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Fair Value Measurements - Summary of Common Stock Warrant Liability and Derivative Liability Valuation Assumptions (Detail) - $ / shares | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Private Placement Common Stock Warrant Liability [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Stock Price | $ 7.97 | $ 7.34
Exercise Price | $ 1.12 | $ 1.12
Remaining term (in years) | 5 years 14 days | 5 years 3 months 14 days
Volatility | 68.00% | 66.00%
Risk-free interest rate | 2.53% | 2.21%
Placement Agent Common Stock Warrant Liability [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Stock Price | $ 7.97 | $ 7.34
Exercise Price | $ 1.12 | $ 1.12
Remaining term (in years) | 3 years 14 days | 3 years 3 months 14 days
Volatility | 84.00% | 82.00%
Risk-free interest rate | 2.37% | 1.99%
JGB Common Stock Warrant Liability [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Stock Price | $ 0 | $ 7.34
Exercise Price | $ 0 | $ 4.67
Remaining term (in years) | 0 years | 4 years 8 months 15 days
Volatility | 0.00% | 69.00%
Risk-free interest rate | 0.00% | 1.89%
Majority Shareholder Common Stock Warrant Liability [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Stock Price | $ 7.97 | $ 7.34
Exercise Price | $ 4.00 | $ 4.00
Remaining term (in years) | 5 years 2 months 15 days | 5 years 5 months 15 days
Volatility | 67.00% | 65.00%
Risk-free interest rate | 2.54% | 2.21%
Derivative Liability [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Stock Price | $ 7.79 | $ 7.34
Remaining term (in years) | 14 days | 2 years 1 month 28 days
Volatility | 45.00% | 69.00%
Risk-free interest rate | 1.70% | 2.14%

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