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Financial Instruments - Fair Value of Financial Instruments Measured at Fair Value on a Recurring Basis (Detail) - TWD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL $ 326.8 $ 3,504.6  
Financial assets at FVTOCI, investments in equity and debt instruments 131,520.9 103,472.4  
Financial assets at FVTOCI 134,776.8 107,067.5  
Level 3 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 4,208.9 3,910.7 $ 5,841.4
Measured at fair value on a recurring basis [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL 326.8 3,504.6  
Financial assets at FVTOCI 134,776.8 107,067.5  
Hedging derivative financial assets 25.9    
Hedging derivative financial liabilities   155.8  
Measured at fair value on a recurring basis [member] | Loans and receivables [member]      
Disclosure of detailed information about financial instruments [line items]      
Notes and accounts receivable, net 3,255.9 3,595.1  
Measured at fair value on a recurring basis [member] | Commercial paper [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments   107.6  
Measured at fair value on a recurring basis [member] | Level 1 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI 12,678.1 11,596.3  
Hedging derivative financial assets 22.4    
Hedging derivative financial liabilities   153.9  
Measured at fair value on a recurring basis [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL 203.0 3,504.6  
Financial assets at FVTOCI 118,013.6 91,560.5  
Hedging derivative financial assets 3.5    
Hedging derivative financial liabilities   1.9  
Measured at fair value on a recurring basis [member] | Level 2 [member] | Loans and receivables [member]      
Disclosure of detailed information about financial instruments [line items]      
Notes and accounts receivable, net 3,255.9 3,595.1  
Measured at fair value on a recurring basis [member] | Level 2 [member] | Commercial paper [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments   107.6  
Measured at fair value on a recurring basis [member] | Level 3 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL 123.8    
Financial assets at FVTOCI 4,085.1 3,910.7  
Measured at fair value on a recurring basis [member] | Forward exchange contracts [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL 162.1 85.3  
Hedging derivative financial assets   23.5  
Financial liabilities at FVTPL, Held for trading 982.3 40.8  
Hedging derivative financial liabilities 1.8    
Measured at fair value on a recurring basis [member] | Forward exchange contracts [member] | Cash flow hedges [member]      
Disclosure of detailed information about financial instruments [line items]      
Hedging derivative financial assets 3.5    
Hedging derivative financial liabilities   1.9  
Measured at fair value on a recurring basis [member] | Forward exchange contracts [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL 162.1 85.3  
Hedging derivative financial assets   23.5  
Financial liabilities at FVTPL, Held for trading 982.3 40.8  
Hedging derivative financial liabilities 1.8    
Measured at fair value on a recurring basis [member] | Forward exchange contracts [member] | Level 2 [member] | Cash flow hedges [member]      
Disclosure of detailed information about financial instruments [line items]      
Hedging derivative financial assets 3.5    
Hedging derivative financial liabilities   1.9  
Measured at fair value on a recurring basis [member] | Convertible Bonds [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL 123.8    
Measured at fair value on a recurring basis [member] | Convertible Bonds [Member] | Level 3 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL 123.8    
Measured at fair value on a recurring basis [member] | Corporate bonds [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 51,790.0 40,753.6  
Measured at fair value on a recurring basis [member] | Corporate bonds [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 51,790.0 40,753.6  
Measured at fair value on a recurring basis [member] | Agency bonds/Agency mortgage-backed securities [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 51,966.5 31,288.8  
Measured at fair value on a recurring basis [member] | Agency bonds/Agency mortgage-backed securities [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 51,966.5 31,288.8  
Measured at fair value on a recurring basis [member] | Asset backed securities [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 10,815.9 15,670.3  
Measured at fair value on a recurring basis [member] | Asset backed securities [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 10,815.9 15,670.3  
Measured at fair value on a recurring basis [member] | Government bonds [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 12,824.2 11,151.3  
Measured at fair value on a recurring basis [member] | Government bonds [member] | Level 1 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 12,678.1 11,006.2  
Measured at fair value on a recurring basis [member] | Government bonds [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, investments in equity and debt instruments 146.1 145.1  
Measured at fair value on a recurring basis [member] | Publicly traded stocks [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, Investments in equity instruments   590.1  
Measured at fair value on a recurring basis [member] | Publicly traded stocks [member] | Level 1 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, Investments in equity instruments   590.1  
Measured at fair value on a recurring basis [member] | Futures Contract [member] | Fair value hedges [member]      
Disclosure of detailed information about financial instruments [line items]      
Hedging derivative financial assets 22.4    
Hedging derivative financial liabilities   153.9  
Measured at fair value on a recurring basis [member] | Futures Contract [member] | Level 1 [member] | Fair value hedges [member]      
Disclosure of detailed information about financial instruments [line items]      
Hedging derivative financial assets 22.4    
Hedging derivative financial liabilities   153.9  
Measured at fair value on a recurring basis [member] | Agency mortgage backed securities [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL 40.9 3,419.3  
Measured at fair value on a recurring basis [member] | Agency mortgage backed securities [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTPL, mandatorily measured at FVTPL 40.9 3,419.3  
Measured at fair value on a recurring basis [member] | Non-publicly traded stocks [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, Investments in equity instruments 4,124.3 3,910.7  
Measured at fair value on a recurring basis [member] | Non-publicly traded stocks [member] | Level 2 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, Investments in equity instruments 39.2    
Measured at fair value on a recurring basis [member] | Non-publicly traded stocks [member] | Level 3 [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets at FVTOCI, Investments in equity instruments $ 4,085.1 $ 3,910.7