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Segment Information - Analysis by Reportable Segment of Revenue and Operating Results of Continuing Operations (Detail) $ in Millions, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 TWD ($)
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 227,514 | $ 7,676 | $ 229,991 | $ 231,795
Income before income tax | 48,009 | $ 1,620 | 49,413 | 51,953
From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 227,514 |  | 229,991 | 231,795
Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 35,730 |  | 36,735 | 34,911
Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 263,244 |  | 266,726 | 266,706
Intersegment Elimination [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | (35,730) |  | (36,735) | (34,911)
Consolidated [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 227,514 |  | 229,991 | 231,795
Domestic Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 24,888 |  | 25,658 | 23,231
Domestic Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 71,137 |  | 72,784 | 72,535
Domestic Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 22,515 |  | 22,669 | 21,401
Domestic Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 93,652 |  | 95,453 | 93,936
Mobile Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 12,433 |  | 13,926 | 19,394
Mobile Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 109,376 |  | 110,801 | 114,877
Mobile Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 2,031 |  | 2,530 | 3,475
Mobile Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 111,407 |  | 113,331 | 118,352
Internet Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 11,118 |  | 10,729 | 9,918
Internet Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 28,917 |  | 28,100 | 25,777
Internet Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,209 |  | 4,734 | 4,701
Internet Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 33,126 |  | 32,834 | 30,478
International Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 1,029 |  | 1,098 | 1,120
International Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 13,552 |  | 14,434 | 15,460
International Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 2,375 |  | 2,680 | 2,120
International Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 15,927 |  | 17,114 | 17,580
Other [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | (1,459) |  | (1,998) | (1,710)
Other [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,532 |  | 3,872 | 3,146
Other [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,600 |  | 4,122 | 3,214
Other [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 9,132 |  | $ 7,994 | $ 6,360

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