Consolidated Statements of Operations (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating expenses      
Research and development $ 41,884 $ 33,221 $ 22,001
General and administrative 8,951 8,002 6,498
Total operating expenses 50,835 41,223 28,499
Loss from operations (50,835) (41,223) (28,499)
Other income (expense)      
Investment and other income (expense), net 76 137 (127)
Interest expense     (309)
Change in fair value of warrant liability 796 2,327 25,520
Total other income (expense), net 872 2,464 25,084
Loss before income taxes (49,963) (38,759) (3,415)
Net loss $ (49,963) $ (38,759) $ (3,415)
Loss per share — basic (in Dollars per share) $ (0.64) $ (0.62) $ (0.06)
Loss per share — diluted (Note 6) (in Dollars per share) $ (0.64) $ (0.62) $ (0.53)
Shares used to compute basic loss per share (in Shares) 77,619,807 62,387,616 53,728,672
Shares used to compute diluted loss per share (in Shares) 77,619,807 62,387,616 54,899,955