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Summary of Significant Accounting Policies (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Segment
Dec. 31, 2012
Dec. 31, 2011
Cash Equivalents and Concentrations of Credit Risk [Abstract]                      
Maximum number of months of investments with original maturities from date of acquisition to be cash equivalents                 3 months    
Calculation of net loss and the number of shares used to compute basic and diluted earnings per share [Abstract]                      
Net loss - basic $ (4,792) $ (7,206) $ (5,482) $ (12,845) $ (10,657) $ (11,813) $ (14,512) $ (14,889) $ (30,324) $ (51,868) $ (30,885)
Adjust for gain on warrant derivative liability                 (1,835) 0 0
Net loss - diluted                 $ (32,159) $ (51,868) $ (30,885)
Net loss per share - basic (in dollars per share) $ (0.04) $ (0.07) $ (0.06) $ (0.15) $ (0.14) $ (0.18) $ (0.26) $ (0.31) $ (0.30) $ (0.85) $ (0.68)
Weighted average shares outstanding - basic (in shares)                 100,809,824 61,275,527 45,236,921
Warrant exercises (in shares)                 4,294,353 0 0
Weighted average shares outstanding - diluted (in dollars per share)                 105,104,177 61,275,527 45,236,921
Net loss per share - diluted (in dollars per share)                 $ (0.31) $ (0.85) $ (0.68)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]                      
Shares excluded from the computation of diluted EPS (in shares)                 13,792,865 10,932,935 6,836,215
Segment Information [Abstract]                      
Number of business segments                 1    
Stock options [Member]
                      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]                      
Shares excluded from the computation of diluted EPS (in shares)                 4,035,143 4,788,887 4,129,749
Unvested restricted shares [Member]
                      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]                      
Shares excluded from the computation of diluted EPS (in shares)                 325,722 501,468 193,532
Warrants [Member]
                      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]                      
Shares excluded from the computation of diluted EPS (in shares)                 9,432,000 5,642,580 2,512,934
Minimum [Member]
                      
Property, Plant and Equipment [Line Items]                      
Estimated useful lives                 3 years    
Maximum [Member]
                      
Property, Plant and Equipment [Line Items]                      
Estimated useful lives                 7 years    
Accounts Receivable [Member] | Customer Concentration Risk [Member]
                      
Concentration Risk [Line Items]                      
Customers individually accounted for greater than 10%                 3    
Product revenue [Member] | Customer Concentration Risk [Member]
                      
Concentration Risk [Line Items]                      
Customers individually accounted for greater than 10%                 4