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Summary of Significant Accounting Policies - Schedule of Estimated Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Stock-based compensation expense $ 1,048 $ 2,820
Research and development [Member]    
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Stock-based compensation expense 86 882
General and administrative [Member]    
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Stock-based compensation expense $ 962 $ 1,938