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Income Tax - Income Tax Recognized in Other Comprehensive Income (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Deferred tax      
Remeasurement on defined benefit pension plans $ 239 $ 305 $ (243)
Change in tax rate - defined benefit pension plans 0 0 (207)
Total income tax expense (benefit) recognized in other comprehensive income $ 239 $ 305 $ (450)