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Income Tax - Items for Which No Deferred Income Tax Assets Have Been Recognized (Detail) - TWD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards $ 341 $ 335
Deductible temporary differences 2 3
2018 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 126 126
2019 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 138 138
2020 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 42 42
2021 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 9 13
2022 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 11 1
2023 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 0 1
2024 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 0 0
2025 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 13 14
2026 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards 0 0
2027 [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Loss carryforwards $ 2 $ 0