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Pension and other postretirement benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Pension Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Service cost $ 4,651 $ 6,346 $ 5,605
Interest cost 13,892 13,388 11,629
Expected return on plan assets (18,774) (21,241) (14,694)
Amortization of prior service costs 106 110 77
Amortization of net loss 4,666 3,980 1,907
Settlements 0 0 4,701
Curtailment cost (benefit) 0 1,235 263
Net periodic benefit cost 4,541 3,818 9,488
Other Postretirement Benefit Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Service cost 1,003 1,970 1,591
Interest cost 5,595 5,985 6,420
Expected return on plan assets 0 0 0
Amortization of prior service costs (2,781) (3,728) (3,844)
Amortization of net loss 3,537 3,814 3,704
Settlements 0 0 0
Curtailment cost (benefit) 0 (4,266) 0
Net periodic benefit cost $ 7,354 $ 3,775 $ 7,871