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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, 2018
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Disclosure of fair value measurement of assets [line items]        
Beginning Balance   $ 363,922,272    
Ending Balance $ 17,447,301 534,061,927 $ 363,922,272  
Available-for-sale [member] | Level 3 of fair value hierarchy [member]        
Disclosure of fair value measurement of assets [line items]        
Beginning Balance 64,964 1,988,549    
Recognized in profit or loss (76) (2,313)    
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) (9,384) (287,248)    
Purchases 19,027 582,436    
Disposals (6,827) (208,978)    
Ending Balance 67,704 2,072,446 1,988,549  
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | Equity Instruments Fair Value Other Comprehensive Income [member]        
Disclosure of fair value measurement of assets [line items]        
Beginning Balance 29,681 908,549 904,790 $ 741,089
Recognized in profit or loss     28 (100,734)
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) (7,323) (224,172) 17,284 (202,565)
Purchases 3,430 105,000 2,649 495,928
Disposals (6,827) (208,978) 16,202 28,928
Ending Balance 18,961 580,399 908,549 $ 904,790
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | FVTPL equity instrument [member]        
Disclosure of fair value measurement of assets [line items]        
Recognized in profit or loss (76) (2,313)    
Purchases 15,597 477,436    
Ending Balance 15,521 475,123    
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | FVOCI Debt Instruments [member]        
Disclosure of fair value measurement of assets [line items]        
Beginning Balance 35,283 1,080,000    
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) (2,061) (63,076)    
Ending Balance $ 33,222 $ 1,016,924 $ 1,080,000