CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net income	$ 76,289	$ 49,991
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	6,691	5,213
Stock-based compensation	6,040	3,135
Deferred income taxes	8,671	7,116
Changes in operating assets and liabilities:
Accounts receivable	(20,674)	(2,567)
Prepaid insurance	(16,517)	(24,897)
Other current assets	(8,893)	(1,402)
Other assets	(809)	(1,952)
Accounts payable	(2,768)	(930)
Payroll taxes and other payroll deductions payable	46,896	(26,674)
Accrued worksite employee payroll expense	33,883	14,072
Accrued health insurance costs	10,679	20,720
Accrued workers’ compensation costs	1,367	6,094
Accrued corporate payroll, commissions and other accrued liabilities	(23,656)	(24,854)
Income taxes payable/receivable	498	7,129
Total adjustments	41,408	(19,797)
Net cash provided by operating activities	117,697	30,194
Marketable securities:
Purchases	(35,538)	(512)
Proceeds from dispositions	5,499	0
Proceeds from maturities	37,360	525
Property and equipment:
Purchases	(5,608)	(6,585)
Net cash provided by (used in) investing activities	1,713	(6,572)
Cash flows from financing activities:
Purchase of treasury stock	(29,037)	(8,565)
Dividends paid	12,386	8,402
Other	1,085	351
Net cash used in financing activities	(40,338)	(16,616)
Net decrease in cash, cash equivalents and restricted cash	79,072	7,006
Cash, cash equivalents and restricted cash beginning of period	535,474	549,612
Cash, cash equivalents and restricted cash end of period	614,546	556,618
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	398,936	354,641
Restricted cash	44,705	41,137
Deposits workers compensation	170,905	160,840
ROU assets obtained in exchange for lease obligations	$ 3,140	$ 0