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Sensitivity analysis for electricity rates (Details) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Increase (decrease) of income before income tax [Member]    
Disclosure of Sensitivity analysis for electricity rates [Line Items]    
1% Increase ₩ 546,499 ₩ 543,045
1% Decrease (546,499) (543,045)
Increase (decrease) of shareholder’s equity [Member]    
Disclosure of Sensitivity analysis for electricity rates [Line Items]    
1% Increase 546,499 543,045
1% Decrease ₩ (546,499) ₩ (543,045)