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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, 2018 TWD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($)
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 215,483 | $ 7,040 | $ 227,514 | $ 229,991
Income before income tax | 44,986 | $ 1,470 | 48,009 | 49,413
From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 215,483 |  | 227,514 | 229,991
Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 30,107 |  | 35,730 | 36,735
Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 245,590 |  | 263,244 | 266,726
Intersegment Elimination [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | (30,107) |  | (35,730) | (36,735)
Consolidated [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 215,483 |  | 227,514 | 229,991
Domestic Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 18,243 |  | 24,888 | 25,658
Domestic Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 66,753 |  | 71,137 | 72,784
Domestic Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 17,125 |  | 22,515 | 22,669
Domestic Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 83,878 |  | 93,652 | 95,453
Mobile Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 15,328 |  | 12,433 | 13,926
Mobile Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 100,937 |  | 109,376 | 110,801
Mobile Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 1,702 |  | 2,031 | 2,530
Mobile Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 102,639 |  | 111,407 | 113,331
Internet Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 11,944 |  | 11,118 | 10,729
Internet Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 29,813 |  | 28,917 | 28,100
Internet Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,038 |  | 4,209 | 4,734
Internet Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 33,851 |  | 33,126 | 32,834
International Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | 1,024 |  | 1,029 | 1,098
International Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 13,435 |  | 13,552 | 14,434
International Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 2,234 |  | 2,375 | 2,680
International Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 15,669 |  | 15,927 | 17,114
Other [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income before income tax | (1,553) |  | (1,459) | (1,998)
Other [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,545 |  | 4,532 | 3,872
Other [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 5,008 |  | 4,600 | 4,122
Other [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 9,553 |  | $ 9,132 | $ 7,994

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