Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Reconciliation of Company's Financial Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail)

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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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Balance at beginning of period $ 38,626,579 $ 9,581,916 $ 13,549,437 $ 12,698,980
Issuance of warrants   12,383,435   12,383,435
(Decrease) increase in fair value (3,246,765) (5,941,144) 21,830,377 (9,058,208)
Balance at end of period $ 35,379,814 $ 16,024,207 $ 35,379,814 $ 16,024,207