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Employee benefit plans - Summary of Pension and Postretirement Life Insurance Plans (Details) - Pension Benefits - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Change in benefit obligation:      
Benefit obligation, beginning $ 17,491 $ 16,338  
Interest cost 544 639 $ 623
Change in experience gain (291) 96  
Change in actuarial assumptions loss 2,159 1,221  
Benefits paid (790) (803)  
Benefit obligation, ending 19,113 17,491 16,338
Change in plan assets:      
Fair value of plan assets, beginning 16,930 14,919  
Actual return on plan assets 1,488 2,814  
Benefits paid (790) (803)  
Fair value of plan assets, ending 17,628 16,930 $ 14,919
Funded status at end of year $ (1,485) $ (561)