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Summary of significant accounting policies - Basis of presentation (Details) - USD ($)
$ in Thousands
Jan. 01, 2019
Sep. 30, 2019
Dec. 31, 2018
Adoption of ASC 842      
Lease, Practical Expedients, Package [true false] true    
Operating lease right-of-use assets   $ 15,647 $ 0
Operating lease liabilities   $ 20,790  
ASC 842      
Adoption of ASC 842      
Operating lease right-of-use assets $ 16,916    
Operating lease liabilities $ 22,338