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Application of New and Amended International Financial Reporting Standards - Additional Information (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Jan. 01, 2018
TWD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2018
USD ($)
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
NONCONTROLLING INTERESTS   $ 9,857 $ 8,474   $ 322
Unappropriated earnings   66,626 54,633   2,177
Tax obligation   8,271 7,996 $ 6,736  
Deferred income tax liabilities   $ 1,992 $ 1,430   $ 65
Remeasured FVOCI [Member]          
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
Total effect on assets $ 1,842        
Total effect on equity 1,843        
NONCONTROLLING INTERESTS (1)        
Reclassification of Impairment [Member]          
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
Total effect on assets 1,516        
Unappropriated earnings 1,516        
Increase (Decrease) Due to Application of IFRS 9 [Member]          
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
Unrealized gain or loss on available-for-sale financial assets 4        
NONCONTROLLING INTERESTS (3)        
Deferred income tax liabilities (1)        
Increase (Decrease) Due to Application of IFRS 9 [Member] | Classification of assets as held for sale [member]          
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
Tax obligation 0        
Total retained earnings [Member]          
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
Unrealized gain or loss on available-for-sale financial assets 6        
Other Equity [Member]          
Disclosure Of Initial Application Of Standards Or Interpretations [Line Items]          
Unrealized gain or loss on available-for-sale financial assets $ 556