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Revenue from Contracts with Customers and Relevant Contract Assets and Liabilities - Detail Analysis of Contract Costs Recognized as Assets (Detail) - KRW (₩) ₩ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Incremental cost of contract establishment [Member] |  |
Statement [Line Items] |  |
Incremental cost of contract establishment | ₩ 1,726,191 | ₩ 1,764,009
Cost of Contract performance [Member] |  |
Statement [Line Items] |  |
Incremental cost of contract establishment | ₩ 78,757 | ₩ 85,234

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- Definition The amount of assets recognised from the costs to obtain or fulfil contracts with customers. The costs to obtain a contract with a customer are the incremental costs of obtaining the contract that the entity would not have incurred if the contract had not been obtained. The costs to fulfil a contract with a customer are the costs that relate directly to a contract or to an anticipated contract that the entity can specifically identify. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 128 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_128_a&doctype=Standard -URIDate 2020-01-01 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | kt_StatementLineItems
Namespace Prefix: | kt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_CategoriesOfAssetsRecognisedFromCostsToObtainOrFulfilContractsWithCustomersAxis=kt_IncrementalCostOfContractEstablishmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_CategoriesOfAssetsRecognisedFromCostsToObtainOrFulfilContractsWithCustomersAxis=kt_CostOfContractPerformanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |