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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, 2018 | Dec. 31, 2017
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | ₩ 24,713 |
Equity contributions in cash |  | ₩ 41,851
Dividend income | 11,146 | 6,212
PT. Mitra Transaksi Indonesia [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | 1,567 |
Equity contributions in cash |  | 5,194
KT-IBKC future investment fund [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (1,050) |
Equity contributions in cash |  | 7,500
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 | 168 |
Equity contributions in cash |  | 864
Gyeonggi-KT Yoojin Superman Fund [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | 1,000 |
Equity contributions in cash |  | 1,000
K-REALTY CR REIT [member] |  |
Disclosure of transactions between related parties [line items] |  |
Dividend income | 8,932 | 5,392
K Bank Inc. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | 26,725 |
Equity contributions in cash |  | 26,543
Korea information & technology investment fund [member] |  |
Disclosure of transactions between related parties [line items] |  |
Dividend income | 1,842 | 739
MOS GS Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (147) |
Dividend income | 8 | 12
MOS Daegu Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (147) |
Dividend income | 8 | 12
MOS Chungcheong Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (153) |
Dividend income | 8 | 12
MOS Gangnam Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (180) |
Dividend income | 10 | 10
MOS GB Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (203) |
Dividend income | 12 | 15
MOS BS Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (183) |
Dividend income | 10 | 10
MOS Honam Co., Ltd. [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (206) |
Dividend income | 10 | ₩ 10
PHI Healthcare Inc [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | 1,000 |
KT-CKP New Media Investment Fund [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (1,229) |
KT-DSC creative economy youth start-up investment fund [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (1,800) |
GE Premier 1st Corporate Restructuring Real Estate Investment Trust Company [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (3,423) |
JB Emerging Market Specialty Investment Private Equity Trust No one[member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | 3,960 |
Dividend income | 202 |
Daiwon Broadcasting Co., Ltd. [Member] |  |
Disclosure of transactions between related parties [line items] |  |
Dividend income | 85 |
Boston Global Film Contents Fund Lp [member] |  |
Disclosure of transactions between related parties [line items] |  |
Equity contributions in cash | (986) |
Gyeonggi KT Green Growth Fund [member] |  |
Disclosure of transactions between related parties [line items] |  |
Dividend income | ₩ 19 |

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