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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of subsidiaries [Line Items]      
Accumulated non-controlling interests at the beginning of the year ₩ 1,352,844    
Profit or loss allocated to non-controlling interests 84,782 ₩ 86,755 ₩ 78,324
Accumulated non-controlling interests at the end of the year ₩ 1,391,764 ₩ 1,352,844  
KT Skylife Co., Ltd. [member]      
Disclosure of subsidiaries [Line Items]      
Non-controlling Interests rate(%) 49.73% 49.73% 49.73%
Accumulated non-controlling interests at the beginning of the year ₩ 329,676 ₩ 316,880 ₩ 297,300
Profit or loss allocated to non-controlling interests 9,395 22,445 27,032
Dividend paid to non-controlling interests (9,817) (8,279) (8,325)
Others (952) (1,370) 873
Accumulated non-controlling interests at the end of the year ₩ 328,302 ₩ 329,676 ₩ 316,880
BC Card Co., Ltd. [member]      
Disclosure of subsidiaries [Line Items]      
Non-controlling Interests rate(%) 30.46% 30.46% 30.46%
Accumulated non-controlling interests at the beginning of the year ₩ 329,338 ₩ 322,921 ₩ 292,931
Profit or loss allocated to non-controlling interests 43,961 47,068 62,943
Dividend paid to non-controlling interests (29,490) (44,637) (22,650)
Others (4,742) 3,986 (10,303)
Accumulated non-controlling interests at the end of the year ₩ 339,067 ₩ 329,338 ₩ 322,921
KT Powertel Co., Ltd. [member]      
Disclosure of subsidiaries [Line Items]      
Non-controlling Interests rate(%) 55.15% 55.15% 55.15%
Accumulated non-controlling interests at the beginning of the year ₩ 51,751 ₩ 50,926 ₩ 70,231
Profit or loss allocated to non-controlling interests 1,165 112 (17,880)
Dividend paid to non-controlling interests     (1,118)
Others 137 713 (307)
Accumulated non-controlling interests at the end of the year ₩ 53,053 ₩ 51,751 ₩ 50,926
KT Hitel Co., Ltd. [member]      
Disclosure of subsidiaries [Line Items]      
Non-controlling Interests rate(%) 32.87% 35.27% 36.30%
Accumulated non-controlling interests at the beginning of the year ₩ 51,798 ₩ 50,689 ₩ 51,136
Profit or loss allocated to non-controlling interests 870 1,274 (608)
Others 478 (165) 161
Accumulated non-controlling interests at the end of the year ₩ 53,146 ₩ 51,798 ₩ 50,689
KT Telecop Co., Ltd. [member]      
Disclosure of subsidiaries [Line Items]      
Non-controlling Interests rate(%) 13.18% 13.18% 13.18%
Accumulated non-controlling interests at the beginning of the year ₩ 103,532 ₩ 103,428 ₩ 104,821
Profit or loss allocated to non-controlling interests 381 19 (1,000)
Others (445) 85 (393)
Accumulated non-controlling interests at the end of the year ₩ 103,468 ₩ 103,532 ₩ 103,428