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Post-employment benefit obligations (Schedule of Principal Actuarial Assumptions) (Details)
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [abstract]    
Discount rate (expense) 4.25% 4.25%
Discount rate (year end disclosures) 3.75% 4.25%
Future salary increases (salaried plan only) 5.00% 5.00%