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Stockholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Apr. 06, 2015
Dec. 15, 2014
Aug. 18, 2014
Jul. 22, 2013
May. 02, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Jun. 29, 2012
Shareholders' Equity                  
Common stock, authorized shares           500,000,000 500,000,000    
Common stock, par value (in dollars per share)           $ 0.01 $ 0.01    
Common stock, issued shares           61,813,995 49,806,131    
Common stock, outstanding shares           61,813,995 49,806,131    
Common stock disclosures                  
Number of shares issued under underwritten public offering 11,500,000   10,235,000 6,900,000          
Number of share's exercise in full of over-allotment option by underwriter 1,500,000   1,335,000 900,000          
Shares price (in dollars per share) $ 20.65   $ 11.25 $ 10.40          
Underwriter's discount and offering expenses $ 14,500   $ 7,100 $ 4,700          
Net proceeds from the sale of shares $ 222,900   $ 108,000 $ 67,000   $ 222,942 $ 109,013 $ 67,017  
Preferred stock                  
Preferred stock, authorized           200,000,000 200,000,000    
Preferred stock, par value           $ 0.01 $ 0.01    
Preferred stock, shares issued           0 0    
Preferred stock, shares outstanding           0 0    
Warrant                  
Warrant disclosures                  
Warrant to purchase common stock, number of shares           0     329,932
Exercise price of warrants (in dollars per share)                 $ 2.94
Fair value of warrant         $ 800        
Warrant Shares issued and delivered         223,431        
Warrant Shares outstanding           0      
Stock options                  
Common stock disclosures                  
Common stock shares reserved for issuance           5,273,722      
Restricted stock units                  
Common stock disclosures                  
Common stock shares reserved for issuance           43,554      
Hercules                  
Common stock disclosures                  
Number of shares issued under underwritten public offering   70,771              
Shares price (in dollars per share)   $ 14.13              
Net proceeds from the sale of shares   $ 1,000