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Retirement Benefit Plans - Fair Value of Plan Assets by Major Categories (Detail) - TWD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure of fair value of plan assets [abstract]    
Cash $ 756.1 $ 707.5
Equity instruments 2,148.1 1,993.3
Debt instruments 1,107.1 1,223.1
Fair value of plan assets $ 4,011.3 $ 3,923.9