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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, 2019 TWD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($)
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 207,520 | $ 6,938 | $ 215,483 | $ 227,514
Income (loss) before income tax | 41,867 | $ 1,400 | 44,986 | 48,009
From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 207,520 |  | 215,483 | 227,514
Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 28,573 |  | 30,107 | 35,730
Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 236,093 |  | 245,590 | 263,244
Intersegment Elimination [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | (28,573) |  | (30,107) | (35,730)
Consolidated [member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 207,520 |  | 215,483 | 227,514
Domestic Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 19,537 |  | 18,243 | 24,888
Domestic Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 65,728 |  | 66,753 | 71,137
Domestic Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 16,065 |  | 17,125 | 22,515
Domestic Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 81,793 |  | 83,878 | 93,652
Mobile Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 11,250 |  | 15,328 | 12,433
Mobile Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 95,469 |  | 100,937 | 109,376
Mobile Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 1,564 |  | 1,702 | 2,031
Mobile Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 97,033 |  | 102,639 | 111,407
Internet Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 12,515 |  | 11,944 | 11,118
Internet Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 30,091 |  | 29,813 | 28,917
Internet Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 3,951 |  | 4,038 | 4,209
Internet Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 34,042 |  | 33,851 | 33,126
International Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 799 |  | 1,024 | 1,029
International Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 11,485 |  | 13,435 | 13,552
International Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 2,079 |  | 2,234 | 2,375
International Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 13,564 |  | 15,669 | 15,927
Other [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | (2,234) |  | (1,553) | (1,459)
Other [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,747 |  | 4,545 | 4,532
Other [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 4,914 |  | 5,008 | 4,600
Other [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 9,661 |  | $ 9,553 | $ 9,132

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