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Pension and other postretirement benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Pension Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Service cost $ 6,346 $ 5,605 $ 4,735
Interest cost 13,388 11,629 8,908
Expected return on plan assets (21,241) (14,694) (10,592)
Amortization of prior service costs 110 77 113
Amortization of net loss 3,980 1,907 3,152
Settlements 0 4,701 0
Curtailment cost (benefit) 1,235 263 (18)
Net periodic benefit cost 3,818 9,488 6,298
Other Postretirement Benefit Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Service cost 1,970 1,591 2,527
Interest cost 5,985 6,420 5,681
Expected return on plan assets 0 0 0
Amortization of prior service costs (3,728) (3,844) (3,995)
Amortization of net loss 3,814 3,704 5,022
Settlements 0 0 0
Curtailment cost (benefit) (4,266) 0 (20)
Net periodic benefit cost $ 3,775 $ 7,871 $ 9,215