CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities
Net income	$ 87,076	$ 113,943
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	17,798	15,510
Stock-based compensation	25,603	17,246
Deferred income taxes	9,161	4,668
Changes in operating assets and liabilities:
Accounts receivable	(203,559)	(8,156)
Prepaid insurance	(6,979)	(13,076)
Other current assets	(7,609)	8,842
Other assets and ROU assets	(5,407)	(6,979)
Accounts payable	(2,140)	(1,680)
Payroll taxes and other payroll deductions payable	(114,985)	(57,756)
Accrued worksite employee payroll expense	195,045	7,209
Accrued health insurance costs	(3,621)	8,534
Accrued workers’ compensation costs	(877)	(357)
Accrued corporate payroll, commissions and other accrued liabilities	13,697	(19,273)
Income taxes payable/receivable	(7,022)	32,228
Total adjustments	(80,081)	918
Net cash provided by operating activities	6,995	114,861
Marketable securities:
Purchases	(30,862)	(25,438)
Proceeds from dispositions	0	484
Proceeds from maturities	30,340	27,625
Property and equipment:
Purchases	(20,708)	(39,466)
Net cash used in investing activities	(21,230)	(36,795)
Cash flows from financing activities:
Purchase of treasury stock	(38,536)	(61,230)
Dividends paid	(32,828)	(31,074)
Borrowings under revolving line of credit	0	100,000
Other	(4,027)	(4,247)
Net cash provided by (used in) financing activities	(67,337)	11,943
Net increase (decrease) in cash, cash equivalents and restricted cash	(81,572)	90,009
Cash, cash equivalents and restricted cash beginning of period	786,699	592,550
Cash, cash equivalents and restricted cash end of period	705,127	682,559
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	455,387	459,399
Restricted cash	46,128	45,216
Deposits workers compensation	203,612	177,944
ROU assets obtained in exchange for lease obligations	$ 15,047	$ 10,196