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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Income taxes        
Federal statutory rate 12.20% 11.80% 11.60% 11.80%
Investment and other tax credits $ 394 $ 276 $ 1,172 $ 819