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Retirement Benefit Plans - Defined Benefit Plans Amounts Recognized in Statements of Financial Position (Detail) - TWD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Surplus (deficit) in plan [abstract]      
Present value of defined benefit obligations $ (901,159) $ (910,081)  
Fair value of plan assets 421,052 389,316  
Net defined benefit liability $ (480,107) $ (520,765) $ (478,526)