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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
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Associates And Joint Ventures [Line Items]      
Share gain from associates and joint ventures ₩ (3,304) ₩ (5,467) ₩ (13,892)
KT Investment Co., Ltd. [member]      
Disclosure Of Associates And Joint Ventures [Line Items]      
Share gain from associates and joint ventures ₩ 52 ₩ 445