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Other Financial Assets and Liabilities - Summary of Valuation Gains and Losses on Derivatives Contracts (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] |  |  |
Valuation gain | ₩ 57 | ₩ 109,436 | ₩ 141,512
Valuation loss | 209,582 | 138 | 1,733
Other comprehensive income | (146,508) | 85,483 | 147,644
Interest rate swap [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Valuation gain | 38 |  |
Valuation loss |  | 148 |
Other comprehensive income | 637 | (142) | (2,858)
Currency swap [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Valuation gain | 19 | 97,158 | 141,512
Valuation loss | 187,468 | (10) | 1,733
Other comprehensive income | (146,752) | 85,479 | 150,255
Currency forwards [member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Valuation gain |  | 12,278 |
Valuation loss | 22,114 |  |
Other comprehensive income | ₩ (393) | ₩ 146 | ₩ 247

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