Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities
Net loss	$ (31,000)	$ (21,610)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	269	82
Stock-based compensation expense	3,260	1,978
Amortization of premium (discount) on marketable securities	345	435
Other non-cash charges	18	12
Loss on disposal of fixed assets	2,278
Premium paid on securities purchased	(22)
Changes in operating assets and liabilities:
Accounts receivable	(6)	(200)
Prepaid expenses, other current and long-term assets	(353)	(718)
Accounts payable	412	(196)
Accrued expenses and other current and long-term liabilities	(517)	1,180
Net cash used in operating activities	(25,316)	(19,037)
Cash flows from investing activities
Purchases of property and equipment	(7,678)	(1,427)
Change in restricted cash		24
Purchases of marketable securities	(10,811)	(99,477)
Sale and redemption of marketable securities	21,997	48,533
Discount received on securities purchased	7
Net cash provided by (used in) investing activities	3,515	(52,347)
Cash flows from financing activities
Payment of debt issuance costs	(42)
Payments on debt		(3,500)
Proceeds from the offering of common stock	77,644
Payments of public offering costs	(31)	(225)
Proceeds from the exercise of stock options	56	213
Proceeds from Employee Stock Purchase Plan	240	138
Net cash provided by (used in) financing activities	77,867	(3,374)
Net decrease in cash and cash equivalents	56,066	(74,758)
Cash and cash equivalents at beginning of period	62,944	103,098
Cash and cash equivalents at end of period	119,010	28,340
Supplemental disclosures of cash flow information:
Cash paid for interest	477	292
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	154	$ 487
Public offering costs included in accounts payable or accrued expenses	$ 236