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Equity-Based Compensation - Schedule of Equity-based Compensation Expense (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Stock-based compensation expense $ 3,500 $ 819
Research and Development [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Stock-based compensation expense 1,819 632
Selling, General and Administrative [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Stock-based compensation expense $ 1,681 $ 187