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Financial Instruments - Fair Value of Financial Instruments Not Measured at Fair Value but for which Fair Value is Disclosed (Detail) - TWD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]    
Financial assets $ 958,471.8 $ 747,870.1
Financial liabilities 748,224.6 534,565.8
Financial assets at amortised cost, class [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets 10,970.2 7,648.8
Financial liabilities at amortised cost [member] | Bonds payable including long-term liabilities-current portion) [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial liabilities 256,705.1 56,900.0
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Financial assets at amortised cost, class [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value 11,053.6 7,718.7
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Financial liabilities at amortised cost [member] | Bonds payable including long-term liabilities-current portion) [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial liabilities, at fair value $ 257,551.2 $ 57,739.1