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Pension and other postretirement benefits - Assumed health care trend rates (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2015
USD ($)
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]  
Medical cost inflation rate 8.00%
Medical cost inflation, years 7 and thereafter 5.00%
Effect of one percentage point increase on total service and interest cost $ 1,403
Effect of one percentage point decrease on total service and interest cost (1,108)
Effect of one percentage point increase on accumulated postretirement benefit obligation 18,170
Effect of one percentage point decrease on accumulated postretirement benefit obligation $ (15,086)