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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Income taxes        
Federal statutory rate 15.10% 14.80% 18.90% 13.40%
Investment and other tax credits $ 273 $ 273 $ 543 $ 546
Deferred tax adjustment     $ 621  
Effective tax rate     16.10% 13.40%