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Fair Value of Financial Assets and Liabilities - Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability and Preferred Stock Repurchase Liability (Detail) - Level 3 [Member] $ in Thousands | 3 Months Ended
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Mar. 31, 2019 USD ($)
Preferred Stock Warrant Liability [Member] |
Fair Value Assets Measured On Recurring and Nonrecurring Basis [Line Items] |
Balance as of December 31, 2018 | $ 4,947
Change in fair value | 288
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger | (5,235)
Balance as of March 31, 2019 | 0
Derivative Liability [Member] |
Fair Value Assets Measured On Recurring and Nonrecurring Basis [Line Items] |
Balance as of December 31, 2018 | 201
Change in fair value | (183)
Balance as of March 31, 2019 | $ 18

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