CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 6,485,871	$ 5,938,818	$ 4,973,070
Payroll taxes, benefits and workers’ compensation costs	5,449,068	4,927,585	4,152,968
Gross profit	1,036,803	1,011,233	820,102
Salaries, wages and payroll taxes	460,715	430,945	379,171
Stock-based compensation	52,996	50,080	40,623
Commissions	46,847	45,672	34,922
Advertising	37,324	37,503	29,097
General and administrative expenses	177,664	156,134	124,413
Depreciation and amortization	42,708	40,660	38,547
Total operating expenses	818,254	760,994	646,773
Operating income	218,549	250,239	173,329
Other income (expense):
Interest income	33,666	9,393	2,447
Interest expense	(27,137)	(14,207)	(7,458)
Income before income tax expense	225,078	245,425	168,318
Income tax expense	53,696	66,075	44,238
Net income	171,382	179,350	124,080
Unrealized gain (loss) on marketable securities, net of tax	91	(73)	(14)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 171,473	$ 179,277	$ 124,066
Basic net income per share of common stock (in dollars per share)	$ 4.53	$ 4.70	$ 3.22
Diluted net income per share of common stock (in dollars per share)	$ 4.47	$ 4.64	$ 3.18