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Employee Benefit - Significant Assumptions Used in Determining Actuarial Present Value of Defined Benefit Obligations (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Net Defined Benefit Liability Asset [Line Items]    
Actuarial assumption of discount rates 2.70%  
Pre-retirement mortality [1] Thailand TMO17 Tables Thailand TMO08 Tables
Bottom of range [Member]    
Disclosure Of Net Defined Benefit Liability Asset [Line Items]    
Actuarial assumption of discount rates   3.10%
Actuarial assumption of expected rates of salary increases 5.00% 5.00%
Top of range [Member]    
Disclosure Of Net Defined Benefit Liability Asset [Line Items]    
Actuarial assumption of discount rates   3.30%
Actuarial assumption of expected rates of salary increases 6.00% 6.00%
[1] TMO represented as Thailand Mortality Ordinary Tables