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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, 2018
Dec. 31, 2017
Disclosure of detailed information about financial instruments [line items]    
Financial assets $ 856,181.4 $ 804,091.0
Financial liabilities 318,672.4 340,543.5
Held-to-maturity [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets   20,821.7
Held-to-maturity [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets   19,338.8
Held-to-maturity [member] | Structured product [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets   1,482.9
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 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 91,800.0 150,201.1
Not measured at fair value but for which the fair value is disclosed [member] | Held-to-maturity [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   21,016.8
Not measured at fair value but for which the fair value is disclosed [member] | Held-to-maturity [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   19,541.4
Not measured at fair value but for which the fair value is disclosed [member] | Held-to-maturity [member] | Structured product [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   1,475.4
Not measured at fair value but for which the fair value is disclosed [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] | Financial assets at amortised cost, class [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value 19,554.5  
Not measured at fair value but for which the fair value is disclosed [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] | 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 93,171.3 152,077.7
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Held-to-maturity [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   21,016.8
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Held-to-maturity [member] | Corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   19,541.4
Not measured at fair value but for which the fair value is disclosed [member] | Level 2 [member] | Held-to-maturity [member] | Structured product [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, at fair value   1,475.4
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 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 $ 93,171.3 $ 152,077.7