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Financial Instruments - Summary of Level 3 Fair Value Were Equity Investments classified as Financial Assets at FVTOCI (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of fair value of investments in equity instruments designated at fair value through other comprehensive income [line items]    
Beginning balance $ 103,472.4  
Ending balance 131,520.9 $ 103,472.4
Level 3 [member]    
Disclosure of fair value of investments in equity instruments designated at fair value through other comprehensive income [line items]    
Beginning balance 3,910.7 5,841.4
Additions 372.3 212.5
Recognized in other comprehensive income 129.5 (2,141.4)
Disposals and proceeds from return of capital of investments (76.5) (175.8)
Transfers out of level 3 (Note) (43.6)  
Effect of exchange rate changes (83.5) 174.0
Ending balance $ 4,208.9 $ 3,910.7