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Quarterly Financial Data - Summary of Quarterly Results of Operations (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2018
Jun. 30, 2018
Quarterly Financial Data [Line Items]            
Change in fair value of derivative liability       $ (18,886) $ (45,164) $ (26,278)
Second Tranche Transaction [Member]            
Quarterly Financial Data [Line Items]            
Change in fair value of derivative liability $ (18,900) $ (2,300) $ (24,000)      
Overstatement (understatement) of change in fair value of derivative   $ 1,200 $ (1,200)