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Net Pension Assets - Summary of Sensitivity Analysis for Actuarial Assumptions (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Discount rate [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Actuarial assumption rate 3.15% 3.37% 3.45%
Compensation - salary increase [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Actuarial assumption rate 4.00% 4.00% 4.00%
Mortality [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Demographic assumptions, Mortality RP - 2000 no collar RP - 2000 no collar RP - 2000 no collar
Termination [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Actuarial assumption rate 13.00% 13.00% 13.00%
Male [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Retirement age 62 62 62
Female [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Retirement age 57 57 57