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Fair Value Measurement - Narrative (Detail) - $ / shares
1 Months Ended 12 Months Ended
Nov. 30, 2019
Apr. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2012
Sep. 30, 2021
Class of Warrant or Right [Line Items]              
Exercise price (in dollars per share)             $ 11.50
Series A Preferred Stock Warrants              
Class of Warrant or Right [Line Items]              
Warrants issued (in shares)   122,400          
Exercise price (in dollars per share)   $ 2.7233          
Series A Preferred Stock Warrants | 2012 Warrants              
Class of Warrant or Right [Line Items]              
Warrants issued (in shares)           1,000,000  
Exercise price (in dollars per share)           $ 2.7233  
Warrants exercise period           10 years  
Warrants exercise period, extended 10 years            
Series A Preferred Stock Warrants | 2015 Warrants              
Class of Warrant or Right [Line Items]              
Warrants issued (in shares)         1,134,653    
Exercise price (in dollars per share)         $ 2.7233    
Warrants exercise period         10 years    
Warrants exercise period, extended 10 years            
Series B Preferred Stock Warrants              
Class of Warrant or Right [Line Items]              
Warrants issued (in shares)     35,412 331,927      
Exercise price (in dollars per share)     $ 7.486 $ 7.486