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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement $ 281.8 $ 271.5 $ 272.2
Cost of revenue [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 177.8 175.3 177.0
Research and development expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 79.1 75.3 73.4
General and administrative expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 20.6 16.7 17.4
Marketing expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement $ 4.3 $ 4.2 $ 4.4