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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, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items]    
Financial assets $ 747,870.1 $ 856,181.4
Financial liabilities 534,565.8 318,672.4
Financial assets at amortised cost, class [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets   21,805.9
Financial assets at amortised cost, class [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets 7,648.8 19,511.8
Financial assets at amortised cost, class [member] | Commercial paper [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets   2,294.1
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 56,900.0 91,800.0
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Financial assets at amortised cost, class [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   21,850.7
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 7,718.7 19,554.5
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Financial assets at amortised cost, class [member] | Commercial paper [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   2,296.2
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 $ 57,739.1 $ 93,171.3