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Income Tax - Information About Unused Loss Carryforwards (Detail)
$ in Millions, $ in Millions
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount $ 3,133 $ 112 $ 3,259
Unused tax losses [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 91    
Unused tax losses [Member] | 2021 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 14    
Unused tax losses [Member] | 2022 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 11    
Unused tax losses [Member] | 2023 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 9    
Unused tax losses [Member] | 2024 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 8    
Unused tax losses [Member] | 2025 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 22    
Unused tax losses [Member] | 2026 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 16    
Unused tax losses [Member] | 2027 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 3    
Unused tax losses [Member] | 2028 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 4    
Unused tax losses [Member] | 2029 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount 2    
Unused tax losses [Member] | 2030 [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Remaining creditable amount $ 2