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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss $ 2,953 $ 2,834 $ 2,822
Operating costs [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss 1,796 1,734 1,732
Marketing expenses [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss 886 847 838
General and administrative expenses [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss 164 156 155
Research and development expenses [Member]      
Disclosure Of Defined Plan [line items]      
Defined benefit plans pension costs recognized in profit and loss $ 107 $ 97 $ 97