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Financial Risk Management - Summary of Cash Outflow and Inflow of Derivatives Settled Gross or Net are Undiscounted Contractual Cash Flow (Detail) - KRW (₩) ₩ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |  |
Outflow | ₩ 1,711,595 | ₩ 2,886,867 | ₩ 2,512,533
Inflow | 1,686,804 | 3,196,870 | 2,606,479
Less than 1 year [member] |  |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |  |
Outflow | 638,171 | 1,174,147 | 335,970
Inflow | 608,270 | 1,302,112 | 276,066
1-5 years [Member] |  |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |  |
Outflow | 546,791 | 1,176,715 | 2,138,379
Inflow | 568,976 | 1,306,199 | 2,284,219
More than 5 years [Member] |  |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |  |
Outflow | 526,633 | 536,005 | 38,184
Inflow | ₩ 509,558 | ₩ 588,559 | ₩ 46,194

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