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Leases - Additional Information (Detail) - KRW (₩) ₩ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Disclosure Of Arrangement Involving Legal Form Of Lease [Abstract] |  |
Cash outflow for leases | ₩ 492,772 | ₩ 532,730
Increase decrease in right of use assets | ₩ 404,293 |

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- Definition The cash outflow for leases. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 16 -IssueDate 2020-01-01 -Paragraph 53 -Subparagraph g -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2020-en-r&anchor=para_53_g&doctype=Standard -URIDate 2020-01-01 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Disclosure Of arrangement involving legal form of lease. + References No definition available. + Details
Name: | kt_DisclosureOfArrangementInvolvingLegalFormOfLeaseAbstract
Namespace Prefix: | kt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Increase Decrease In Right Of Use Assets + References No definition available. + Details
Name: | kt_IncreaseDecreaseInRightOfUseAssets
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration