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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in benefit obligation:      
Benefit obligation, beginning $ 16,703 $ 17,380  
Interest cost 650 665 $ 696
Change in experience gain (13) (322)  
Change in actuarial assumptions loss (gain) 395 (256)  
Benefits paid (800) (764)  
Benefit obligation, ending 16,935 16,703 17,380
Change in plan assets:      
Fair value of plan assets, beginning 12,644 12,331  
Actual return on plan assets 1,420 1,023  
Employer contributions   54  
Benefits paid (800) (764)  
Fair value of plan assets, ending 13,264 12,644 $ 12,331
Funded status at end of year $ (3,671) $ (4,059)