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Disclosure of additional information about defined benefit plans (Details) - Pension obligations [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Statement [Line Items]    
Opening defined benefit obligation $ 349,165 $ 337,004
Current service cost 10,707 10,768
Past service cost related to the new collective bargaining agreement 10,442 0
Interest cost 12,602 13,415
Benefits paid from plan (33,721) (32,644)
Benefits paid from employer (999) (1,424)
Participant contributions 93 93
Effects of movements in exchange rates 24,440 10,348
Arising from changes in demographic assumptions 1,598 0
Arising from changes in financial assumptions 9,402 14,955
Arising from experience adjustments (675) (3,350)
Closing defined benefit obligation $ 383,054 $ 349,165