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Preferred Stock - Additional Information (Detail)
 1 Months Ended 12 Months Ended
May 31, 2017
USD ($)
$ / shares
shares
Feb. 29, 2016
shares
May 31, 2015
shares
Sep. 30, 2014
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Aug. 28, 2017
Class Of Stock [Line Items]              
Exchange ratio of common stock             0.5532
Preferred stock, conversion basis         one-for-one    
Preferred stock, conversion percentage         100.00%    
Total consideration from stock sold and issued to investors | $           $ 40,433,000  
Preferred stock dividends declared         $ 0 $ 0  
Preferred stock dividends paid         0 0  
Preferred stock, par value         0.001 $ 0.001  
Public offering price per share threshold for conversion of preferred stock         $ 12.09    
Aggregate proceeds threshold for conversion of preferred stock | $         $ 50,000,000    
Preferred stock, shares issued | shares         0 0  
Letter Agreement | The Gates Foundation              
Class Of Stock [Line Items]              
Minimum percentage of initial public offering over price paid upon redemption           200.00%  
Private Synlogic              
Class Of Stock [Line Items]              
Preferred stock, par value         $ 0.0001    
Series C Preferred Stock              
Class Of Stock [Line Items]              
Stock sold and issued to investors | shares 2,882,679            
Stock sold and issued to investors, per share $ 8.06            
Total consideration from stock sold and issued to investors | $ $ 40,400,000            
Offering costs | $ $ 1,600,000            
Series A Preferred Stock              
Class Of Stock [Line Items]              
Vote of preferred stock holders required for approval         70.00%    
Series A Preferred Stock | Letter Agreement | The Gates Foundation              
Class Of Stock [Line Items]              
Stock sold and issued to investors | shares   361,884 218,646 201,163      
Preferred stock, shares issued | shares       781,693      
Proceeds from issuance of stock to be spend on research | $           $ 5,000,000