CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Revenues(1)	$ 1,550,887	$ 1,439,160	$ 4,905,668	$ 4,449,104
Payroll taxes, benefits and workers’ compensation costs	1,292,956	1,194,607	4,090,952	3,678,909
Gross profit	257,931	244,553	814,716	770,195
Salaries, wages and payroll taxes	113,074	109,525	348,557	323,486
Stock-based compensation	15,210	13,341	41,676	38,818
Commissions	10,773	11,068	33,828	32,121
Advertising	7,240	9,790	29,775	30,812
General and administrative expenses	41,504	38,115	132,699	115,215
Depreciation and amortization	10,666	10,083	31,903	30,367
Total operating expenses	198,467	191,922	618,438	570,819
Operating income	59,464	52,631	196,278	199,376
Other income (expense):
Interest income	7,950	2,808	24,693	3,901
Interest expense	(7,047)	(4,082)	(19,939)	(8,698)
Income before income tax expense	60,367	51,357	201,032	194,579
Income tax expense	16,035	13,688	49,211	53,427
Net income	$ 44,332	$ 37,669	$ 151,821	$ 141,152
Basic net income per share of common stock (in dollars per share)	$ 1.17	$ 0.99	$ 4.00	$ 3.70
Diluted net income per share of common stock (in dollars per share)	$ 1.16	$ 0.98	$ 3.94	$ 3.66
Revenue Composition [Abstract]
Gross billings	$ 10,067,191	$ 9,528,695	$ 31,762,946	$ 29,111,243
Worksite employee payroll cost	8,516,304	8,089,535	26,857,278	24,662,139
Revenues(1)	$ 1,550,887	$ 1,439,160	$ 4,905,668	$ 4,449,104