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Income Tax - Items for Which No Deferred Income Tax Assets Have Been Recognized (Detail) - TWD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards $ 29 $ 58
2022 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 0 10
2023 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 0 8
2024 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 1 8
2025 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 15 19
2026 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 8 8
2027 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 3 3
2028 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 1 1
2029 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 1 1
2030 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 0 0
2031 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards $ 0 $ 0