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Summary of Significant Accounting Policies - Leases (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2019
USD ($)
ft²
Jan. 01, 2019
USD ($)
Recent Accounting Pronouncements    
Operating lease right-of-use asset $ 15,704  
Operating lease liability $ 16,596  
Practical Expedients, Package true  
Practical Expedient, Use of Hindsight false  
Finance Lease deferred leasing inventives costs $ 12,600  
Waltham Massachusetts Facility    
Recent Accounting Pronouncements    
Area of leased space | ft² 27,000  
Burlington Massachusetts Facility    
Recent Accounting Pronouncements    
Area of leased space | ft² 19,000  
ASU No 2016-02 Leases | Adjustment    
Recent Accounting Pronouncements    
Operating lease right-of-use asset   $ 12,800
Operating lease liability   $ 13,400