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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | ₩ 25,603 | ₩ 24,713 | ₩ 41,851
Dividend income | 15,354 | 11,146 | 6,212
Repayment | 30,385 |  |
PT. Mitra Transaksi Indonesia [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash |  | 1,567 | 5,194
KT-IBKC future investment fund [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | 3,750 | (1,050) | 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 | 864
Gyeonggi-KT Yoojin Superman Fund [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | 1,000 | 1,000 | 1,000
K-REALTY CR REIT [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Dividend income | 10,928 | 8,932 | 5,392
Repayment | 30,385 |  |
K Bank Inc. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | 21,782 | 26,725 | 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 | (174) | (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) | (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 | 69 | 202 |
Daiwon Broadcasting Co., Ltd. [Member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Dividend income | 77 | 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 |
KT Philippines co. Ltd. {member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | 99 |  |
Virtua Realm Sendirian Berhad [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | 550 |  |
KIF Investment Fund [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Dividend income | 4,280 |  |
KT-Smart Factory Investment Fund [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | 2,800 |  |
KT-SB Venture Investment Fund [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Equity contributions in cash | ₩ (2,404) |  |

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