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Retirement Benefit Plans - Movements in Fair Value of Plan Assets (Detail) - Fair value of plan assets [member] - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of fair value of plan assets [line items]      
Balance, beginning of year $ 3,923.9 $ 3,929.1 $ 3,870.1
Interest income 63.7 59.1 73.6
Remeasurement:      
Return on plan assets (excluding amounts included in net interest expense) 71.3 (29.3) (45.7)
Contributions from employer 226.7 226.9 226.0
Benefits paid from plan assets (274.3) (261.9) (194.9)
Balance, end of year $ 4,011.3 $ 3,923.9 $ 3,929.1