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Income Tax - Items for Which No Deferred Income Tax Assets Have Been Recognized (Detail) - TWD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards $ 237 $ 215
Deductible temporary differences 1 2
2019 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 136 138
2020 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 41 42
2021 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 12 9
2022 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 10 11
2023 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 9 0
2024 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 2 0
2025 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 15 13
2026 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 8 0
2027 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 3 2
2028 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards $ 1 $ 0