Investments in Associates and Joint Ventures - Summary of Reconciliation of Carrying Amount of Interests in the Associates and Joint Ventures (Detail) - KRW (₩) ₩ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure of associates and joint ventures [line items]
Carrying amount	₩ 279,431	₩ 284,075
Korea Information & Technology Fund [member]
Disclosure of associates and joint ventures [line items]
Net assets	₩ 418,601	₩ 404,908
Percentage of ownership	33.30%	33.30%
Share in net assets	₩ 139,534	₩ 134,969
Carrying amount	139,534	134,969
KT-SB Venture Investment [member]
Disclosure of associates and joint ventures [line items]
Net assets	₩ 5,884	₩ 9,471
Percentage of ownership	50.00%	50.00%
Share in net assets	₩ 2,942	₩ 4,736
Carrying amount	2,942	4,736
Mongolian Telecommunications [Member]
Disclosure of associates and joint ventures [line items]
Net assets		₩ 15,610
Percentage of ownership		40.00%
Share in net assets		₩ 6,244
Carrying amount		6,244
KT Wibro Infra Co., Ltd. [member]
Disclosure of associates and joint ventures [line items]
Net assets		₩ 269,769
Percentage of ownership		26.20%
Share in net assets		₩ 70,679
Carrying amount		52,200
KT Wibro Infra Co., Ltd. [member] \| Material reconciling items [member]
Disclosure of associates and joint ventures [line items]
Share in net assets		(18,479)
KT-CKP New Media Investment Fund [member]
Disclosure of associates and joint ventures [line items]
Net assets	₩ 4,620	₩ 8,967
Percentage of ownership	49.70%	49.70%
Share in net assets	₩ 2,294	₩ 4,454
Carrying amount	2,294	₩ 4,454
KT-IBKC future investment fund [member]
Disclosure of associates and joint ventures [line items]
Net assets	₩ 21,649
Percentage of ownership	50.00%	50.00%
Share in net assets	₩ 10,825
Carrying amount	10,825
K Bank Inc. [member]
Disclosure of associates and joint ventures [line items]
Net assets	₩ 233,775
Percentage of ownership	10.00%
Share in net assets	₩ 42,108
Carrying amount	₩ 42,108

The amount of investments accounted for using the equity method. The equity method is a method of accounting whereby the investment is initially recognised at cost and adjusted thereafter for the post-acquisition change in the investor's share of net assets of the investee. The investor's profit or loss includes its share of the profit or loss of the investee. The investor's other comprehensive income includes its share of the other comprehensive income of the investee. [Refer: At cost [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2017-01-01
-Paragraph 24
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2017-03-01&anchor=para_24_a&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2017-01-01
-Paragraph B16
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2017-03-01&anchor=para_B16&doctype=Appendix&seq=2
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 54
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_54_e&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_InvestmentAccountedForUsingEquityMethod
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of assets less the amount of liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 1
-IssueDate 2017-01-01
-Paragraph IG63
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2017-03-01&anchor=para_IG63&doctype=Implementation%20Guidance
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_NetAssetsLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The proportion of ownership interest in an associate attributable to the entity. [Refer: Associates [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 27
-IssueDate 2017-01-01
-Paragraph 17
-Subparagraph b
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2017-03-01&anchor=para_17_b_iii&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 27
-IssueDate 2017-01-01
-Paragraph 16
-Subparagraph b
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2017-03-01&anchor=para_16_b_iii&doctype=Standard
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2017-01-01
-Paragraph 21
-Subparagraph a
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2017-03-01&anchor=para_21_a_iv&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ProportionOfOwnershipInterestInAssociate
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information about associate and joint ventures associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	kt_DisclosureOfAssociatesAndJointVenturesLineItems
Namespace Prefix:	kt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Share of net assets (liabilities) in joint ventures and associates.

+ References

No definition available.

+ Details

Name:	kt_ShareOfNetAssetsLiabilitiesInJointVenturesAndAssociates
Namespace Prefix:	kt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_KoreaInformationAndTechnologyFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_KTSBVentureInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_MongolianTelecommunicationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_KTWibroInfraCorporationLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentConsolidationItemsAxis=ifrs-full_MaterialReconcilingItemsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_KTCKPNewMediaInvestmentFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_KTIBKCFutureInvestmentFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInAssociatesAxis=kt_KBankIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: