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Retirement Benefit Plans - Movements in Present Value of Defined Benefit Obligation (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Remeasurement:      
Actuarial loss arising from experience adjustments $ 911.7 $ 68.3 $ 1,413.8
Actuarial (gain) loss arising from changes in financial assumptions (281.5) 571.3 (160.8)
Present value of defined benefit obligation [member]      
Disclosure of defined benefit plans [line items]      
Balance, beginning of year 17,995.1 17,484.0 16,585.5
Current service cost 153.0 139.1 134.3
Interest expense 242.8 304.0 120.8
Remeasurement:      
Actuarial loss arising from experience adjustments 911.7 68.3 1,413.8
Actuarial (gain) loss arising from changes in financial assumptions (281.5) 571.3 (160.8)
Benefits paid from plan assets (679.1) (556.5) (585.3)
Benefits paid directly by the Company (9.8) (15.1) (24.3)
Balance, end of year $ 18,332.2 $ 17,995.1 $ 17,484.0