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Financial Instruments by Category - Summary of Gains or Losses Arising From Financial Instruments by Category (Parenthetical) (Detail) - KRW (₩) ₩ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest income recognized as operating revenue | ₩ 93,078 | ₩ 115,686 | ₩ 70,035
BC Card Co., Ltd. [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest income recognized as operating revenue | 15,561 | 15,867 | ₩ 14,380
Operating income (expense) | ₩ 11,049 | ₩ (1,987) |

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