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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
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