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Related Party Transactions - Summary of Significant Transactions with Related Parties (Detail) - KRW (₩) ₩ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of transactions between related parties [line items] |  |  |
Sales | ₩ 24,312 | ₩ 37,282 | ₩ 8,009
Purchases | 128,252 | 156,259 | 177,666
KT Wibro Infra Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales |  |  | 11
Purchases |  |  | 391
Smart Channel Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales |  |  | 766
K-Realty CR-REITs No.1 [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 2,088 | 2,233 | 1,989
Purchases | 31,984 | 35,532 | 37,469
MOS GS Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 493 | 704 | 663
Purchases | 12,023 | 16,946 | 17,361
MOS Daegu Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 229 | 335 | 291
Purchases | 8,775 | 8,514 | 12,220
MOS Chungcheong Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 540 | 455 | 408
Purchases | 9,159 | 15,542 | 13,469
MOS Gangnam Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 333 | 484 | 412
Purchases | 11,549 | 16,380 | 15,797
MOS GB Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 1,378 | 987 | 891
Purchases | 16,519 | 21,651 | 21,802
MOS BS Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 324 | 460 | 441
Purchases | 11,193 | 15,957 | 15,346
MOS Honam Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 331 | 493 | 418
Purchases | 10,499 | 14,294 | 14,389
Others [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 2,888 | 1,149 | 1,719
Purchases | 9,547 | 11,384 | ₩ 29,422
K Bank Inc. [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | 15,705 | 29,939 |
Purchases | 7,004 | 59 |
NgeneBio [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Sales | ₩ 3 | ₩ 43 |

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