CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net income	$ 114,372	$ 133,952
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	27,715	23,329
Stock-based compensation	35,965	38,110
Deferred income taxes	7,950	6,757
Changes in operating assets and liabilities:
Accounts receivable	(186,757)	(62,933)
Prepaid insurance	(9,493)	(24,341)
Other current assets	(13,832)	9,721
Other assets and ROU assets	(4,153)	(10,862)
Accounts payable	(1,953)	1,451
Payroll taxes and other payroll deductions payable	(145,098)	(99,196)
Accrued worksite employee payroll expense	200,054	69,937
Accrued health insurance costs	7,681	23,459
Accrued workers’ compensation costs	(864)	(2,273)
Accrued corporate payroll, commissions and other accrued liabilities	22,428	(32,238)
Income taxes payable/receivable	(7,478)	9,977
Total adjustments	(59,529)	(27,378)
Net cash provided by operating activities	54,843	106,574
Marketable securities:
Purchases	(41,413)	(42,007)
Proceeds from dispositions	0	484
Proceeds from maturities	42,600	41,860
Property and equipment:
Purchases	(23,603)	(68,770)
Net cash used in investing activities	(22,416)	(68,433)
Cash flows from financing activities:
Purchase of treasury stock	(49,770)	(91,216)
Dividends paid	(50,198)	(46,522)
Borrowings under revolving line of credit	0	100,000
Other	(4,812)	(6,302)
Net cash used in financing activities	(95,156)	(31,436)
Net increase (decrease) in cash, cash equivalents and restricted cash	(62,729)	6,705
Cash, cash equivalents and restricted cash beginning of period	786,699	592,550
Cash, cash equivalents and restricted cash end of period	723,970	599,255
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	467,921	372,356
Restricted cash	47,813	45,265
Deposits workers compensation	208,236	181,634
ROU assets obtained in exchange for lease obligations	$ 16,275	$ 18,364