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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, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items] |  |  |
Interest income recognized as operating revenue | ₩ 270,571 | ₩ 282,704 | ₩ 244,796
Interest expense recognized as operating expense | 263,579 | 278,427 | 296,874
Profit and loss from foreign currency transaction | 17,493 | 24,596 | 17,175
Gain (loss) on valuation | 40,822 | 15,429 | (5)
BC Card Co., Ltd. [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Interest income recognized as operating revenue | 21,018 | 21,021 | 20,854
Interest expense recognized as operating expense | 548 | 21 | 1,456
Profit and loss from foreign currency transaction | 17,006 | 20,422 | 19,687
Operating expenses from foreign currency translation | 5 | 0 | 56
Dividend income | ₩ 2,250 | ₩ 1,598 | ₩ 2,059

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