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Australia Research and Development Tax Incentive - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 01, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Research and Development Tax Incentive [Line Items]            
Unrealized foreign currency exchange losses       $ 13,000 $ 45,000  
Zafgen Australia Pty Limited [Member]            
Research and Development Tax Incentive [Line Items]            
Unrealized foreign currency exchange losses         100,000  
Zafgen Australia Pty Limited [Member] | Australia [Member]            
Research and Development Tax Incentive [Line Items]            
Tax incentive receivable   $ 200,000   200,000   $ 1,500,000
Zafgen Australia Pty Limited [Member] | Maximum [Member]            
Research and Development Tax Incentive [Line Items]            
Unrealized foreign currency exchange losses   100,000 $ 100,000 100,000    
Research and Development [Member] | Zafgen Australia Pty Limited [Member]            
Research and Development Tax Incentive [Line Items]            
Percentage of refundable tax incentive 41.00%         43.50%
Reduction to research and development expenses     $ 300,000 $ 200,000 $ 1,300,000  
Percentage of reduction in research and development costs       41.00% 43.50%  
Research and Development [Member] | Zafgen Australia Pty Limited [Member] | Maximum [Member]            
Research and Development Tax Incentive [Line Items]            
Reduction to research and development expenses   $ 100,000