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Retirement Benefit Plans - Summary of Sensitivity Analysis (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
The expected contributions to the plan for the next year $ 1,932 $ 2,076
Bottom of range [Member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
The average duration of the defined benefit obligations 6 years 4 months 24 days 6 years 6 months
Top of range [Member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
The average duration of the defined benefit obligations 13 years 14 years