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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Income taxes        
Federal statutory rate 11.80% 12.00% 11.20% 11.80%
Investment and other tax credits $ 390 $ 273 $ 778 $ 543