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Transactions between related parties (Details) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dongducheon Dream Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales ₩ 7,071 ₩ 6,093 ₩ 17,041
Electricity purchase 689,510 828,547 813,440
Outstanding balances for related party transactions [Abstract]      
Trade receivables 323 332  
Trade payables 85,920 74,651  
Non-trade receivables and others 267 298  
Non-trade payables and others 12 10  
Korea Gas Corporation      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 116,484 99,933 88,011
Purchase of power generation fuel 4,049,486 5,191,243 3,245,519
Outstanding balances for related party transactions [Abstract]      
Trade receivables 9,267 9,439  
Trade payables 462,138 601,092  
Non-trade receivables and others 38 498  
Non-trade payables and others 560 314  
Daegu Photovoltaic Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 327 128 349
REC purchase 3,626 3,745 3,646
Outstanding balances for related party transactions [Abstract]      
Trade payables 69 83  
Jeongam Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 1,198 724 30
Outstanding balances for related party transactions [Abstract]      
Trade receivables 165 151  
Non-trade payables and others 6 2  
Korea Power Engineering Service Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 348 840 1,317
Service purchase 1,774 1,765 1,292
Outstanding balances for related party transactions [Abstract]      
Trade receivables 74 0  
Non-trade receivables and others 74 0  
KS Solar Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 0 0 5
REC purchase 0 0 900
Heang Bok Do Si Photovoltaic Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 2 2 2
Rental fee and others 450 455 570
Korea Electric Power Industrial Development Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 11,085 10,422 14,044
Electricity metering service fee 245,057 261,668 289,293
Outstanding balances for related party transactions [Abstract]      
Trade receivables 243 278  
Trade payables 3,756 0  
Non-trade receivables and others 43 51  
Non-trade payables and others 3,827 13,102  
Goseong Green Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 19,245 10,024 24,069
Electricity purchase 32 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 103 0  
Non-trade receivables and others 5,285 0  
Non-trade payables and others 65,626 43,020  
Gangneung Eco Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 7,446 4,928 2,391
Service purchase 0 19 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 115 1  
Non-trade receivables and others 65 99  
Shin Pyeongtaek Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 21,218 8,468 9,025
Electricity purchase 119,162 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1,013 123  
Trade payables 64,696 0  
Non-trade receivables and others 2,281 242  
Non-trade payables and others 61 82  
Naepo Green Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 154 160 185
Outstanding balances for related party transactions [Abstract]      
Trade receivables 18 17  
Noeul Green Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 30 30 32
REC purchase 22,767 18,282 15,862
Outstanding balances for related party transactions [Abstract]      
Trade receivables 3 3  
Non-trade payables and others 1,855 8,774  
Samcheok Eco Materials Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 627 605 237
Electricity purchase 9,425 3,819 14
Outstanding balances for related party transactions [Abstract]      
Trade receivables 59 61  
YTN Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 2,257 2,043 1,987
Electricity purchase 333 322 731
Outstanding balances for related party transactions [Abstract]      
Trade receivables 97 98  
Non-trade payables and others 0 11  
Busan Green Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 18 19 120
Electricity purchase 13,811 25,123 12,189
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1 1  
Non-trade payables and others 606 10,882  
Gunsan Bio Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 4,755 3,611 0
Electricity purchase 3,022 0 0
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 0 3,458  
Korea Electric Vehicle Charge Service      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 1,029 578 700
Electricity purchase 536 605 1,093
Outstanding balances for related party transactions [Abstract]      
Trade receivables 137 69  
Non-trade receivables and others 529 354  
Non-trade payables and others 0 56  
Tamra Offshore Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 0 58 55
Electricity purchase 0 8,371 2,105
Daegu clean Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 0 126 421
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 0 17  
Cheongna Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 9,922 11,688 7,980
Service purchase 58,747 58 59
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1,074 183  
Non-trade payables and others   0  
YaksuESS Co., Ltd [Member]      
Disclosure of transactions between related parties [Line Items]      
Service purchase 1,191 0 0
Outstanding balances for related party transactions [Abstract]      
Trade payables 525 0  
Gangwon Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 2,433 2,402 994
Electricity purchase 24,116 25,407 25,968
