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Warrants and Derivative Liabilities - Schedule of Assumptions Used to Calculate the Fair Value of Warrants (Detail) - USD ($) $ in Millions |  |  |  | 3 Months Ended
--- | --- | --- | --- | ---
Dec. 19, 2014 | Nov. 13, 2014 | Oct. 09, 2013 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013
Senior Convertible Note Warrant |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  | 0.66% | 0.96% | 0.64% | 0.74% | 0.73% | 1.00% | 1.07% | 0.98% | 1.11% | 1.17% | 1.02% | 1.13% | 0.67%
Expected volatility |  |  |  | 76.76% | 76.68% | 73.39% | 71.61% | 70.42% | 72.38% | 76.20% | 83.50% | 80.99% | 75.60% | 71.98% | 71.90% | 71.74%
Term (years) |  |  |  | 1 year 6 months 4 days | 1 year 9 months 4 days | 2 years 4 days | 2 years 3 months 4 days | 2 years 6 months 4 days | 2 years 9 months 4 days | 3 years 4 days | 3 years 3 months 4 days | 3 years 6 months 4 days | 3 years 9 months 4 days | 4 years 4 days | 4 years 3 months 7 days | 4 years 6 months 4 days
Fair value |  |  |  | $ 0.4 | $ 0.3 | $ 0.1 | $ 0.2 | $ 0.3 | $ 0.5 | $ 1.5 | $ 2.3 | $ 2.2 | $ 2.2 | $ 2.5 | $ 3.0 | $ 3.4
Senior Convertible Note Warrant | Post Modification |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 1.05% |  |  |  |  |  |  |  |  |  |  |  |  |
Expected volatility |  |  | 71.45% |  |  |  |  |  |  |  |  |  |  |  |  |
Term (years) |  |  | 3 years 11 months 27 days |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  | $ 3.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Convertible Note Warrant | Pre Modification |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 1.05% |  |  |  |  |  |  |  |  |  |  |  |  |
Expected volatility |  |  | 71.45% |  |  |  |  |  |  |  |  |  |  |  |  |
Term (years) |  |  | 3 years 11 months 27 days |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  | $ 2.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Hercules Warrant |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate | 1.74% |  |  | 1.08% | 1.65% | 1.31% | 1.63% | 1.41% | 1.73% |  |  |  |  |  |  |
Expected volatility | 70.26% |  |  | 70.25% | 73.57% | 75.32% | 72.57% | 74.60% | 77.43% |  |  |  |  |  |  |
Term (years) | 5 years 6 months 11 days |  |  | 4 years 3 months | 4 years 6 months | 4 years 9 months | 5 years | 5 years 3 months | 5 years 6 months |  |  |  |  |  |  |
Fair value | $ 0.2 |  |  | $ 0.2 | $ 0.2 | $ 0.1 | $ 0.2 | $ 0.2 | $ 0.2 |  |  |  |  |  |  |
November 2014 Warrant |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  | 1.64% |  | 0.98% | 1.51% | 1.17% | 1.44% | 1.28% | 1.61% |  |  |  |  |  |  |
Expected volatility |  | 72.86% |  | 69.88% | 70.02% | 73.02% | 74.18% | 75.96% | 78.00% |  |  |  |  |  |  |
Term (years) |  | 5 years |  | 3 years 7 months 13 days | 3 years 10 months 13 days | 4 years 1 month 13 days | 4 years 4 months 13 days | 4 years 7 months 13 days | 4 years 10 months 13 days |  |  |  |  |  |  |
Fair value |  | $ 4.3 |  | $ 2.6 | $ 2.1 | $ 1.3 | $ 1.8 | $ 2.5 | $ 3.2 |  |  |  |  |  |  |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Risk-free interest rate assumption used in valuing an instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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