Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities
Net loss	$ (16,815)	$ (9,174)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	186	40
Stock-based compensation expense	1,641	1,008
Amortization of premium (discount) on marketable securities	196	109
Other non-cash charges	9	12
Loss on disposal of fixed assets	2,278
Changes in operating assets and liabilities:
Accounts receivable	39	(403)
Prepaid expenses, other current and long-term assets	(572)	(987)
Accounts payable	(1,009)	593
Accrued expenses and other current and long-term liabilities	51	(1,176)
Net cash used in operating activities	(13,996)	(9,978)
Cash flows from investing activities
Purchases of property and equipment	(3,094)	(533)
Change in restricted cash		24
Purchases of marketable securities	(3,006)	(89,425)
Sale and redemption of marketable securities	16,097	27,465
Net cash provided by (used in) investing activities	9,997	(62,469)
Cash flows from financing activities
Payment of public offering costs		(225)
Payments on debt		(3,500)
Payment of debt issuance costs	(42)
Proceeds from the exercise of stock options		37
Net cash used in financing activities	(42)	(3,688)
Net decrease in cash and cash equivalents	(4,041)	(76,135)
Cash and cash equivalents at beginning of period	62,944	103,098
Cash and cash equivalents at end of period	58,903	26,963
Supplemental disclosures of cash flow information:
Cash paid for interest	237	$ 292
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	$ 2,450