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License and Acquisition Agreements - Regeneron License Agreement (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2019
Dec. 31, 2018
License and Acquisition Agreements      
Research and development expense   $ 135,001 $ 86,597
Regeneron | License Agreement      
License and Acquisition Agreements      
Upfront payment for exchange of rights $ 5,000    
Uncured period which causes termination of the agreement, due to insolvency or bankruptcy or for material breach of the agreement by the other party that remains uncured 90 days    
Research and development expense     0
Notice period to terminate the agreement under payment-related breaches 30 days    
Consecutive suspension of activities period for termination of agreement 12 months    
Period after which the agreement can be terminated 18 months    
Notice period to terminate the agreement 180 days    
Notice period to terminate the agreement after US marketing approval of the developed product 1 year    
Notice period to terminate the agreement if products are having safety concerns 3 months    
Regeneron | Clinical Supply Agreement      
License and Acquisition Agreements      
Research and development expense related to purchase of drug materials   $ 6,854 $ 1,835
Maximum | Regeneron | License Agreement      
License and Acquisition Agreements      
Milestone payment to be paid upon regulatory milestone achievement $ 27,500