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Pension and other postretirement benefits - Change in benefit obligation (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Pension Plans [Member]          
Change in benefit obligation [Roll Forward]          
Benefit obligation at beginning of year $ 328,047 $ 326,571 $ 326,571 $ 336,292  
Service cost 1,224 1,270 4,651 6,346 $ 5,605
Interest cost 3,332 3,480 13,892 13,388 11,629
Plan amendments     0 0  
Actuarial loss (gain)     12,761 (11,429)  
Medicare Part D     0 0  
Benefits paid     (29,828) (19,247)  
Curtailment     0 1,221  
Benefit obligation at end of year     328,047 326,571 336,292
Other Postretirement Benefit Plans [Member]          
Change in benefit obligation [Roll Forward]          
Benefit obligation at beginning of year 133,856 132,550 132,550 158,781  
Service cost 247 330 1,003 1,970 1,591
Interest cost $ 1,366 $ 1,456 5,595 5,985 6,420
Plan amendments     0 (1,758)  
Actuarial loss (gain)     1,919 (18,150)  
Medicare Part D     38 0  
Benefits paid     (7,249) (5,878)  
Curtailment     0 (8,400)  
Benefit obligation at end of year     $ 133,856 $ 132,550 $ 158,781