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Information About Non-controlling Interests - Summary of Changes in Accumulated Non-controlling Interests (Detail) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of subsidiaries [line items]      
Accumulated non-controlling interests at the beginning of the year ₩ 1,520,160    
Profit or loss allocated to non-controlling interests 45,367 ₩ 50,165 ₩ 73,841
Accumulated non-controlling interests at the end of the year ₩ 1,539,989 ₩ 1,520,160  
KT Skylife Co., Ltd. [member]      
Disclosure of subsidiaries [line items]      
Non-controlling Interests rate(%) 49.70% 49.70% 49.70%
Accumulated non-controlling interests at the beginning of the year ₩ 375,906 ₩ 374,150 ₩ 328,302
Profit or loss allocated to non-controlling interests 22,171 10,029 23,405
Dividend paid to non-controlling interests (8,279) (8,279) (8,279)
Others (898) 6 30,722
Accumulated non-controlling interests at the end of the year ₩ 388,900 ₩ 375,906 ₩ 374,150
BC Card Co., Ltd. [member]      
Disclosure of subsidiaries [line items]      
Non-controlling Interests rate(%) 30.50% 30.50% 30.50%
Accumulated non-controlling interests at the beginning of the year ₩ 417,475 ₩ 345,547 ₩ 339,067
Profit or loss allocated to non-controlling interests 9,899 37,795 28,418
Dividend paid to non-controlling interests (22,787) (18,900) (35,924)
Others 7,239 53,033 13,986
Accumulated non-controlling interests at the end of the year ₩ 411,826 ₩ 417,475 ₩ 345,547
KT Powertel Co., Ltd. [member]      
Disclosure of subsidiaries [line items]      
Non-controlling Interests rate(%) 55.20% 55.20% 55.20%
Accumulated non-controlling interests at the beginning of the year ₩ 54,276 ₩ 52,865 ₩ 53,053
Profit or loss allocated to non-controlling interests 2,151 1,751 (3,058)
Dividend paid to non-controlling interests (478)    
Others (202) (340) 2,870
Accumulated non-controlling interests at the end of the year ₩ 55,747 ₩ 54,276 ₩ 52,865
KT Hitel Co., Ltd. [member]      
Disclosure of subsidiaries [line items]      
Non-controlling Interests rate(%) 32.90% 32.90% 32.90%
Accumulated non-controlling interests at the beginning of the year ₩ 54,709 ₩ 52,336 ₩ 53,146
Profit or loss allocated to non-controlling interests (1,840) 1,720 454
Others (2,563) 653 (1,264)
Accumulated non-controlling interests at the end of the year ₩ 50,306 ₩ 54,709 ₩ 52,336
KT Telecop Co., Ltd. [member]      
Disclosure of subsidiaries [line items]      
Non-controlling Interests rate(%) 13.20% 13.20% 13.20%
Accumulated non-controlling interests at the beginning of the year ₩ 3,650 ₩ 103,357 ₩ 103,468
Profit or loss allocated to non-controlling interests 152 (588) 59
Others (208) (99,119) (170)
Accumulated non-controlling interests at the end of the year ₩ 3,594 ₩ 3,650 ₩ 103,357