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Fair Value Information - Schedule of Reconciliations for Financial Assets (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Disclosure Of Fair Value Measurement Of Assets [Line Items]      
Effect of retrospective application of IFRS 9     $ 883
Level 3 [Member]      
Disclosure Of Fair Value Measurement Of Assets [Line Items]      
Foreign currency denominated monetary assets $ 4,550 $ 4,468  
Effect of retrospective application of IFRS 9   4,468  
Acquisition 290    
Recognized in profit or loss under “Other gains and losses” (26)    
Recognized in other comprehensive income under “Unrealized gain or loss on investments in equity instruments at fair value through other comprehensive income” (175)    
Proceeds from return of investees (7)    
Level 3 [Member] | Measured at Fair Value Through Profit or Loss [Member]      
Disclosure Of Fair Value Measurement Of Assets [Line Items]      
Foreign currency denominated monetary assets 517 543  
Effect of retrospective application of IFRS 9   543  
Recognized in profit or loss under “Other gains and losses” (26)    
Unrealized loss in 2018 (26)    
Level 3 [Member] | FVOCI [Member]      
Disclosure Of Fair Value Measurement Of Assets [Line Items]      
Foreign currency denominated monetary assets 4,033 3,925  
Effect of retrospective application of IFRS 9   $ 3,925  
Acquisition 290    
Recognized in other comprehensive income under “Unrealized gain or loss on investments in equity instruments at fair value through other comprehensive income” (175)    
Proceeds from return of investees $ (7)