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Capital Stock (Details) - USD ($) $ / shares in Units, $ in Thousands |  |  |  | 1 Months Ended | 11 Months Ended | 12 Months Ended | 23 Months Ended |  |
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Feb. 27, 2015 | Dec. 08, 2014 | Sep. 27, 2012 | Feb. 28, 2015 | Dec. 31, 2013 | Aug. 31, 2013 | Sep. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Feb. 28, 2010 | Jun. 30, 2009 | Dec. 31, 2012 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 18, 2013 | Aug. 23, 2013
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Offering price (in dollars per share) |  | $ 4.94 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warrant outstanding (in shares) |  |  |  |  |  |  |  |  |  |  |  |  | 18,165,000 | 21,768,000 |  |  |  |
Capital units issued upon the underwriters 30-day over-allotment option (in shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,425,000 |
Proceeds from issuance of common stock |  |  |  |  |  |  |  |  |  |  |  |  | $ 481 | $ 41,442 | $ 84,338 |  |  |
Reclassification of contingent warrant liability to equity upon exercise of warrants |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,552 | $ 2,526 | $ 6,171 |  |  |
Hercules Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) | $ 3.31 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercisable period of warrants | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate number of unregistered shares of common stock called by warrants (in shares) | 181,268 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of warrant liability | $ 500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Electric Capital Corporation Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  |  | $ 3.54 |  |  |  |  |  | $ 1.14 |  |  |  |  |  |  |  |  |
Exercisable period of warrants |  |  | 5 years |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |
Aggregate number of unregistered shares of common stock called by warrants (in shares) |  |  | 39,346 |  |  |  |  |  | 263,158 |  |  |  |  |  |  |  |  |
Fair value of warrant liability |  |  | $ 100 |  |  |  |  |  | $ 200 |  |  |  |  |  |  |  |  |
Five Year Warrants Issued in February 2015 [Member] | Hercules Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  | $ 3.31 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercisable period of warrants |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate number of unregistered shares of common stock called by warrants (in shares) |  |  |  | 181,268 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Two Year Warrants Issued in December 2014 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warrant outstanding (in shares) |  |  |  |  |  |  |  |  |  |  |  |  | 8,097,165 | 8,097,165 |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7.90 |  |  |  |
Warrant term |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |
Fair value of warrant liability |  | $ 10,300 |  |  |  |  |  |  |  |  |  |  | $ 3,000 | $ 5,200 |  |  |  |
Gain (loss) on revaluation of warrant liability |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200 |  |  |  |  |
Five Year Warrants Issued in September 2012 [Member] | General Electric Capital Corporation Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  |  |  |  | $ 3.54 |  |  |  |  |  |  |  |  |  |  |
Exercisable period of warrants |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |
Aggregate number of unregistered shares of common stock called by warrants (in shares) |  |  |  |  |  |  | 39,346 |  |  |  |  |  |  |  |  |  |  |
Five Year Warrants Issued in March 2012 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warrant outstanding (in shares) |  |  |  |  |  |  |  | 14,834,577 |  |  |  |  | 9,585,153 | 12,109,418 |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  |  |  |  |  | $ 1.76 |  |  |  |  |  |  |  |  |  |
Warrant term |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |
Fair value of warrant liability |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,500 | $ 26,700 |  |  |  |
Gain (loss) on revaluation of warrant liability |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,400 |  |  |  |  |
Warrants exercise (in shares) |  |  |  |  |  |  |  |  |  |  |  |  | 2,524,265 |  |  |  |  |
Cashless exercise of warrants (in shares) |  |  |  |  |  |  |  |  |  |  |  |  | 2,523,515 |  |  |  |  |
Exercise of warrants (in shares) |  |  |  |  |  |  |  |  |  |  |  |  | 1,410,474 |  |  |  |  |
Gain (loss) on revaluation of warrant liability related to exercised warrants |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200 |  |  |  |  |
Reclassification of contingent warrant liability to equity upon exercise of warrants |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,600 |  |  |  |  |
Five Year Warrants Issued in December 2011 [Member] | General Electric Capital Corporation Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  |  |  |  |  |  | $ 1.14 |  |  |  |  |  |  |  |  |
Exercisable period of warrants |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |
Aggregate number of unregistered shares of common stock called by warrants (in shares) |  |  |  |  |  |  |  |  | 263,158 |  |  |  |  |  |  |  |  |
Five Year Warrants Issued in February 2010 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warrant outstanding (in shares) |  |  |  |  |  |  |  |  |  | 1,260,000 |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  |  |  |  |  |  |  | $ 10.50 |  |  |  |  |  |  |  |
Warrant term |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |
Warrants Issued to Private Investors [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warrant outstanding (in shares) |  |  |  |  |  |  |  |  |  |  | 347,826 |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  |  |  |  |  |  |  |  |  |  | $ 19.50 |  |  |  |  |  |  |
Warrant term |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |
Underwritten Offering [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock per capital unit (in shares) |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross proceeds from issuance of capital units |  | $ 40,000 |  |  | $ 57,400 | $ 31,600 |  |  |  |  |  |  |  |  |  |  |  |
Offering expense |  | $ 2,300 |  |  | $ 3,800 | $ 2,200 |  |  |  |  |  |  |  |  |  |  |  |
Immediate term for warrants exercisable (in years) |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warrant outstanding (in shares) |  | 8,097,165 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of warrants (in dollars per share) |  | $ 7.90 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital units issued (in shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,925,000 | 8,736,187
Capital units issued upon the underwriters 30-day over-allotment option (in shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,139,502
Period to purchase additional stock |  |  |  |  | 30 days | 30 days |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of capital units (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.25 | $ 3.62
2011 ATM Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of shares of common stock (in shares) |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  | 7,572,327 |  |
Proceeds from issuance of common stock |  |  |  |  |  |  |  |  |  |  |  | $ 500 |  |  |  | $ 14,600 |  |
2015 ATM Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of shares of common stock (in shares) |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |
Sales commission paid per transaction (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |

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