CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net income	$ 113,943	$ 104,845
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	15,510	13,599
Stock-based compensation	17,246	14,296
Deferred income taxes	4,668	8,260
Changes in operating assets and liabilities:
Accounts receivable	(8,156)	(23,512)
Prepaid insurance	(13,076)	(16,058)
Other current assets	8,842	(2,073)
Other assets and ROU assets	6,979	(2,033)
Accounts payable	(1,680)	(4,074)
Payroll taxes and other payroll deductions payable	(57,756)	(16,472)
Accrued worksite employee payroll expense	7,209	43,553
Accrued health insurance costs	8,534	(14,777)
Accrued workers’ compensation costs	(357)	2,133
Accrued corporate payroll, commissions and other accrued liabilities	(19,273)	(22,878)
Income taxes payable/receivable	32,228	(12,381)
Total adjustments	918	(32,417)
Net cash provided by operating activities	114,861	72,428
Marketable securities:
Purchases	(25,438)	(60,609)
Proceeds from dispositions	484	5,499
Proceeds from maturities	27,625	55,110
Property and equipment:
Purchases	(39,466)	(17,207)
Net cash used in investing activities	(36,795)	(17,207)
Cash flows from financing activities:
Purchase of treasury stock	(61,230)	(38,796)
Dividends paid	(31,074)	(24,740)
Borrowings under revolving line of credit	100,000	25,000
Other	(4,247)	(3,763)
Net cash provided by (used in) financing activities	11,943	(34,773)
Net increase in cash, cash equivalents and restricted cash	90,009	20,448
Cash, cash equivalents and restricted cash beginning of period	592,550	535,474
Cash, cash equivalents and restricted cash end of period	682,559	555,922
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	459,399	324,926
Restricted cash	45,216	43,268
Deposits workers compensation	177,944	187,728
ROU assets obtained in exchange for lease obligations	$ 10,196	$ 11,894