CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
Revenues(1)	$ 1,439,160	$ 1,209,628	$ 4,449,104	$ 3,681,834
Payroll taxes, benefits and workers’ compensation costs	1,194,607	1,011,149	3,678,909	3,032,356
Gross profit	244,553	198,479	770,195	649,478
Salaries, wages and payroll taxes	109,525	89,232	323,486	286,669
Stock-based compensation	13,341	10,362	38,818	35,965
Commissions	11,068	8,724	32,121	24,694
Advertising	9,790	9,507	30,812	23,804
General and administrative expenses	38,115	31,134	115,215	91,981
Depreciation and amortization	10,083	9,917	30,367	27,715
Total operating expenses	191,922	158,876	570,819	490,828
Operating Income (Loss), Total	52,631	39,603	199,376	158,650
Other income (expense):
Interest income	2,808	251	3,901	2,230
Interest expense	(4,082)	(1,963)	(8,698)	(5,537)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total	51,357	37,891	194,579	155,343
Income tax expense	13,688	10,595	53,427	40,971
Net income	37,669	27,296	141,152	114,372
Less distributed and undistributed earnings allocated to participating securities	0	(39)	(27)	(219)
Net income allocated to common shares	$ 37,669	$ 27,257	$ 141,125	$ 114,153
Basic net income per share of common stock (in dollars per share)	$ 0.99	$ 0.71	$ 3.70	$ 2.97
Diluted net income per share of common stock (in dollars per share)	$ 0.98	$ 0.70	$ 3.66	$ 2.94
Revenue Composition [Abstract]
Gross billings	$ 9,528,695	$ 7,994,006	$ 29,111,243	$ 23,682,279
Worksite employee payroll cost	8,089,535	6,784,378	24,662,139	20,000,445
Revenues(1)	$ 1,439,160	$ 1,209,628	$ 4,449,104	$ 3,681,834