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Commitments and Contingencies - Additional Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 123 Months Ended
Oct. 10, 2016
Mar. 31, 2017
Mar. 31, 2017
Incept      
Commitments and Contingencies      
Payments for Royalties     $ 111
Regeneron | Collaboration Agreement      
Commitments and Contingencies      
Payments made under the collaboration agreement   $ 0  
Regeneron | Collaboration Agreement | Maximum      
Commitments and Contingencies      
Reimbursable clinical development costs $ 25,000    
Potential increase in reimbursable clinical development costs $ 5,000