Outstanding balances for related party transactions [Abstract]      
Trade receivables 7 10  
Trade payables 2,979 2,547  
Gwangyang Green Energy Co., Ltd      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 122 874 0
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 1,334 0  
Hyundai Green Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Design service 13,106 14,031 14,280
Electricity purchase 507,662 480,815 458,378
Outstanding balances for related party transactions [Abstract]      
Trade receivables 502 390  
Trade payables 41,587 38,072  
Korea Power Exchange      
Disclosure of transactions between related parties [Line Items]      
Service sales 7,670 6,854 8,446
Trading Fees 120,509 137,489 207,855
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1,142 847  
Non-trade receivables and others 114 229  
Non-trade payables and others 4,160 1,214  
Hyundai Energy      
Disclosure of transactions between related parties [Line Items]      
Service sales 3,437 1,718 15,627
Electricity purchase 2,847 3,445 87,607
Outstanding balances for related party transactions [Abstract]      
Trade receivables 128 57  
Trade payables 175 259  
Non-trade receivables and others 12,532 9,664  
Non-trade payables and others 9,178 8,021  
Taebaek Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 991 614 813
REC purchase 7,070 8,582 6,534
Outstanding balances for related party transactions [Abstract]      
Trade receivables   0  
Trade payables 358 453  
Non-trade payables and others 243 291  
Taeback Guinemi Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 77 0 0
Electricity purchase 349 0 0
Outstanding balances for related party transactions [Abstract]      
Trade payables 190 0  
Non-trade receivables and others 2 0  
Pyeongchang Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Design service 1,189 1,166 1,176
Service purchase 5,188 6,994 4,033
Outstanding balances for related party transactions [Abstract]      
Trade receivables 4 4  
Non-trade payables and others 0 25  
Daeryun Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 1,879 1,731 1,796
Electricity purchase 144,862 184,063 146,189
Outstanding balances for related party transactions [Abstract]      
Trade receivables 144 178  
Trade payables 19,843 15,845  
Changjuk Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 927 758 788
Electricity purchase 6,994 7,929 6,981
Outstanding balances for related party transactions [Abstract]      
Trade receivables   0  
Trade payables 447 455  
Non-trade payables and others 280 273  
GS Donghae Electric Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 15,792 7,238 11,204
Electricity purchase 681,379 780,233 351,367
Outstanding balances for related party transactions [Abstract]      
Trade receivables 175 245  
Trade payables 69,811 69,820  
Non-trade receivables and others 367 2,537  
Non-trade payables and others 16 59  
KNH Solar Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 17 17 17
Electricity purchase 3,377 3,586 3,947
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1 1  
Non-trade payables and others 0 0  
S-Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 6,535 9,118 12,852
Electricity purchase 522,469 507,875 457,329
Outstanding balances for related party transactions [Abstract]      
Trade receivables 89 922  
Trade payables 53,705 56,897  
Non-trade receivables and others 43 34  
Non-trade payables and others 0 16  
PND solar., Ltd      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 407 12 0
Service purchase 557 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 0 3  
Hyundai Eco Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 117 0 0
Electricity purchase 6,810 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 16 0  
Hadong Mineral Fiber Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service purchase 0 60 0
Green Biomass Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 0 0 0
Woodchip purchase 0 440 1,345
Outstanding balances for related party transactions [Abstract]      
Non-trade payables and others   0  
YeongGwang Yaksu Wind Electric. Co., Ltd      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 81 0 0
Electricity purchase 6,511 2,437 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 5 0  
Trade payables 0 2,679  
Korea Energy Solutions Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 52 0 0
Electricity purchase 15,000 0 0
Suwon New Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 5,606 0 0
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 2,939 0  
9 Associates (Overseas)      
Disclosure of transactions between related parties [Line Items]      
Electricity sales and others 98,450 60,318 21,864
Electricity purchase and Others 0 31 72
4 Associates (Overseas)      
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 1,112 455  
Ecollite Co., Ltd.      
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 210 210  
Daegu Green Power      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 1,151 1,022 1,131
Electricity purchase 296,464 287,008 252,024
Outstanding balances for related party transactions [Abstract]      
Trade receivables 113 110  
Trade payables 28,980 32,609  
Non-trade receivables and others 1 12  
Daejung Offshore Wind Power Co., Ltd. [Member]      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 1 1 1
KAPES, Inc.      
