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Financial Instruments by Category - Summary of Gains or Losses Arising From Financial Instruments by Category (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 | ₩ 270,571 | ₩ 282,704 | ₩ 244,796
Interest expense | (263,579) | (278,427) | (296,874)
Total |  | 214,063 | (116,643)
Gain (loss) on valuation | 40,822 | 15,429 | (5)
Other comprehensive income (loss) for the year | 23,255 | 172,338 | (54,628)
Reclassified to profit or loss from other comprehensive income for the year | 111,431 | (44,684) |
Financial liabilities designated as at fair value through profit or loss [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gain (loss) on disposal | 799 | 2,664 |
Gain (loss) on valuation | 119 |  |
Derivatives used for hedging [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gain on transactions | 1,141 |  | 20,678
Gain (loss) on valuation | (161,003) | 4,949 | 42,195
Other comprehensive income (loss) for the year | (81,671) | 20,742 | (2,810)
Reclassified to profit or loss from other comprehensive income for the year | 107,786 | (5,080) | (28,388)
Financial liabilities at amortized cost [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Interest expense | (220,945) | (223,974) | (296,894)
Loss on repayment |  |  | (15)
Gain (loss) foreign currency transaction | (10,717) | (20,958) | (30,956)
Gain (loss) on foreign currency translation | 141,849 | (75,502) | (66,050)
Total | 5,481 |  |
Financial assets at amortized cost [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Interest income | 55,742 | 79,838 | 93,233
Gain (loss) foreign currency transaction | (19,244) | 32,293 | 19,396
Gain (loss) on foreign currency translation | (3,895) | (474) | (2,901)
Gain (loss) on disposal | 138 | (43) | 44
Gain (loss) on valuation | (140,474) | (59,947) | (110,544)
Financial assets at fair value through profit or loss [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Interest income | 6,548 | 5,634 | 9,194
Dividend income | 4,379 | 1,096 | 1,207
Gain (loss) foreign currency transaction | (38) | (27) |
Gain (loss) on disposal | (329) | 5,115 | 1,713
Gain (loss) on valuation | 59,044 | 4,334 | 10,768
Financial assets at fair value through other comprehensive income [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Interest income | 227,736 | 217,355 | 163,390
Dividend income | 56 | 2,312 | 1,704
Gain (loss) on disposal | (8,152) | (11,247) | (13,818)
Impairment loss |  | (304) | (2,416)
Other comprehensive income (loss) for the year | 41,997 | 167,152 | 43,811
Derivative financial liabilities [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gain on transactions | 6,050 | 6,332 | 7,272
Gain (loss) on valuation | (2,707) | 56,537 | 22,065
Other comprehensive income (loss) for the year | (2,373) | 46,806 | 20,078
Reclassified to profit or loss from other comprehensive income for the year | ₩ 3,645 | (39,604) | (15,891)
Held-to-maturity financial assets [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gain (loss) on valuation |  | ₩ (1,936) | ₩ (2,708)

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