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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | ₩ 37,282 | ₩ 8,009 | ₩ 30,791
Purchases | 156,259 | 177,666 | 325,144
KT Service Bukbu [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 2,143
Purchases |  |  | 28,550
Information Technology Solution Nambu Corporation [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 2,707
Purchases |  |  | 24,025
Information Technology Solution Seobu Corporation [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 2,324
Purchases |  |  | 20,031
Information Technology Solution Busan Corporation [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 1,496
Purchases |  |  | 14,049
KT Service Nambu [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 1,972
Purchases |  |  | 21,133
Information Technology Solution Honam Corporation [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 2,050
Purchases |  |  | 29,538
Information Technology Solution Daegu Corporation [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  |  | 1,256
Purchases |  |  | 18,272
KT Wibro Infra Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  | 11 | 11
Purchases |  | 391 | 814
Smart Channel Co., Ltd. [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales |  | 766 | 6,545
Purchases |  |  | 4,722
K-Realty CR-REITs No.1 [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 2,233 | 1,989 | 2,133
Purchases | 35,532 | 37,469 | 38,167
MOS GS Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 704 | 663 | 752
Purchases | 16,946 | 17,361 | 17,474
MOS Daegu Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 335 | 291 | 357
Purchases | 8,514 | 12,220 | 12,227
MOS Chungcheong Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 455 | 408 | 310
Purchases | 15,542 | 13,469 | 12,735
MOS Gangnam Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 484 | 412 | 454
Purchases | 16,380 | 15,797 | 15,829
MOS GB Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 987 | 891 | 964
Purchases | 21,651 | 21,802 | 21,582
MOS BS Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 460 | 441 | 453
Purchases | 15,957 | 15,346 | 15,482
MOS Honam Co., Ltd. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 493 | 418 | 470
Purchases | 14,294 | 14,389 | 17,004
Others [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 1,149 | 1,719 | 4,394
Purchases | 11,384 | ₩ 29,422 | ₩ 13,510
K Bank Inc. [member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | 29,939 |  |
Purchases | 59 |  |
NgeneBio [Member] |  |  |
Disclosure of transactions between related parties [Line Items] |  |  |
Sales | ₩ 43 |  |

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