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Common Stock Warrant Liability (Tables)
 3 Months Ended
Mar. 31, 2013
Common Stock Warrant Liability [Abstract]  
Estimated fair value of warrants accounted for derivative liabilities
Selected terms and estimated fair value of warrants accounted for as derivative liabilities at March 31, 2013 are as follows:

           
Fair Value of Warrants
(in thousands)
 
Issuance
Date
 
Number of
Warrant Shares
  
Exercise
Price
 
Warrant
Expiration
Date
 
Issuance
Date
  
March 31,
2013
 
                
5/13/2009
  466,667  $17.25 
5/13/2014
 $3,360  $– 
2/23/2010
  916,669   12.75 
2/23/2015
  5,701   33 
2/22/2011
  4,948,750   2.80 
2/22/2016
  8,004   6,110