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Retirement Benefit Plans - Summary of Defined Benefit Plans Amounts Included in Consolidated Balance Sheets (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
TWD ($)
Dec. 31, 2014
TWD ($)
Disclosure of net defined benefit liability (asset) [abstract]            
Present value of the defined benefit obligation $ 7,910,638 $ 266,891 $ 8,389,884      
Fair value of plan assets (4,341,373) (146,470) (4,417,367)      
Present value of unfunded defined benefit obligation 3,569,265 120,421 3,972,517 $ 134,026 $ 3,999,947 $ 4,171,806
Recorded under other payables (24,638) (831) (22,273)      
Recorded under other current assets 182,421 6,154 15,542      
Net defined benefit liability $ 3,727,048 $ 125,744 $ 3,965,786