Fair Value Measurements - Reconciliation of Company's Financial Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Warrant Liabilities [Member] - USD ($) |
3 Months Ended | 6 Months Ended | ||||
|---|---|---|---|---|---|---|
Jun. 30, 2019 |
Jun. 30, 2018 |
Jun. 30, 2019 |
Jun. 30, 2018 |
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| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] | ||||||
| Balance at beginning of period | $ 2,982,034 | $ 25,823,996 | $ 20,781,663 | $ 13,549,437 | ||
| Increase (decrease) in fair value | [1] | 1,363,868 | 12,802,583 | (16,435,761) | 25,077,142 | |
| Balance at end of period | $ 4,345,902 | $ 38,626,579 | $ 4,345,902 | $ 38,626,579 | ||
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