CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 171,382	$ 179,350	$ 124,080
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	42,708	40,660	38,547
Stock-based compensation	52,996	50,080	40,623
Deferred income taxes	(4,814)	(10,641)	4,711
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Accounts receivable	(71,114)	(109,458)	(120,560)
Prepaid insurance and related assets	4,693	(421)	(1,121)
Other current assets	(14,215)	(5,206)	(13,851)
Other assets and ROU assets	23,523	2,600	5,240
Accounts payable	2,961	1,320	209
Payroll taxes and other payroll deductions payable	10,288	88,193	89,932
Accrued worksite employee payroll costs	45,797	103,744	74,817
Accrued health insurance costs	(6,942)	3,401	17,316
Accrued workers’ compensation costs	(9,787)	(10,114)	(197)
Accrued corporate payroll, commissions and other accrued liabilities	(44,620)	(5,179)	14,716
Income taxes payable/receivable	(4,368)	19,362	(14,307)
Total adjustments	27,106	168,341	136,075
Net cash provided by operating activities	198,488	347,691	260,155
Marketable securities:
Purchases	(47,983)	(46,748)	(58,202)
Proceeds from maturities	38,635	44,955	60,045
Proceeds from dispositions	27,735	0	0
Proceeds from Sale of Property, Plant, and Equipment [Abstract]
Property and equipment purchases	(40,117)	(30,329)	(32,856)
Net cash used in investing activities	(21,730)	(32,122)	(31,013)
Cash flows from financing activities:
Purchase of treasury stock	(131,519)	(73,285)	(69,725)
Dividends paid	(84,219)	(76,592)	(144,179)
Other	60,726	8,727	5,831
Net cash used in financing activities	(155,012)	(141,150)	(208,073)
Net increase in cash, cash equivalents, restricted cash and funds held for clients	21,746	174,419	21,069
Cash, cash equivalents, restricted cash and funds held for clients beginning of period	1,013,919	839,500	786,699
Cash, cash equivalents, restricted cash and funds held for clients end of period	1,035,665	1,013,919	839,500
Supplemental cash flow information:
Income Taxes Paid, Net	62,879	57,354	53,835
Interest Paid, Excluding Capitalized Interest, Operating Activities	22,067	13,402	7,268
ROU assets obtained in exchange for lease obligations	22,177	9,736	19,572
Excise tax liability accrued for common stock repurchases	$ 544	0	0
Previously Reported [Member]
Cash flows from financing activities:
Cash, cash equivalents, restricted cash and funds held for clients beginning of period		$ 807,768
Cash, cash equivalents, restricted cash and funds held for clients end of period			$ 807,768