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Retirement Benefit Plans - Fair Value of Plan Assets by Major Categories (Detail) - TWD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Disclosure of fair value of plan assets [abstract]    
Cash $ 713.2 $ 756.1
Equity instruments 2,313.8 2,148.1
Debt instruments 1,274.6 1,107.1
Fair value of plan assets $ 4,301.6 $ 4,011.3