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Stock-Based Compensation - Summary of Stock-Based Compensation Expense for Stock Options Granted that was Recorded in the Company's Results of Operations (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock-based compensation expense    
Stock-based compensation expense $ 1,357 $ 1,128
Cost of Product Revenue [Member]    
Stock-based compensation expense    
Stock-based compensation expense 22 30
Research and Development [Member]    
Stock-based compensation expense    
Stock-based compensation expense 369 302
Selling, General and Administrative [Member]    
Stock-based compensation expense    
Stock-based compensation expense $ 966 $ 796