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Revenue - Summary of Disaggregation of Revenue (Detail) - TWD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | $ 206,396 | $ 206,360 | $ 214,461
Domestic Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 68,804 | 65,066 | 66,103
Mobile Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 90,143 | 95,390 | 100,877
Internet Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 31,809 | 29,834 | 29,650
International Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 8,682 | 11,475 | 13,424
Other [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 6,958 | 4,595 | 4,407
Mobile services [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 56,724 | 58,703 | 63,906
Mobile services [Member] | Mobile Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 56,724 | 58,703 | 63,906
Sales of product [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 39,391 | 41,593 | 41,289
Sales of product [Member] | Domestic Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 2,214 | 1,957 | 1,731
Sales of product [Member] | Mobile Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 32,112 | 35,545 | 35,702
Sales of product [Member] | Internet Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 107 | 41 | 4
Sales of product [Member] | International Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 313 | 265 | 251
Sales of product [Member] | Other [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 4,645 | 3,785 | 3,601
Local telephone and domestic long distance telephone services [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 26,475 | 27,929 | 29,996
Local telephone and domestic long distance telephone services [Member] | Domestic Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 26,475 | 27,929 | 29,996
Broadband access and domestic leased line services [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 22,420 | 22,116 | 22,453
Broadband access and domestic leased line services [Member] | Domestic Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 22,420 | 22,116 | 22,453
Data communications internet services [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 21,447 | 21,003 | 21,137
Data communications internet services [Member] | Internet Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 21,447 | 21,003 | 21,137
International network and leased line services |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 3,884 | 7,066 | 8,724
International network and leased line services | International Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 3,884 | 7,066 | 8,724
Others [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 36,055 | 27,950 | 26,956
Others [Member] | Domestic Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 17,695 | 13,064 | 11,923
Others [Member] | Mobile Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 1,307 | 1,142 | 1,269
Others [Member] | Internet Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 10,255 | 8,790 | 8,509
Others [Member] | International Fixed Communications Business [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | 4,485 | 4,144 | 4,449
Others [Member] | Other [Member] |  |  |
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items] |  |  |
Revenue from contracts with customers | $ 2,313 | $ 810 | $ 806

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- Definition The amount of revenue from contracts with customers. A customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 113 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_113_a&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 114 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_114&doctype=Standard -URIDate 2020-01-01 + Details
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