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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about financial instruments [line items]      
Interest income recognized as operating revenue ₩ 282,704 ₩ 244,796 ₩ 93,078
Interest expense recognized as operating expense 268,225 296,874 302,464
Profit and loss from foreign currency transaction 24,596 17,175 79,653
BC Card Co., Ltd. [member]      
Disclosure of detailed information about financial instruments [line items]      
Interest income recognized as operating revenue 21,018 21,021 15,561
Interest expense recognized as operating expense 548 21 0
Profit and loss from foreign currency transaction 17,006 20,422 11,409
Operating expenses from foreign currency translation ₩ 5 ₩ 0 ₩ 3