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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
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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 |

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- Definition 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]] + References 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
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- 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
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- 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
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