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Post-employment Benefits - Actuarial assumptions (Details) - Y
Dec. 31, 2017
Dec. 31, 2016
Post-employment Benefits    
Actuarial assumption of retirement age 65 65
Discount rate (as a percent) 7.60% 6.70%
Salary scale (as a percent) 5.00% 5.00%
Inflation rate (as a percent) 3.50% 3.50%