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Net Income and Other Comprehensive Income (Loss) - Other Gains and Losses (Detail) $ in Millions, $ in Millions | 12 Months Ended
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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)

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- Definition Gain losses on disposal of financial instruments. + References No definition available. + Details
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- Definition The net gain arising from foreign exchange differences. [Refer: Foreign exchange gain (loss)] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 21 -IssueDate 2016-01-13 -Paragraph 52 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=21&date=2016-03-30&anchor=para_52_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The gains (losses) that the entity does not separately disclose in the same statement or note. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2016-01-13 -Paragraph 102 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_102&doctype=Standard -URIDate 2016-03-31 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2016-01-13 -Paragraph 103 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_103&doctype=Standard -URIDate 2016-03-31 + Details
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