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Financial Instruments - Summary of Financial Assets Measured at Level 3 Fair Value (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Disclosure of fair value measurement of assets [line items]        
Beginning Balance $ 357,930,576   $ 364,986,892  
Ending Balance 363,922,272 $ 12,278,079 357,930,576 $ 364,986,892
Level 3 of fair value hierarchy [member] | Financial assets available-for-sale, category [member]        
Disclosure of fair value measurement of assets [line items]        
Beginning Balance 904,790 30,526 741,089 778,866
Purchases 2,649 89 495,928 2,010
Total gain or loss in profit or loss 28 1 (100,734) (15,891)
Total gain or loss in other comprehensive income 17,284 583 (202,565) 21,195
Disposals (16,202) (547) (28,928) (45,091)
Ending Balance $ 908,549 $ 30,652 $ 904,790 $ 741,089