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Retirement Benefit Plans - Amounts Arising from Defined Benefit Obligation in Consolidated Statements of Financial Position (Detail)
$ in Millions, $ in Millions
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Disclosure of defined benefit plans [abstract]      
Present value of defined benefit obligation $ 12,774.6   $ 12,480.5
Fair value of plan assets (3,923.9)   (3,929.1)
Net defined benefit liability $ 8,850.7 $ 298.6 $ 8,551.4