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Retirement Benefit Plans - Summary of Sensitivity Analysis (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2018
TWD ($)
yr
Dec. 31, 2017
TWD ($)
yr
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
The expected contributions to the plan for the next year | $ $ 2,237 $ 4,393
Bottom of range [Member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
The average duration of the defined benefit obligation 6.5 6.8
Top of range [Member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
The average duration of the defined benefit obligation 12.1 12.5