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Net Employee Defined Benefit Liability - Summary of Principal Assumptions Used In Determining Pension Benefit Obligations (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of sensitivity analysis for actuarial assumptions [abstract]    
Discount rate 2.47% 2.68%
Rate of increase in salaries 3.51% 4.50%
Rate of inflation, pre-retirement 2.40% 2.45%
Rate of increase in future pensions payment 3.53% 3.58%
Life expectancy at age 65 for current pensioners:    
Male 22 years 22 years
Female 24 years 24 years
Life expectancy at age 65 for current members aged 40:    
Male 23 years 24 years
Female 26 years 26 years