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Property and equipment - Summary by type (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Property and equipment    
Total property and equipment $ 504,063 $ 502,646
Less: accumulated depreciation and amortization (239,434) (240,287)
Total property and equipment, net 264,629 262,359
Leasehold improvements    
Property and equipment    
Total property and equipment 408,360 418,023
Software    
Property and equipment    
Total property and equipment 44,575 42,281
Construction in progress    
Property and equipment    
Total property and equipment 37,027 27,291
Computer equipment    
Property and equipment    
Total property and equipment 12,332 13,245
Furniture, fixtures and office equipment    
Property and equipment    
Total property and equipment $ 1,769 $ 1,806