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Postretirement benefits (cash flows) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Postretirement benefits [Line Items]      
Company contributions $ 0 $ 0 $ 0
Cash surrender value of insurance polices which fund pension plan 6,869 $ 6,615  
Postretirement benefit plan [Member]      
Expected benefit payments      
2019 7,679    
2020 7,906    
2021 7,730    
2022 7,219    
2023 6,686    
2024 - 2028 26,205    
Pension plan [Member]      
Expected benefit payments      
2019 320    
2020 320    
2021 310    
2022 300    
2023 300    
2024 - 2028 $ 1,320