Quarterly Consolidated Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Revenues	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Costs and expenses:
Research and development	17,166	24,610	26,611	21,175	19,881	20,424	19,525	18,198	89,562	78,028	83,274
Selling, general and administrative	25,232	13,872	10,249	6,375					55,724	21,794	11,004
General and administrative					7,066	2,728	7,394	4,606
Loss from Operations	(42,398)	(38,482)	(36,860)	(27,550)	(26,947)	(23,152)	(26,919)	(22,804)	(145,286)	(99,822)	(94,278)
Other Loss
Interest and investment income/(expense), net	(3,007)	(1,674)	(1,673)	(7,036)	(3,469)	(4,291)	(5,207)	(4,317)	(13,390)	(17,284)	(2,875)
Debt conversion expense	(14,543)	0	0	(25,615)					(40,158)	0	0
Tax credits	126	0	0	0					121	0	83
Change in fair value of derivative instruments-warrants	(45)	(87)	(23)	260	(30)	1,446	(1,542)	(268)	106	(394)	1,362
Total Other Loss	(17,469)	(1,761)	(1,696)	(32,391)	(3,499)	(2,845)	(6,749)	(4,585)	(53,321)	(17,678)	(1,430)
Net Loss	$ (59,867)	$ (40,243)	$ (38,556)	$ (59,941)	$ (30,446)	$ (25,997)	$ (33,668)	$ (27,389)	$ (198,607)	$ (117,500)	$ (95,708)
Weighted Average Common Shares Outstanding—basic and diluted (in shares)	190,093,786	179,786,580	168,127,144	117,626,669	113,678,306	111,328,339	100,343,637	96,683,525	164,437,548	105,570,960	94,276,178
Net Loss per Common Share—basic and diluted (in dollars per share)	$ (0.31)	$ (0.22)	$ (0.23)	$ (0.51)	$ (0.27)	$ (0.23)	$ (0.34)	$ (0.28)	$ (1.21)	$ (1.11)	$ (1.02)