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Income Tax - Income Tax Expense Recognized in Other Comprehensive Income (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income tax relating to components of other comprehensive income [abstract]      
Related to remeasurement of defined benefit obligation $ (30.4) $ 103.3 $ 30.6
Related to unrealized gain/loss on investments in equity instruments at FVTOCI 9.4 91.8  
Related to gain/loss on cash flow hedges   0.6 (0.6)
Related to unrealized gain/loss on available-for-sale financial assets     (2.9)
Deferred income tax benefit (expense) $ (21.0) $ 195.7 $ 27.1