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Warrants and Derivative Liabilities - Schedule of Assumptions Used to Calculate the Fair Value of Exchanged Warrants (Detail) - November 2014 Warrant $ in Millions | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($)
--- | --- | --- | --- | --- | --- | --- | --- | ---
Derivative [Line Items] |  |  |  |  |  |  |  |
Fair value | $ 3.6 | $ 1.3 | $ 1.6 | $ 1.1 | $ 0.4 | $ 0.8 | $ 0.9 | $ 1.8
Risk-free interest rate |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Measurement inputs | 0.0261 | 0.0262 | 0.0240 | 0.0220 | 0.0187 | 0.0149 | 0.0144 | 0.0141
Expected annual dividend yield |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Measurement inputs | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0
Expected volatility |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Measurement inputs | 0.7029 | 0.6366 | 0.6740 | 0.6586 | 0.6586 | 0.6564 | 0.6721 | 0.6653
Term (years) |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Term (years) | 10 months 15 days | 1 year 1 month 13 days | 1 year 4 months 13 days | 1 year 7 months 13 days | 1 year 10 months 13 days | 2 years 1 month 13 days | 2 years 4 months 13 days | 2 years 7 months 13 days

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