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Retirement Benefit Plans - Pension Costs of Defined Benefit Plans Recognized in Profit or Loss by Categories (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement $ 204.9 $ 259.6 $ 281.8
Cost of revenue [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 126.3 157.8 177.8
Research and development expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 57.3 72.7 79.1
General and administrative expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 18.2 25.1 20.6
Marketing expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement $ 3.1 $ 4.0 $ 4.3