Unaudited Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Series D-1 Redeemable Convertible Preferred Stock [Member]
Cash flows from financing activities:
Proceeds from issuance of Series D-1 redeemable convertible preferred stock		$ 4,000
Series D Redeemable Convertible Preferred Stock [Member]
Cash flows from financing activities:
Proceeds from exercise of warrants		2
Net income (loss)	6,834	(35,716)
Loss on extinguishment of debt	4,740
Write-off of deferred offering costs	445
Stock-based compensation expense	2,279	964
Non-cash interest expense	2,443	1,280
Amortization of debt issuance costs and debt discount	835	954
Depreciation and amortization expense	55	40
Fair value adjustment	(31,041)	24,423
Prepaid expenses and other current assets	1,934	976
Other long-term assets	(2)
Accounts payable and accrued expenses	(693)	(2,175)
Other long-term liabilities	32
Net cash used in operating activities	(12,139)	(9,254)
Purchases of property and equipment	(219)	(40)
Restricted cash for collateral	(1,100)
Purchases of marketable securities and long-term investments	(65,627)
Maturity of marketable securities	10,645
Net cash used in investing activities	(56,301)	(40)
Proceeds from issuance of debt, net of discounts and commissions	82,800
Repayment of term notes and related accrued interest	(18,621)
Proceeds from initial public offering, net of discounts and commissions		59,892
Payment of deferred offering costs		(668)
Payment of debt and stock issuance costs	(673)
Repayment of obligations under capital lease	(15)	(15)
Proceeds from exercise of warrants	141	51
Net cash provided by financing activities	63,632	63,262
Net (decrease) increase in cash and cash equivalents	(4,808)	53,968
Cash and equivalents, beginning of period	32,318	10,255
Cash and equivalents, end of period	27,510	64,223
Supplemental cash flow information:
Cash paid for interest	182
Unpaid offering costs charged to equity		1,870
Transfer of warrants to equity upon exercise	755
Fixed assets included in accounts payable and accrued expenses	50
Conversion of preferred stock into common stock upon initial public offering		28,209
Reclassification of 2013 warrants to equity		1,110
Fixed assets financed under a capital lease agreement	$ 548	$ 42