CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Cash flows from operating activities
Net income	$ 103,483	$ 87,076
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	20,284	17,798
Stock-based compensation	25,477	25,603
Deferred income taxes	14,064	9,161
Changes in operating assets and liabilities:
Accounts receivable	(172,551)	(203,559)
Prepaid insurance	(41,654)	(6,979)
Other current assets	(17,874)	(7,609)
Other assets and ROU assets	4,324	(5,407)
Accounts payable	(378)	(2,140)
Payroll taxes and other payroll deductions payable	(164,247)	(114,985)
Accrued worksite employee payroll costs	253,109	195,045
Accrued health insurance costs	12,642	(3,621)
Accrued workers’ compensation costs	(11,400)	(877)
Accrued corporate payroll, commissions and other accrued liabilities	(575)	13,697
Income taxes payable/receivable	6,729	(7,022)
Total adjustments	(80,698)	(80,081)
Net cash provided by operating activities	22,785	6,995
Marketable securities:
Purchases	(17,268)	(30,862)
Proceeds from maturities	17,665	30,340
Property and equipment:
Purchases	(9,043)	(20,708)
Net cash used in investing activities	(8,646)	(21,230)
Cash flows from financing activities:
Purchase of treasury stock	(56,805)	(38,536)
Dividends paid	(37,097)	(32,828)
Other	(2,270)	(4,027)
Net cash used in financing activities	(91,632)	(67,337)
Net decrease in cash, cash equivalents and restricted cash	(77,493)	(81,572)
Cash, cash equivalents and restricted cash beginning of period	807,768	786,699
Cash, cash equivalents and restricted cash end of period	730,275	705,127
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	510,869	455,387
Restricted cash	53,329	46,128
Deposits workers compensation	166,077	203,612
ROU assets obtained in exchange for lease obligations	$ 2,678	$ 15,047