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Commitments and contingencies - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]        
Data cost incurred $ 1,447 $ 1,154 $ 4,080 $ 3,333
Total capital commitment under certain data licensing agreements $ 20,331   $ 20,331