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Segment Information - Main Products and Service Revenues (Detail) $ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Disclosure of products and services [Line Items] |  |  |  |
Revenues | $ 207,609 | $ 7,393 | $ 207,520 | $ 215,483
Mobile services [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | 56,724 |  | 58,703 | 63,906
Local telephone and domestic long distance telephone services [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | 26,475 |  | 27,929 | 29,996
Sales of products [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | 39,391 |  | 41,593 | 41,289
Broadband access and domestic leased line services [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | 22,420 |  | 22,116 | 22,453
Data communications internet services [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | 21,447 |  | 21,003 | 21,137
International network and leased line services revenue [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | 3,884 |  | 7,066 | 8,724
Others [Member] |  |  |  |
Disclosure of products and services [Line Items] |  |  |  |
Revenues | $ 37,268 |  | $ 29,110 | $ 27,978

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