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Held-to-maturity Financial Assets - Summary of Held-to-maturity Financial Assets (Detail)
$ in Millions, $ in Millions
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Disclosure of financial assets [line items]      
Held-to-maturity investments $ 20,821.7   $ 38,917.7
Current portion 1,988.4 $ 67.1 16,610.1
Noncurrent portion 18,833.3 $ 635.4 22,307.6
Corporate bonds [member]      
Disclosure of financial assets [line items]      
Held-to-maturity investments 19,338.8   23,849.7
Structured product [member]      
Disclosure of financial assets [line items]      
Held-to-maturity investments $ 1,482.9   1,609.9
Commercial paper [member]      
Disclosure of financial assets [line items]      
Held-to-maturity investments     8,628.2
Negotiable certificate of deposit [member]      
Disclosure of financial assets [line items]      
Held-to-maturity investments     $ 4,829.9