CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Cash flows from operating activities
Net income	$ 141,152	$ 114,372
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	30,367	27,715
Stock-based compensation	38,818	35,965
Deferred income taxes	4,144	7,950
Changes in operating assets and liabilities:
Accounts receivable	(45,394)	(186,757)
Prepaid insurance	(19,083)	(9,493)
Other current assets	1,839	(13,832)
Other assets and ROU assets	1,985	(4,153)
Accounts payable	(946)	(1,953)
Payroll taxes and other payroll deductions payable	(115,763)	(145,098)
Accrued worksite employee payroll costs	85,333	200,054
Accrued health insurance costs	14,857	7,681
Accrued workers’ compensation costs	(11,541)	(864)
Accrued corporate payroll, commissions and other accrued liabilities	(12,636)	22,428
Income taxes payable/receivable	12,868	(7,478)
Total adjustments	(19,122)	(59,529)
Net cash provided by operating activities	122,030	54,843
Marketable securities:
Purchases	(35,618)	(41,413)
Proceeds from maturities	32,215	42,600
Property and equipment:
Purchases	(16,448)	(23,603)
Net cash used in investing activities	(19,851)	(22,416)
Cash flows from financing activities:
Purchase of treasury stock	(63,420)	(49,770)
Dividends paid	(56,866)	(50,198)
Other	3,130	(4,812)
Net cash used in financing activities	(123,416)	(95,156)
Net decrease in cash, cash equivalents, restricted cash and funds held for clients	(21,237)	(62,729)
Cash, cash equivalents, restricted cash and funds held for clients beginning of period	839,500	786,699
Cash, cash equivalents, restricted cash and funds held for clients end of period	818,263	723,970
Supplemental Cash Flow Elements [Abstract]
ROU assets obtained in exchange for lease obligations	$ 4,513	$ 16,275