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Goodwill and intangible assets - Future Amortization Expense for Intangible Assets (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]      
Remainder of 2018 $ 5,253    
2019 6,982    
2020 6,880    
2021 6,880    
2022 6,880    
Thereafter 7,454    
Total $ 40,329 $ 42,082 $ 47,344