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Sensitivity analysis for provisions assumptions (Details) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
PCBs [Member] | Discount rate [Member]    
Disclosure of Sensitivity analysis for provisions assumptions [Line Items]    
0.1% Increase ₩ (811) ₩ (817)
0.1% Decrease 816 822
PCBs [Member] | Inflation rate [Member]    
Disclosure of Sensitivity analysis for provisions assumptions [Line Items]    
0.1% Increase 826 834
0.1% Decrease (822) (830)
Nuclear plants [Member] | Discount rate [Member]    
Disclosure of Sensitivity analysis for provisions assumptions [Line Items]    
0.1% Increase (262,949) (209,277)
0.1% Decrease 270,370 215,139
Nuclear plants [Member] | Inflation rate [Member]    
Disclosure of Sensitivity analysis for provisions assumptions [Line Items]    
0.1% Increase 287,926 240,115
0.1% Decrease (280,249) (233,553)
Spent fuel [Member] | Discount rate [Member]    
Disclosure of Sensitivity analysis for provisions assumptions [Line Items]    
0.1% Increase (51,015) (52,353)
0.1% Decrease 52,997 54,387
Spent fuel [Member] | Inflation rate [Member]    
Disclosure of Sensitivity analysis for provisions assumptions [Line Items]    
0.1% Increase 53,763 55,173
0.1% Decrease ₩ (51,823) ₩ (53,182)