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Investments in Associates and Joint Ventures - Summary of Changes in Investments in Associates and Joint Ventures (Parenthetical) (Detail) - KRW (₩) ₩ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of information about unconsolidated structured entities controlled by investment entity [line items] |  |  |
Share gain from associates and joint ventures | ₩ (5,467) | ₩ (13,892) | ₩ 2,599
KT Investment Co., Ltd. [member] |  |  |
Disclosure of information about unconsolidated structured entities controlled by investment entity [line items] |  |  |
Share gain from associates and joint ventures | ₩ 445 | ₩ 1,588 |

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- Definition The entity's share of the profit (loss) of associates and joint ventures accounted for using the equity method. [Refer: Associates [member]; Investments accounted for using equity method; Joint ventures [member]; Profit (loss)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 8 -IssueDate 2018-01-01 -Paragraph 23 -Subparagraph g -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2018-03-01&anchor=para_23_g&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 8 -IssueDate 2018-01-01 -Paragraph 28 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2018-03-01&anchor=para_28_e&doctype=Standard -URIDate 2018-03-16 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Note Effective on first application of IFRS 9 -Name IFRS -Number 4 -IssueDate 2018-01-01 -Paragraph 39M -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=4&date=2018-03-02&anchor=para_39M_b&doctype=Standard&book=b -URIDate 2018-03-16 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 82 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_82_c&doctype=Standard -URIDate 2018-03-16 + Details
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