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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, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Disclosure of defined benefit plans [abstract]        
Current service cost $ 145.0   $ 132.8 $ 134.5
Net interest expense 126.5   139.4 144.4
Components of defined benefit costs recognized in profit or loss 271.5   272.2 278.9
Remeasurement on the net defined benefit liability:        
Return on plan assets (excluding amounts included in net interest expense) 29.3   45.7 (13.7)
Actuarial loss arising from experience adjustments 483.9   38.2 297.1
Actuarial loss (gain) arising from changes in financial assumptions (258.5)   694.6 544.3
Actuarial loss arising from changes in demographic assumptions     278.7  
Components of defined benefit costs recognized in other comprehensive income 254.7 $ 8.6 1,057.2 827.7
Total $ 526.2   $ 1,329.4 $ 1,106.6