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Provisions - Summary of Provisions (Detail) - Sales returns and allowances [member]
$ in Millions
12 Months Ended
Dec. 31, 2017
TWD ($)
Disclosure of other provisions [line items]  
Balance, beginning of year $ 18,037.8
Provision 44,833.6
Payment (48,884.7)
Effect of exchange rate changes (24.9)
Balance, end of year $ 13,961.8