Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (24,288)us-gaap_NetIncomeLoss	$ (15,943)us-gaap_NetIncomeLoss
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash interest expense	83us-gaap_PaidInKindInterest	59us-gaap_PaidInKindInterest
Depreciation	508us-gaap_Depreciation	412us-gaap_Depreciation
Stock-based compensation	1,960us-gaap_ShareBasedCompensation	1,260us-gaap_ShareBasedCompensation
Restructuring charges	10us-gaap_RestructuringCharges	(8)us-gaap_RestructuringCharges
Non-cash changes in the fair value of contingent consideration payable	1,000	505
Changes in operating assets and liabilities:
Receivable due from collaboration agreements		589
Prepaid expenses and other current assets	418	3,893
Other non-current assets	(397)
Accounts payable and accrued expenses	(551)	(987)
Non-current liabilities	282
Net cash used in operating activities	(20,975)	(10,220)
Investing activities
Sale and redemption of marketable securities	40,739	11,465
Purchases of marketable securities	(18,321)	(17,227)
Purchases of property and equipment	(753)	(40)
Net cash provided by/ (used in) investing activities	21,665	(5,802)
Financing activities
Payments of secured loan agreement		(100)us-gaap_RepaymentsOfSecuredDebt
Proceeds from exercise of stock options	4,063	15
Net cash provided by/(used in) financing activities	4,063	(85)
Net increase/(decrease) in cash and cash equivalents	4,753	(16,107)
Cash and cash equivalents at beginning of period	24,074	43,640
Cash and cash equivalents at end of period	28,827	27,533
Supplemental disclosures of cash flow information
Cash paid during the period for interest	$ 209us-gaap_InterestPaid	$ 208us-gaap_InterestPaid

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