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Income taxes (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Income taxes          
Federal statutory rate 19.40% 11.80% 21.00% 21.00% 35.00%