CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income	$ 74,551	$ 49,646
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	10,693	8,659
Stock-based compensation	8,887	9,806
Deferred income taxes	(255)	8,465
Changes in operating assets and liabilities:
Accounts receivable	(17,773)	8,715
Prepaid insurance	(6,866)	(12,029)
Other current assets	4,351	1,533
Other assets	(1,957)	(2,375)
Accounts payable	(2,324)	(1,057)
Payroll taxes and other payroll deductions payable	(89,815)	(57,826)
Accrued worksite employee payroll expense	28,194	13,342
Accrued health insurance costs	(1,948)	219
Accrued workers’ compensation costs	9,926	12,832
Accrued corporate payroll, commissions and other accrued liabilities	(15,183)	(11,974)
Income taxes payable/receivable	16,808	(1,929)
Total adjustments	(57,262)	(23,619)
Net cash provided by operating activities	17,289	26,027
Marketable securities:
Purchases	(11,849)	(919)
Marketable Securities Proceeds From Dispositions	5,439	0
Proceeds from maturities	1,125	805
Property and equipment:
Purchases	(14,025)	(20,802)
Net cash used in investing activities	(19,310)	(20,916)
Cash flows from financing activities:
Purchase of treasury stock	(16,227)	(25,528)
Dividends paid	(16,786)	(11,579)
Other	1,110	830
Net cash used in financing activities	(31,903)	(36,277)
Net decrease in cash, cash equivalents and restricted cash	(33,924)	(31,166)
Begin of Period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	549,612	472,609
End of Period Cash, cash equivalents and restricted cash	515,688	441,443
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	308,711	241,890
Restricted cash	41,827	41,703
Deposits workers compensation	$ 165,150	$ 157,850