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Related Party Transactions - Summary of Fund Transactions with Related Parties (Detail) - KRW (₩) ₩ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | ₩ 41,851 | ₩ 38,675
Dividend income | 6,212 | 7,554
KT-DSC creative economy youth start-up investment fund [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash |  | 6,000
PT. Mitra Transaksi Indonesia [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | 5,194 | 16,626
K-Realty rental housing REIT [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash |  | 5,500
AI Research institute [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash |  | 3,000
KT-IBKC future investment fund [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | 7,500 | 3,750
Gyeonggi-KT Yoojin Superman Fund [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | 1,000 | 1,000
FUNDA Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash |  | 2,799
K-Realty CR-REITs No.1 [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income |  | 4,186
Korea information & technology investment fund [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 739 | 3,201
Daiwon Broadcasting Co., Ltd. [Member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income |  | 85
Others [Member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income |  | ₩ 82
CHAMP IT Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | 750 |
Korea Electronic Vehicle Charging Service [Member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | 864 |
K-REALTY CR REIT [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 5,392 |
K Bank Inc. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Equity contributions in cash | 26,543 |
MOS GS Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 12 |
MOS Daegu Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 12 |
MOS Chungcheong Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 12 |
MOS Gangnam Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 10 |
MOS GB Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 15 |
MOS BS Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | 10 |
MOS Honam Co., Ltd. [member] |  |
Disclosure of transactions between related parties [Line Items] |  |
Dividend income | ₩ 10 |

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