CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Costs and Expenses:
Research and development	$ 24,550	$ 20,946	$ 24,479	$ 13,299	$ 16,449	$ 10,782	$ 9,055	$ 14,344	$ 83,274	$ 50,630	$ 29,294
Purchased in-process research and development											1,000us-gaap_ResearchAndDevelopmentInProcess
General and administrative	3,041us-gaap_GeneralAndAdministrativeExpense	2,506us-gaap_GeneralAndAdministrativeExpense	2,279us-gaap_GeneralAndAdministrativeExpense	3,178us-gaap_GeneralAndAdministrativeExpense	2,908us-gaap_GeneralAndAdministrativeExpense	2,692us-gaap_GeneralAndAdministrativeExpense	2,803us-gaap_GeneralAndAdministrativeExpense	3,278us-gaap_GeneralAndAdministrativeExpense	11,004us-gaap_GeneralAndAdministrativeExpense	11,681us-gaap_GeneralAndAdministrativeExpense	7,941us-gaap_GeneralAndAdministrativeExpense
Loss from Operations	(27,591)us-gaap_OperatingIncomeLoss	(23,452)us-gaap_OperatingIncomeLoss	(26,758)us-gaap_OperatingIncomeLoss	(16,477)us-gaap_OperatingIncomeLoss	(19,357)us-gaap_OperatingIncomeLoss	(13,474)us-gaap_OperatingIncomeLoss	(11,858)us-gaap_OperatingIncomeLoss	(17,622)us-gaap_OperatingIncomeLoss	(94,278)us-gaap_OperatingIncomeLoss	(62,311)us-gaap_OperatingIncomeLoss	(38,235)us-gaap_OperatingIncomeLoss
Other Income/(Loss)
Amortization of deferred financing costs	(411)us-gaap_AmortizationOfFinancingCosts								(411)us-gaap_AmortizationOfFinancingCosts
Interest and investment income/ (expense), net (includes interest expense of $2,500 on Senior Convertible Debentures for year 2014)	(2,511)us-gaap_InvestmentIncomeNet	19us-gaap_InvestmentIncomeNet	(1)us-gaap_InvestmentIncomeNet	29us-gaap_InvestmentIncomeNet					(2,464)us-gaap_InvestmentIncomeNet	38us-gaap_InvestmentIncomeNet	218us-gaap_InvestmentIncomeNet
Tax credits			83us-gaap_OtherNonoperatingIncome						83us-gaap_OtherNonoperatingIncome		506us-gaap_OtherNonoperatingIncome
Change in fair value of derivative instruments-warrants	(42)us-gaap_UnrealizedGainLossOnDerivatives	425us-gaap_UnrealizedGainLossOnDerivatives	756us-gaap_UnrealizedGainLossOnDerivatives	223us-gaap_UnrealizedGainLossOnDerivatives	(484)us-gaap_UnrealizedGainLossOnDerivatives	(77)us-gaap_UnrealizedGainLossOnDerivatives	1,803us-gaap_UnrealizedGainLossOnDerivatives	(1,093)us-gaap_UnrealizedGainLossOnDerivatives	1,362us-gaap_UnrealizedGainLossOnDerivatives	149us-gaap_UnrealizedGainLossOnDerivatives	(1,933)us-gaap_UnrealizedGainLossOnDerivatives
Total Other (Loss)/ Income	(2,964)us-gaap_NonoperatingIncomeExpense	444us-gaap_NonoperatingIncomeExpense	838us-gaap_NonoperatingIncomeExpense	252us-gaap_NonoperatingIncomeExpense	(494)us-gaap_NonoperatingIncomeExpense	(63)us-gaap_NonoperatingIncomeExpense	1,819us-gaap_NonoperatingIncomeExpense	(1,076)us-gaap_NonoperatingIncomeExpense	(1,430)us-gaap_NonoperatingIncomeExpense	187us-gaap_NonoperatingIncomeExpense	(1,209)us-gaap_NonoperatingIncomeExpense
Net loss	$ (30,555)	$ (23,008)	$ (25,920)	$ (16,225)	$ (19,851)	$ (13,537)	$ (10,039)	$ (18,698)	$ (95,708)	$ (62,124)	$ (39,444)
Weighted Average Common Shares Outstanding
Basic and Diluted (in shares)	96,190,332	94,738,048	94,069,703	92,056,124	90,182,115	90,182,115	87,482,939	72,789,006	94,276,178	85,220,458	61,702,277
Net Loss per Common Share, Basic and Diluted
Net Loss per Common Share, Basic and Diluted (in dollars per share)	$ (0.32)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.24)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.28)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.18)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.22)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.15)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.11)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.26)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.02)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.73)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.64)us-gaap_EarningsPerShareBasicAndDiluted

Amount of noncash expense included in interest expense to issue debt and obtain financing associated with the related debt instruments. Alternate captions include noncash interest expense.

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