Quarterly Consolidated Financial Data (Unaudited) (Details) (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											1,000
General and administrative	3,041	2,506	2,279	3,178	2,908	2,692	2,803	3,278	11,004	11,681	7,941
Loss from Operations	(27,591)	(23,452)	(26,758)	(16,477)	(19,357)	(13,474)	(11,858)	(17,622)	(94,278)	(62,311)	(38,235)
Amortization of deferred financing costs	(411)								(411)
Interest and investment income/ (expense), net (includes interest expense of $2,500 on Senior Convertible Debentures for year 2014)	(2,511)	19	(1)	29					(2,464)	38	218
Tax credits			83						83		506
Interest and investment income					11	14	16	17
Interest expense					(21)
Change in fair value of derivative instruments-warrants	(42)	425	756	223	(484)	(77)	1,803	(1,093)	1,362	149	(1,933)
Total Other (Loss)/ Income	(2,964)	444	838	252	(494)	(63)	1,819	(1,076)	(1,430)	187	(1,209)
Net loss	(30,555)	(23,008)	(25,920)	(16,225)	(19,851)	(13,537)	(10,039)	(18,698)	(95,708)	(62,124)	(39,444)
Interest and investment income/ (expense), net (includes interest expense of $2,500 on Senior Convertible Debentures for year 2014), interest expense									$ 2,500
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 (in dollars per share)	$ (0.32)	$ (0.24)	$ (0.28)	$ (0.18)	$ (0.22)	$ (0.15)	$ (0.11)	$ (0.26)	$ (1.02)	$ (0.73)	$ (0.64)