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Pension and other postretirement benefits - Change in benefit obligation (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Pension Plans [Member]        
Change in benefit obligation [Roll Forward]        
Benefit obligation at beginning of year   $ 336,292 $ 238,319  
Service cost   6,346 5,605 $ 4,735
Interest cost   13,388 11,629 8,908
Plan amendments   0 (5,308)  
Medicare Part D   0 0  
Actuarial loss (gain)   (11,429) 32,772  
Acquisition   0 84,743  
Benefits paid   (19,247) (11,895)  
Curtailment   1,221 0  
Settlements $ (19,573) 0 (19,573)  
Benefit obligation at end of year 336,292 326,571 336,292 238,319
Other Postretirement Benefit Plans [Member]        
Change in benefit obligation [Roll Forward]        
Benefit obligation at beginning of year   158,781 134,652  
Service cost   1,970 1,591 2,527
Interest cost   5,985 6,420 5,681
Plan amendments   (1,758) 0  
Medicare Part D   0 273  
Actuarial loss (gain)   (18,150) 17,669  
Acquisition   0 2,857  
Benefits paid   (5,878) (4,681)  
Curtailment   (8,400) 0  
Settlements   0 0  
Benefit obligation at end of year $ 158,781 $ 132,550 $ 158,781 $ 134,652