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Available-for-sale Financial Assets Non Current - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Disclosure of financial assets [Line Items]        
Impairment loss on available-for-sale financial assets $ 0 $ 0 $ 577,000,000 $ 107,000,000
Carrying value of non-listed available-for-sale financial assets disposed 5,000,000   9,000,000 2,000,000
Gains (losses) from disposal of available-for-sale assets 3,000,000   1,000,000 (400,000)
Taiwania Capital Buffalo Fund Co Ltd [Member]        
Disclosure of financial assets [Line Items]        
Amount invested in investment entity $ 300,000,000      
Ownership percentage in investment entity 12.90% 12.90%    
Domestic listed stocks [Member]        
Disclosure of financial assets [Line Items]        
Impairment loss on available-for-sale financial assets $ 0   577,000,000 26,000,000
Non-listed stocks [Member] | Ceased operations [Member]        
Disclosure of financial assets [Line Items]        
Impairment loss on available-for-sale financial assets       77,000,000
Non-listed stocks [Member] | Profit recession or deficit [Member]        
Disclosure of financial assets [Line Items]        
Impairment loss on available-for-sale financial assets $ 0   $ 0 $ 4,000,000