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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement $ 271.5 $ 272.2 $ 278.9
Cost of revenue [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 175.3 177.0 189.5
Research and development expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 75.3 73.4 81.3
General and administrative expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement 16.7 17.4 3.1
Marketing expenses [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit cost recognized in the income statement $ 4.2 $ 4.4 $ 5.0