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Pension and other postretirement benefits - Change in benefit obligation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in benefit obligation [Roll Forward]      
Plan amendments $ (18,768)    
Defined Benefit Pension Plans [Member]      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 328,047 $ 326,571  
Service cost 4,384 4,651 $ 6,346
Interest cost 13,310 13,892 13,388
Plan amendments 0 0  
Actuarial loss (gain) 18,523 12,761  
Medicare Part D 0 0  
Benefits paid (20,156) (29,828)  
Benefit obligation at end of year 344,108 328,047 326,571
Other Postretirement Benefit Plans [Member]      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 133,856 132,550  
Service cost 801 1,003 1,970
Interest cost 5,317 5,595 5,985
Plan amendments (27,395) 0  
Actuarial loss (gain) 14,698 1,919  
Medicare Part D 406 38  
Benefits paid (6,919) (7,249)  
Benefit obligation at end of year $ 120,764 $ 133,856 $ 132,550