CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net income	$ 31,695	$ 15,497	$ 13,147
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	52,817	84,403	73,838
Deferred income taxes	14,954	43,842	(6,680)
Stock-based compensation	17,923	10,960	6,113
Excess tax benefit from equity incentive plan activity	(20,670)	(9,663)	(15,610)
Accretion of workers compensation and leases fair value adjustment	(639)	(1,090)	(1,427)
Changes in operating assets and liabilities:
Restricted cash and investments	(17,991)	(6,880)	(6,118)
Prepaid expenses and other current assets	1,313	(7,389)	(7,723)
Workers compensation receivables	3,152	(5,413)	9,876
Other assets	(14,527)	8,004	4,052
Accounts payable	287	5,212	976
Prepaid income taxes	24,494	(23,387)	6,394
Other current liabilities	5,616	7,749	13,186
Other liabilities	31,483	29,822	4,149
Worksite employee related assets	261,750	(862,699)	(304,265)
Worksite employee related liabilities	(261,058)	862,931	310,813
Net cash provided by operating activities	130,599	151,899	100,721
Investing activities
Acquisitions of businesses	(4,750)	0	(194,998)
Purchase of debt securities	(41,939)	(24,875)	(7,750)
Proceeds from maturity of debt securities	27,557	0	1,000
Purchase of property and equipment	(18,557)	(20,552)	(10,690)
Net cash used in investing activities	(37,689)	(45,427)	(212,438)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	0	217,796	0
Realized tax benefit of deductible IPO transaction costs	822	1,939	0
Proceeds from issuance of common stock on exercised options	7,166	2,193	7,109
Proceeds from issuance of common stock on employee stock purchase plan	5,315	3,393	0
Excess tax benefit from equity incentive plan activity	20,670	9,663	15,610
Borrowings under notes payable	0	0	970,000
Repayment of notes payable	(45,312)	(273,550)	(451,679)
Payment of debt issuance costs	0	(11,060)	(25,697)
Payments of special dividend	0	0	(357,582)
Repayments under capital leases	(250)	(306)	(778)
Repurchase of common stock	(48,364)	(15,009)	(5,963)
Awards effectively repurchased for required employee withholding taxes	(799)	(1,431)	(8,643)
Net cash provided by (used in) financing activities	(60,752)	(66,372)	142,377
Effect of exchange rate changes on cash and cash equivalents	(321)	(115)	(53)
Net increase in cash and cash equivalents	31,837	39,985	30,607
Cash and cash equivalents at beginning of period	134,341	94,356	63,749
Cash and cash equivalents at end of period	166,178	134,341	94,356
Supplemental disclosures of cash flow information
Cash paid for interest	15,224	32,051	30,534
Cash paid for income taxes, net	2,005	(3,809)	8,070
Supplemental schedule of noncash investing and financing activities
Payable for purchase of property and equipment	344	1,290	1,302
Allowance for tenant improvements	$ 1,257	$ 0	$ 0