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Fair Value of Financial Instruments (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Level 3 [Member]
Mar. 31, 2012
Level 3 [Member]
Mar. 31, 2013
Minimum [Member]
Level 3 [Member]
Dec. 31, 2012
Minimum [Member]
Level 3 [Member]
Mar. 31, 2013
Maximum [Member]
Level 3 [Member]
Dec. 31, 2012
Maximum [Member]
Level 3 [Member]
Mar. 31, 2013
Recurring [Member]
Dec. 31, 2012
Recurring [Member]
Mar. 31, 2013
Recurring [Member]
Level 1 [Member]
Dec. 31, 2012
Recurring [Member]
Level 1 [Member]
Mar. 31, 2013
Recurring [Member]
Level 2 [Member]
Dec. 31, 2012
Recurring [Member]
Level 2 [Member]
Mar. 31, 2013
Recurring [Member]
Level 3 [Member]
Dec. 31, 2012
Recurring [Member]
Level 3 [Member]
Mar. 31, 2013
Recurring [Member]
Money Markets [Member]
Dec. 31, 2012
Recurring [Member]
Money Markets [Member]
Mar. 31, 2013
Recurring [Member]
Money Markets [Member]
Level 1 [Member]
Dec. 31, 2012
Recurring [Member]
Money Markets [Member]
Level 1 [Member]
Mar. 31, 2013
Recurring [Member]
Money Markets [Member]
Level 2 [Member]
Dec. 31, 2012
Recurring [Member]
Money Markets [Member]
Level 2 [Member]
Mar. 31, 2013
Recurring [Member]
Money Markets [Member]
Level 3 [Member]
Dec. 31, 2012
Recurring [Member]
Money Markets [Member]
Level 3 [Member]
Mar. 31, 2013
Recurring [Member]
Certificate of Deposit [Member]
Dec. 31, 2012
Recurring [Member]
Certificate of Deposit [Member]
Mar. 31, 2013
Recurring [Member]
Certificate of Deposit [Member]
Level 1 [Member]
Dec. 31, 2012
Recurring [Member]
Certificate of Deposit [Member]
Level 1 [Member]
Mar. 31, 2013
Recurring [Member]
Certificate of Deposit [Member]
Level 2 [Member]
Dec. 31, 2012
Recurring [Member]
Certificate of Deposit [Member]
Level 2 [Member]
Mar. 31, 2013
Recurring [Member]
Certificate of Deposit [Member]
Level 3 [Member]
Dec. 31, 2012
Recurring [Member]
Certificate of Deposit [Member]
Level 3 [Member]
Mar. 31, 2013
Recurring [Member]
Common Stock Warrant Liability [Member]
Dec. 31, 2012
Recurring [Member]
Common Stock Warrant Liability [Member]
Mar. 31, 2013
Recurring [Member]
Common Stock Warrant Liability [Member]
Level 1 [Member]
Dec. 31, 2012
Recurring [Member]
Common Stock Warrant Liability [Member]
Level 1 [Member]
Mar. 31, 2013
Recurring [Member]
Common Stock Warrant Liability [Member]
Level 2 [Member]
Dec. 31, 2012
Recurring [Member]
Common Stock Warrant Liability [Member]
Level 2 [Member]
Mar. 31, 2013
Recurring [Member]
Common Stock Warrant Liability [Member]
Level 3 [Member]
Dec. 31, 2012
Recurring [Member]
Common Stock Warrant Liability [Member]
Level 3 [Member]
Assets [Abstract]                                                                                
Fair Value                 $ 23,277 $ 23,777 $ 23,277 $ 23,777 $ 0 $ 0 $ 0 $ 0 $ 22,877 $ 23,377 $ 22,877 $ 23,377 $ 0 $ 0 $ 0 $ 0 $ 400 $ 400 $ 400 $ 400 $ 0 $ 0 $ 0 $ 0                
Liabilities [Abstract]                                                                                
Fair Value                                                                 6,143 6,305 0 0 0 0 6,143 6,305
Fair value measurements of common stock warrants using significant unobservable inputs (level 3)                                                                                
Balance at beginning of period 6,305   6,305 6,996                                                                        
Exercise of warrants       (126)                                                                        
Change in fair value of common stock warrant liability 162 (3,434) (162) 3,434                                                                        
Balance at end of period $ 6,143   $ 6,143 $ 10,304                                                                        
Exercisable period of warrants 5 years                                                                              
Significant Unobservable Input Assumptions of Level 3 Valuations [Abstract]                                                                                
Historical Volatility (in hundredths) 101.00%       60.00% 56.00% 79.00% 80.00%                                                                
Expected Term (in years) 6 years       1 year 1 month 6 days 1 year 4 months 24 days 2 years 10 months 24 days 3 years 2 months 12 days                                                                
Risk-free interest rate (in hundredths) 0.12%       0.14% 0.16% 0.36% 0.36%