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Related Party Transactions - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Novartis International Pharmaceutical Ltd.    
Related Party Transaction [Line Items]    
Payments to related parties $ 0 $ 0
Silverstein Foundation | Funding Agreement    
Related Party Transaction [Line Items]    
Fund received from related parties $ 0 $ 500,000