Condensed Consolidated Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net loss	$ (9,589)	$ (14,758)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	494	294
Stock-based compensation	998	803
Changes in operating assets and liabilities:
Other current assets and prepaid expenses	18	162
Accounts payable	(361)	111
Accrued expenses	(1,476)	(80)
Other liabilities	(40)	(38)
Net cash used in operating activities	(9,956)	(13,506)
Cash flows from investing activities:
Change in restricted cash	213	59
Proceeds from sale of equipment	2	0
Purchases of property and equipment	(125)	(674)
Net cash provided by (used in) investing activities	90	(615)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	4,325	0
Proceeds from exercise of stock options	32	139
Deferred financing costs	(30)	0
Net cash provided by financing activities	4,327	139
Effect of exchange rate changes on cash and cash equivalents	(1)	1
Net change in cash and cash equivalents	(5,540)	(13,981)
Cash and cash equivalents, beginning of period	83,416	102,238
Cash and cash equivalents, end of period	77,876	88,257
Supplemental disclosure of noncash investing and financing activities:
Purchases of property and equipment included in liabilities	0	934
Stock offering costs included in liabilities	65	0
Change in stock option early exercise repurchase liability	$ 23	$ 26