CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities
Net income	$ 133,952	$ 130,704
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	23,329	20,929
Stock-based compensation	38,110	20,813
Deferred income taxes	6,757	8,659
Changes in operating assets and liabilities:
Accounts receivable	(62,933)	(83,267)
Prepaid insurance	(24,341)	(13,750)
Other current assets	9,721	1,283
Other assets and ROU assets	10,862	(2,709)
Accounts payable	1,451	(5,249)
Payroll taxes and other payroll deductions payable	(99,196)	(80,549)
Accrued worksite employee payroll expense	69,937	81,478
Accrued health insurance costs	23,459	(1,914)
Accrued workers’ compensation costs	(2,273)	6,087
Accrued corporate payroll, commissions and other accrued liabilities	(32,238)	(22,830)
Income taxes payable/receivable	9,977	(12,609)
Total adjustments	(27,378)	(83,628)
Net cash provided by operating activities	106,574	47,076
Marketable securities:
Purchases	(42,007)	(90,909)
Proceeds from dispositions	484	5,799
Proceeds from maturities	41,860	85,480
Property and equipment:
Purchases	(68,770)	(35,968)
Net cash used in investing activities	(68,433)	(35,598)
Cash flows from financing activities:
Purchase of treasury stock	(91,216)	(153,668)
Dividends paid	(46,522)	(36,777)
Borrowings under revolving line of credit	100,000	95,000
Other	(6,302)	(7,215)
Net cash used in financing activities	(31,436)	(88,230)
Net increase (decrease) in cash, cash equivalents and restricted cash	6,705	(76,752)
Cash, cash equivalents and restricted cash beginning of period	592,550	535,474
Cash, cash equivalents and restricted cash end of period	599,255	458,722
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	372,356	243,439
Restricted cash	45,265	45,251
Deposits workers compensation	181,634	170,032
ROU assets obtained in exchange for lease obligations	$ 18,364	$ 21,980