CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Income Statement [Abstract]
Revenues(1)	$ 1,769,652	$ 1,577,837
Payroll taxes, benefits and workers’ compensation costs	1,437,506	1,292,063
Gross profit	332,146	285,774
Salaries, wages and payroll taxes	124,541	107,439
Share-Based Payment Arrangement, Expense	11,110	9,846
Commissions	11,017	10,310
Advertising	5,940	8,595
General and administrative expenses	48,034	41,005
Depreciation and amortization	10,497	10,184
Total operating expenses	211,139	187,379
Operating Income (Loss), Total	121,007	98,395
Other income (expense):
Interest income	8,777	148
Interest expense	(6,205)	(1,925)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total	123,579	96,618
Income tax expense	28,984	26,734
Net income	94,595	69,884
Less distributed and undistributed earnings allocated to participating securities	0	(47)
Net income allocated to common shares	$ 94,595	$ 69,837
Basic net income per share of common stock (in dollars per share)	$ 2.49	$ 1.82
Diluted net income per share of common stock (in dollars per share)	$ 2.45	$ 1.80
Revenue Composition [Abstract]
Gross billings	$ 11,451,262	$ 10,357,905
Worksite employee payroll cost	9,681,610	8,780,068
Revenues(1)	$ 1,769,652	$ 1,577,837