CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net income	$ 61,922	$ 62,092
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	8,047	7,602
Stock-based compensation	11,822	6,552
Deferred income taxes	22,946	12,560
Changes in operating assets and liabilities:
Accounts receivable	(168,498)	(54,966)
Prepaid insurance	(47,506)	(35,107)
Other current assets	(14,257)	3,068
Increase (Decrease) in Other Noncurrent Assets	(1,508)	(3,403)
Accounts payable	(362)	1,675
Payroll taxes and other payroll deductions payable	(67,441)	(9,003)
Accrued worksite employee payroll expense	168,498	34,905
Accrued health insurance costs	39,451	25,661
Accrued workers’ compensation costs	(3,393)	3,467
Accrued corporate payroll, commissions and other accrued liabilities	(5,421)	(46,531)
Income taxes payable/receivable	(5,345)	8,409
Total adjustments	(59,951)	(38,305)
Net cash provided by operating activities	1,971	23,787
Marketable securities:
Purchases	(10,585)	(8,689)
Proceeds from maturities	10,580	24,000
Property and equipment:
Purchases	(12,072)	(15,625)
Net cash used in investing activities	(12,077)	(314)
Cash flows from financing activities:
Purchase of treasury stock	(29,686)	(61,203)
Dividends paid	(15,461)	(15,557)
Borrowings under revolving line of credit	0	100,000
Other	(2,751)	(2,363)
Net cash provided by (used in) financing activities	(42,396)	25,603
Net increase (decrease) in cash, cash equivalents and restricted cash	(52,502)	49,076
Cash, cash equivalents and restricted cash beginning of period	786,699	592,550
Cash, cash equivalents and restricted cash end of period	734,197	641,626
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	494,777	404,728
Restricted cash	46,353	48,349
Deposits workers compensation	193,067	188,549
ROU assets obtained in exchange for lease obligations	$ 12,104	$ 6,787