CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 151,821	$ 141,152
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	31,903	30,367
Stock-based compensation	41,676	38,818
Deferred income taxes	5,467	4,144
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Accounts receivable	(33,931)	(45,394)
Prepaid insurance and related assets	(9,554)	(19,083)
Other current assets	584	1,839
Other assets and ROU assets	3,740	(1,985)
Accounts payable	(525)	(946)
Payroll taxes and other payroll deductions payable	(38,024)	(115,763)
Accrued worksite employee payroll costs	46,534	85,333
Accrued health insurance costs	(2,642)	14,857
Accrued workers’ compensation costs	(7,500)	(11,541)
Accrued corporate payroll, commissions and other accrued liabilities	(42,757)	(12,636)
Income taxes payable/receivable	(6,521)	12,868
Total adjustments	(11,550)	(19,122)
Net cash provided by operating activities	140,271	122,030
Marketable securities:
Purchases	(39,596)	(35,618)
Proceeds from maturities	32,105	32,215
Proceeds from dispositions	27,735	0
Proceeds from Sale of Property, Plant, and Equipment [Abstract]
Property and equipment purchases	(22,526)	(16,448)
Net cash used in investing activities	(2,282)	(19,851)
Cash flows from financing activities:
Purchase of treasury stock	(131,473)	(63,420)
Dividends paid	(62,969)	(56,866)
Other	(314)	(3,130)
Net cash used in financing activities	(194,756)	(123,416)
Net decrease in cash, cash equivalents, restricted cash and funds held for clients	(56,767)	(21,237)
Cash, cash equivalents, restricted cash and funds held for clients beginning of period	1,013,919	839,500
Cash, cash equivalents, restricted cash and funds held for clients end of period	957,152	818,263
Supplemental cash flow information:
ROU assets obtained in exchange for lease obligations	11,678	4,513
Excise tax liability accrued for common stock repurchases	$ 545	$ 0