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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Income taxes        
Federal statutory rate 15.30% 15.50% 17.70% 14.20%
Investment and other tax credits $ 276 $ 276 $ 819 $ 821
Deferred tax adjustment     $ 621  
Effective tax rate     15.80% 14.20%