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Net Income and Other Comprehensive Income (Loss) - Other Gains and Losses (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Analysis Of Income And Expense [Abstract]        
Net foreign currency exchange gains (losses) $ (88)   $ 181 $ 63
Gain on disposal of financial instruments 3 $ 0 0 0
Valuation gain (loss) on financial assets and liabilities at fair value through profit or loss, net 1 0 (1) 0
Gain (loss) on disposal of investments accounted for using equity method 0 0 (2) 4
Impairment loss on investments accounted for using equity method 0 0 0 (8)
Impairment loss on available-for-sale financial assets 0 0 (577) (107)
Others (48)   (49) (180)
Other gains and losses $ (132) $ (4) $ (448) $ (228)