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Held-to-maturity Financial Assets - Summary of Held-to-maturity Financial Assets (Detail)
$ in Millions
Dec. 31, 2017
TWD ($)
Disclosure of financial assets [line items]  
Held-to-maturity investments $ 20,821.7
Current portion 1,988.4
Noncurrent portion 18,833.3
Corporate bonds [member]  
Disclosure of financial assets [line items]  
Held-to-maturity investments 19,338.8
Structured product [member]  
Disclosure of financial assets [line items]  
Held-to-maturity investments $ 1,482.9