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Common Stock - Summary of Share-Based Compensation Expense (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]  
Total stock-based compensation $ 348
Research and development [Member]  
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]  
Total stock-based compensation 107
General and administrative [Member]  
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]  
Total stock-based compensation $ 241