CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net income	$ 130,704	$ 110,758
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	20,929	16,335
Stock-based compensation	20,813	14,656
Deferred income taxes	8,659	(591)
Changes in operating assets and liabilities:
Accounts receivable	(83,267)	(66,020)
Prepaid insurance	(13,750)	(2,973)
Other current assets	1,283	2,717
Other assets	(2,709)	(1,760)
Accounts payable	(5,249)	(1,131)
Payroll taxes and other payroll deductions payable	(80,549)	(99,161)
Accrued worksite employee payroll expense	81,478	72,713
Accrued health insurance costs	(1,914)	7,383
Accrued workers’ compensation costs	6,087	19,405
Accrued corporate payroll, commissions and other accrued liabilities	(22,830)	(11,887)
Income taxes payable/receivable	(12,609)	9,885
Total adjustments	(83,628)	(40,429)
Net cash provided by operating activities	47,076	70,329
Marketable securities:
Purchases	(90,909)	(54,754)
Proceeds from dispositions	5,799	16,299
Proceeds from maturities	85,480	2,650
Property and equipment:
Purchases	(35,968)	(21,519)
Net cash used in investing activities	(35,598)	(57,324)
Cash flows from financing activities:
Purchase of treasury stock	(153,668)	(16,236)
Dividends paid	(36,777)	(25,170)
Borrowings under revolving line of credit	95,000	0
Other	(7,215)	(1,613)
Net cash used in financing activities	(88,230)	(39,793)
Net decrease in cash, cash equivalents and restricted cash	(76,752)	(26,788)
Cash, cash equivalents and restricted cash beginning of period	535,474	549,612
Cash, cash equivalents and restricted cash end of period	458,722	522,824
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	243,439	328,299
Restricted cash	45,251	42,257
Deposits workers compensation	170,032	152,268
ROU assets obtained in exchange for lease obligations	$ 21,980	$ 0