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Retirement Benefit Plans - Pension Costs Recognized in Profit and Loss for Defined Benefit Plans (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss $ 2,001 $ 2,859 $ 2,953
Operating costs [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss 1,205 1,726 1,796
Marketing expenses [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss 603 866 886
General and administrative expenses [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss 121 164 164
Research and development expenses [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss $ 72 $ 103 $ 107