CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Costs and Expenses:
Research and development	$ 19,881	$ 20,424	$ 19,525	$ 18,198	$ 24,550	$ 20,946	$ 24,479	$ 13,299	$ 78,028	$ 83,274	$ 50,630
Selling, general and administrative	7,066	2,728	7,394	4,606					21,794	11,004	11,681
General and administrative					3,041	2,506	2,279	3,178
Loss from Operations	(26,947)	(23,152)	(26,919)	(22,804)	(27,591)	(23,452)	(26,758)	(16,477)	(99,822)	(94,278)	(62,311)
Other Income /(Loss):
Interest and investment income/(expense), net	(3,469)	(4,291)	(5,207)	(4,317)	(2,511)	19	(1)	29	(17,284)	(2,875)	38
Tax credits							83			83
Change in fair value of derivative instruments-warrants	(30)	1,446	(1,542)	(268)	(42)	425	756	223	(394)	1,362	149
Total Other Income /(Loss)	(3,499)	(2,845)	(6,749)	(4,585)	(2,964)	444	838	252	(17,678)	(1,430)	187
Net loss	$ (30,446)	$ (25,997)	$ (33,668)	$ (27,389)	$ (30,555)	$ (23,008)	$ (25,920)	$ (16,225)	$ (117,500)	$ (95,708)	$ (62,124)
Weighted Average Common Shares Outstanding
Basic and diluted (in shares)	113,678,306	111,328,339	100,343,637	96,683,525	96,190,332	94,738,048	94,069,703	92,056,124	105,570,960	94,276,178	85,220,458
Net Loss per Common Share, Basic and Diluted
Net Loss per Common Share, basic and diluted (in dollars per share)	$ (0.27)	$ (0.23)	$ (0.34)	$ (0.28)	$ (0.32)	$ (0.24)	$ (0.28)	$ 0.18	$ (1.11)	$ (1.02)	$ (0.73)