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Revenue - Additional Information (Detail) $ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Revenue From Contract With Customer Line Item |  |  |  |
Amortization of incremental costs of obtaining contracts | $ 772 | $ 27 | $ 1,173 | $ 1,941
Telecommunications Business [Member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | $ 29,501 |  |  |
Remaining performance obligations, timing of satisfaction | The Company expects to recognize such revenue of $18,729 million, $9,082 million and $1,690 million in 2021, 2022 and 2023, respectively. | The Company expects to recognize such revenue of $18,729 million, $9,082 million and $1,690 million in 2021, 2022 and 2023, respectively. |  |
Telecommunications Business [Member] | Year 1 [Member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | $ 18,729 |  |  |
Telecommunications Business [Member] | Later than one year and not later than two years [member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | 9,082 |  |  |
Telecommunications Business [Member] | Later than two years and not later than three years [member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | 1,690 |  |  |
Project Business [Member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | $ 18,555 |  |  |
Remaining performance obligations, timing of satisfaction | The Company recognizes revenues when the project business contract is completed and accepted by customers. The Company expects to recognize such revenue of $7,977 million, $5,897 million and $4,681 million in 2021, 2022 and 2023, respectively. | The Company recognizes revenues when the project business contract is completed and accepted by customers. The Company expects to recognize such revenue of $7,977 million, $5,897 million and $4,681 million in 2021, 2022 and 2023, respectively. |  |
Project Business [Member] | Year 1 [Member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | $ 7,977 |  |  |
Project Business [Member] | Later than one year and not later than two years [member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | 5,897 |  |  |
Project Business [Member] | Later than two years and not later than three years [member] |  |  |  |
Revenue From Contract With Customer Line Item |  |  |  |
Transaction price allocated to performance obligations | $ 4,681 |  |  |

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- Definition The amount of amortisation for assets recognised from the costs incurred to obtain or fulfil contracts with customers. [Refer: Assets recognised from costs to obtain or fulfil contracts with customers; Amortisation expense] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 128 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_128_b&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The explanation of when the entity expects to recognise the transaction price allocated to the remaining performance obligations as revenue. [Refer: Revenue from contracts with customers; Transaction price allocated to remaining performance obligations] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 120 -Subparagraph b -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_120_b_ii&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of the transaction price allocated to the performance obligations that are unsatisfied (or partially unsatisfied) as of the end of the reporting period. The transaction price is the amount of consideration to which an entity expects to be entitled in exchange for transferring promised goods or services to a customer, excluding amounts collected on behalf of third parties (for example, some sales taxes). [Refer: Performance obligations [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 120 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_120_a&doctype=Standard -URIDate 2020-01-01 + Details
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