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Revenue from Contracts with Customers and Relevant Contract Assets and Liabilities - Detail Analysis of Contract with Customer Assets and Liabilities (Parenthetical) (Detail) - KRW (₩) ₩ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Disclosure of revenue from contracts with customers [line items] |  |  |
Contract assets | [1] | ₩ 586,438 | ₩ 557,041
Contract liabilities | [1] | 384,133 | 365,610
long term construction contract [Member] |  |  |
Disclosure of revenue from contracts with customers [line items] |  |  |
Contract assets |  | 86,234 | 98,288
Contract liabilities |  | ₩ 29,574 | ₩ 47,832
[1] | The amounts include adjustments arising from adoption of IFRS 15 (Note 26).

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- Definition The amount of an entity’s right to consideration in exchange for goods or services that the entity has transferred to a customer, when that right is conditioned on something other than the passage of time (for example, the entity’s future performance). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 105 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_105&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 116 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_116_a&doctype=Standard -URIDate 2020-01-01 + Details
Name: | ifrs-full_ContractAssets
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The amount of an entity’s obligation to transfer goods or services to a customer for which the entity has received consideration (or the amount is due) from the customer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 105 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_105&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2020-01-01 -Paragraph 116 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2020-en-r&anchor=para_116_a&doctype=Standard -URIDate 2020-01-01 + Details
Name: | ifrs-full_ContractLiabilities
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Disclosure of revenue from contracts with customers [line items] + References No definition available. + Details
Name: | kt_DisclosureOfRevenueFromContractsWithCustomersLineItems
Namespace Prefix: | kt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_TypesOfContractsAxis=kt_LongTermConstructionContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |