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Income taxes (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2019
Income taxes          
Income tax expense $ 13,057,000 $ 15,437,000 $ 42,541,000 $ 48,741,000  
Effective income tax rate 29.00% 25.00% 26.00% 29.00%  
Recorded net income tax benefits no longer deductible         $ 6,228,000