Investments in Associates and Joint Ventures - Summary of Changes in Investments in Associates and Joint Ventures (Parenthetical) (Detail) - KRW (₩) ₩ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of associates and joint ventures [line items]
Share gain from associates and joint ventures	₩ (13,892)	₩ 2,599	₩ 6,144
KT Investment Co., Ltd. [member]
Disclosure of associates and joint ventures [line items]
Share gain from associates and joint ventures	₩ 1,588	₩ 52

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)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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