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Fair Value Information - Schedule of Reconciliations for Financial Assets (Detail) - Level 3 [Member] - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Fair Value Measurement Of Assets [Line Items]        
Foreign currency denominated monetary assets $ 5,116 $ 5,478 $ 4,550 $ 4,468
Acquisition   300 290  
Recognized in profit or loss under “Other gains and losses” (101) (39) (26)  
Recognized in other comprehensive income under “Unrealized gain or loss on financial assets at fair value through other comprehensive income” (263) 676 (175)  
Proceeds from return of investees     (7)  
Proceed from return of investments   (9)    
Reclassified from investments accounted for using equity method 2      
Measured at Fair Value Through Profit or Loss [Member]        
Disclosure Of Fair Value Measurement Of Assets [Line Items]        
Foreign currency denominated monetary assets 677 778 517 543
Acquisition   300    
Recognized in profit or loss under “Other gains and losses” (101) (39) (26)  
Unrealized loss (101) (39) (26)  
FVOCI [Member]        
Disclosure Of Fair Value Measurement Of Assets [Line Items]        
Foreign currency denominated monetary assets 4,439 4,700 4,033 $ 3,925
Acquisition   0 290  
Recognized in other comprehensive income under “Unrealized gain or loss on financial assets at fair value through other comprehensive income” (263) 676 (175)  
Proceeds from return of investees     $ (7)  
Proceed from return of investments   $ (9)    
Reclassified from investments accounted for using equity method $ 2