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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest income | ₩ 93,078 | ₩ 115,686 | ₩ 70,035
Interest expense | (302,464) | (337,219) | (385,925)
Dividend income | 6,212 | 7,554 |
Total | (150,420) | (308,677) | (385,127)
Other comprehensive income for the year | (68,288) | (5,670) | (68,835)
Reclassified to profit or loss from other comprehensive income for the year | 86,479 | (79,384) |
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 |  | (632) | (850)
Gain (loss) on valuation | (3,078) | 33 | (2,006)
Derivatives used for hedging [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Gain (loss) on transactions |  | 8,329 | (273)
Gain (loss) on valuation | (145,885) | (138) | (1,733)
Other comprehensive income for the year | (66,624) | 4,295 | 11,513
Reclassified to profit or loss from other comprehensive income for the year | 91,698 | (3,956) | (9,959)
Financial liabilities at amortized cost [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest expense | (302,464) | (337,219) | (385,925)
Gain (loss) foreign currency transaction | 62,347 | (7,518) | (23,416)
Gain(loss) foreign currency translation | 225,695 | (112,864) | (166,254)
Loans and receivables [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest income | 108,608 | 129,813 | 85,603
Gain (loss) foreign currency transaction | (11,949) | (7,493) | (365)
Gain(loss) on foreign currency translation | (12,354) | 3,083 | 1,921
Gain (loss) on disposal | (20,351) | (15,838) | (2,539)
Gain (loss) on valuation | (44,219) | (92,589) | (141,555)
Financial assets at fair value through profit or loss [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Dividend income | 1 |  |
Gain (loss) on disposal | 153 | 186 | 368
Gain (loss) on valuation | (464) | (7,184) |
Derivative liabilities for hedging purpose [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Gain (loss) on transactions | (58,569) |  | (5,157)
Gain (loss) on valuation | (63,640) | 109,436 | 141,512
Other comprehensive income for the year | (44,429) | 60,501 | 100,401
Reclassified to profit or loss from other comprehensive income for the year | 50,231 | (71,915) | (88,003)
Available-for-sale [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest income | 453 | 40 | 73
Dividend income | 5,174 | 3,926 | 7,733
Gain (loss) on disposal | 89,598 | 22,695 | 131,045
Impairment loss | (6,137) | (966) | (1,471)
Other comprehensive income for the year | 51,235 | 10,925 | 47,381
Reclassified to profit or loss from other comprehensive income for the year | ₩ (55,450) | (3,840) | (83,397)
Held-to-maturity financial assets [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Interest income |  | ₩ 213 | ₩ 226

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