CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net income	$ 110,758	$ 68,848
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	16,335	13,355
Stock-based compensation	14,656	16,390
Deferred income taxes	(591)	5,820
Changes in operating assets and liabilities:
Accounts receivable	(66,020)	(32,007)
Prepaid insurance	(2,973)	(3,937)
Other current assets	2,717	266
Other assets	(1,760)	(3,195)
Accounts payable	(1,131)	(1,119)
Payroll taxes and other payroll deductions payable	(99,161)	(62,310)
Accrued worksite employee payroll expense	72,713	41,488
Accrued health insurance costs	7,383	8,183
Accrued workers’ compensation costs	19,405	20,180
Accrued corporate payroll, commissions and other accrued liabilities	(11,887)	(7,639)
Income taxes payable/receivable	9,885	4,071
Total adjustments	(40,429)	(454)
Net cash provided by operating activities	70,329	68,394
Marketable securities:
Purchases	(54,754)	(1,337)
Proceeds from dispositions	16,299	0
Proceeds from maturities	2,650	1,181
Property and equipment:
Purchases	(21,519)	(26,518)
Net cash used in investing activities	(57,324)	(26,674)
Cash flows from financing activities:
Purchase of treasury stock	(16,236)	(27,172)
Dividends paid	(25,170)	(17,831)
Other	1,613	1,177
Net cash used in financing activities	(39,793)	(43,826)
Net decrease in cash, cash equivalents and restricted cash	(26,788)	(2,106)
Cash, cash equivalents and restricted cash beginning of period	549,612	472,609
Cash, cash equivalents and restricted cash end of period	522,824	470,503
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	328,299	285,934
Restricted cash	42,257	41,748
Deposits workers compensation	$ 152,268	$ 142,821