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Employee benefit plans - Summary of Pension and Postretirement Life Insurance Plans (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Change in plan assets:      
Fair value of plan assets, beginning $ 12,839    
Employer contributions 778 $ 872  
Fair value of plan assets, ending 12,331 12,839  
Pension Benefits [Member]      
Change in benefit obligation:      
Benefit obligation, beginning 17,869 14,211  
Interest cost 696 678 $ 646
Change in experience loss (gain) (41) (250)  
Change in assumptions loss (gain) (367) 3,944  
Benefits paid (777) (714)  
Benefit obligation, ending 17,380 17,869 14,211
Change in plan assets:      
Fair value of plan assets, beginning 12,839 12,417  
Actual return on plan assets 29 895  
Employer contributions 240 241  
Benefits paid (777) (714)  
Fair value of plan assets, ending 12,331 12,839 12,417
Funded status at end of year $ (5,049) $ (5,030)  
Postretirement Life Insurance Benefits [Member]      
Change in benefit obligation:      
Service cost     35
Interest cost     $ 111