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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [Line Items]      
Defined benefit plans pension costs recognized in profit and loss $ 2,834 $ 2,822 $ 2,914
Operating costs [Member]      
Disclosure of defined benefit plans [Line Items]      
Defined benefit plans pension costs recognized in profit and loss 1,734 1,732 1,794
Marketing expenses [Member]      
Disclosure of defined benefit plans [Line Items]      
Defined benefit plans pension costs recognized in profit and loss 847 838 856
General and administrative expenses [Member]      
Disclosure of defined benefit plans [Line Items]      
Defined benefit plans pension costs recognized in profit and loss 156 155 162
Research and development expenses [Member]      
Disclosure of defined benefit plans [Line Items]      
Defined benefit plans pension costs recognized in profit and loss $ 97 $ 97 $ 102