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Financial Instruments - Categories of Financial Instruments (Detail) - TWD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Financial assets    
Held-to-maturity financial assets $ 0 $ 2,140
Loans and receivables (Note a) 68,983 70,040
Available-for-sale financial assets 5,751 4,764
Financial liabilities    
Measured at FVTPL Held for trading 1 1
Measured at amortized cost (Note b) 39,725 40,553
Cash flow hedges [Member]    
Financial liabilities    
Hedging derivative financial liabilities $ 1 $ 1