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Leases - Lexington, Massachusetts (Details)
$ in Thousands
1 Months Ended
Jul. 31, 2013
item
Dec. 31, 2017
ft²
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Leases        
Future minimum lease payments     $ 39,682  
Current deferred rent     697 $ 684
Lease for facility in Lexington, Massachusetts, United States        
Leases        
Lease term (in years) 10 years      
Number of subsequent renewals | item 2      
Renewal term (in years) 5 years      
Future minimum lease payments     14,000  
Total deferred rent     6,100 6,200
Current deferred rent     $ 700 $ 700
Lease for facility in Lexington, Massachusetts, United States | Forecast        
Leases        
Area of facility subject to lease (in square feet) | ft²   53,000