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Summary of Stock-Based Compensation Recognized (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Total stock-based compensation $ 606 $ 120
Research and development    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Total stock-based compensation 115 20
General and Administrative    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Total stock-based compensation [1] $ 491 $ 100
[1] 2018 includes $0.1 million in modification stock-based compensation expense related to a Board member’s departure.