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Retirement Benefit Plans - Amounts Recognized in Consolidated Statements of Profit or Loss and Other Comprehensive Income of Defined Benefit Plans (Detail)
$ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Disclosure of defined benefit plans [abstract]        
Current service cost $ 135.6   $ 137.7 $ 145.0
Net interest expense 124.0   144.1 126.5
Components of defined benefit costs recognized in profit or loss 259.6   281.8 271.5
Remeasurement on the net defined benefit liability:        
Return on plan assets (excluding amounts included in net interest expense) (124.4)   (71.3) 29.3
Actuarial loss (gain) arising from experience adjustments (438.0)   334.7 483.9
Actuarial gain arising from changes in demographic assumptions (233.2)      
Actuarial loss (gain) arising from changes in financial assumptions 541.7   597.8 (258.5)
Components of defined benefit costs recognized in other comprehensive income (253.9) $ (8.5) 861.2 254.7
Total $ 5.7   $ 1,143.0 $ 526.2