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Retirement Benefit Plans - Movements in Present Value of Defined Benefit Obligation (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Remeasurement:      
Actuarial loss arising from experience adjustments $ 334.7 $ 483.9 $ 38.2
Actuarial loss (gain) arising from changes in financial assumptions 597.8 (258.5) 694.6
Actuarial loss arising from changes in demographic assumptions     278.7
Present value of defined benefit obligation [member]      
Disclosure of defined benefit plans [line items]      
Balance, beginning of year 12,774.6 12,480.5 11,318.1
Current service cost 137.7 145.0 132.8
Interest expense 207.8 185.6 213.0
Remeasurement:      
Actuarial loss arising from experience adjustments 334.7 483.9 38.2
Actuarial loss (gain) arising from changes in financial assumptions 597.8 (258.5) 694.6
Actuarial loss arising from changes in demographic assumptions     278.7
Benefits paid from plan assets (274.3) (261.9) (194.9)
Benefits paid directly by the Company (115.6)    
Balance, end of year $ 13,662.7 $ 12,774.6 $ 12,480.5