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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, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 207,609 | $ 7,393 | $ 207,520 | $ 215,483
Income (loss) before income tax | 42,826 | $ 1,525 | 41,867 | 44,986
From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 207,609 |  | 207,520 | 215,483
Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 28,676 |  | 28,573 | 30,107
Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 236,285 |  | 236,093 | 245,590
Intersegment Elimination [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | (28,676) |  | (28,573) | (30,107)
Consolidated [member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 207,609 |  | 207,520 | 215,483
Domestic Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 22,504 |  | 19,537 | 18,243
Domestic Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 69,469 |  | 65,728 | 66,753
Domestic Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 15,930 |  | 16,065 | 17,125
Domestic Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 85,399 |  | 81,793 | 83,878
Mobile Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 8,777 |  | 11,250 | 15,328
Mobile Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 90,230 |  | 95,469 | 100,937
Mobile Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 1,536 |  | 1,564 | 1,702
Mobile Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 91,766 |  | 97,033 | 102,639
Internet Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 13,120 |  | 12,515 | 11,944
Internet Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 32,115 |  | 30,091 | 29,813
Internet Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 3,966 |  | 3,951 | 4,038
Internet Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 36,081 |  | 34,042 | 33,851
International Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | 829 |  | 799 | 1,024
International Fixed Communications Business [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 8,695 |  | 11,485 | 13,435
International Fixed Communications Business [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 1,875 |  | 2,079 | 2,234
International Fixed Communications Business [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 10,570 |  | 13,564 | 15,669
Other [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Income (loss) before income tax | (2,404) |  | (2,234) | (1,553)
Other [Member] | From External Customers [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 7,100 |  | 4,747 | 4,545
Other [Member] | Intersegment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | 5,369 |  | 4,914 | 5,008
Other [Member] | Segment [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Revenues | $ 12,469 |  | $ 9,661 | $ 9,553

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