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Employees - Summary of Assumptions, Weighted by Liabilities for Valuation of Defined Benefit Plans Which cover Approximately 96% of Total Pension Plan (Detail)
Dec. 31, 2017
Dec. 31, 2016
Defined benefit pension plans [member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 2.50% 2.60%
Inflation 2.50% 2.50%
Rate of increase in salaries 2.80% 2.90%
Rate of increase for pensions in payment (where provided) 2.40% 2.40%
Rate of increase for pensions in deferment (where provided) 2.60% 2.70%
Other post-employment benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 4.20% 4.80%
Rate of increase in salaries 3.00% 3.00%
Long-term medical cost inflation 5.30% 5.30%