,377,014	$ 13,792,437	$ 13,248,684
Worksite employee payroll cost	5,362,317	5,333,698	11,569,588	11,052,358
Revenues(1)	$ 993,366	$ 1,043,316	$ 2,222,849	$ 2,196,326