Disclosure of transactions between related parties [Line Items]      
Commission 504 648 1,420
Service purchase 142,511 77,758 164,165
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1 0  
Trade payables   0  
Non-trade receivables and others 0 253  
Non-trade payables and others 50,738 338  
Dangjin Eco Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Technical fee 4 3,424 670
Outstanding balances for related party transactions [Abstract]      
Trade receivables 2 0  
Non-trade receivables and others 0 18  
Non-trade payables and others 0 0  
Honam Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 657 415 552
Electricity purchase 5,306 7,700 5,962
Outstanding balances for related party transactions [Abstract]      
Trade payables 347 432  
Non-trade payables and others 1,671 3,573  
Seokmun Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 1,495 1,897 1,765
REC purchase 26,370 31,759 21,674
Outstanding balances for related party transactions [Abstract]      
Trade receivables 51 39  
Non-trade receivables and others 136 303  
Non-trade payables and others 4,750 5,043  
Incheon New Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Construction revenue 354 568 539
Outstanding balances for related party transactions [Abstract]      
Trade receivables 0 128  
KW Nuclear Components Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 721 839 644
Outstanding balances for related party transactions [Abstract]      
Trade receivables 4 4  
Non-trade receivables and others 15 31  
Chun-cheon Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 2,397 2,471 4,855
REC purchase 313,471 320,954 194,136
Outstanding balances for related party transactions [Abstract]      
Trade receivables 260 20  
Trade payables 23,438 33,796  
Non-trade receivables and others 5,362 308  
Yeonggwangbaeksu Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 1,744 1,540 1,654
Electricity purchase 11,739 11,366 11,124
Outstanding balances for related party transactions [Abstract]      
Trade receivables 6 7  
Trade payables 671 736  
Non-trade receivables and others 157 0  
Non-trade payables and others 1,526 789  
Yeonggwang Wind Power Co., Ltd      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 705 46 0
Electricity purchase 11,940 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 10 10  
Non-trade receivables and others 54 0  
Non-trade payables and others 2,432 0  
KEPCO-Uhde Inc.      
Disclosure of transactions between related parties [Line Items]      
Service sales 15 17 34
Outstanding balances for related party transactions [Abstract]      
Non-trade payables and others 9 4  
Busan Shinho Solar Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 452 362 87
REC purchase 7,843 7,901 7,984
Outstanding balances for related party transactions [Abstract]      
Trade receivables 2 2  
Trade payables 148 175  
Non-trade payables and others 733 717  
Daesan Green Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 39 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 6 0  
Cheong-Song Noraesan Wind Power Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 6 0 0
Outstanding balances for related party transactions [Abstract]      
Trade receivables 1 0  
Saemangeum Solar Power Co., Ltd      
Disclosure of transactions between related parties [Line Items]      
Service sales 2,230 0 0
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 2,197 0  
Jaeun Resident Wind Power Plant Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 105 0 0
25 Joint ventures (Overseas)      
Disclosure of transactions between related parties [Line Items]      
Electricity sales and others 1,275,279 206,054 131,344
Electricity purchase and Others 7,232 1,404 2,785
14 Joint ventures (Overseas)      
Outstanding balances for related party transactions [Abstract]      
Trade receivables 51,700 30,208  
Non-trade receivables and others 13,652 740  
Non-trade payables and others 87,404 18,379  
Global Trade Of Power System Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service purchase 502 565 414
Expressway Solar-light Power      
Disclosure of transactions between related parties [Line Items]      
Electricity purchase 3,067 3,513 2,941
Yeongwol Energy Station Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 0 373 830
REC purchase 0 4,019 14,256
Outstanding balances for related party transactions [Abstract]      
Trade receivables 0 0  
DS POWER Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 0 565 5,819
Outstanding balances for related party transactions [Abstract]      
Trade receivables 0 0  
Ulleungdo Natural Energy Co., Ltd.      
Disclosure of transactions between related parties [Line Items]      
Service sales 0 292 1,013
Service purchase 0 63 119
Korea Development Bank      
Disclosure of transactions between related parties [Line Items]      
Electricity sales 3,948 3,524 3,239
Dividend paid 20 166,876 418,407
Interest income 2,091 4,438 1,685
Interest expense 4,063 5,043 ₩ 4,573
Outstanding balances for related party transactions [Abstract]      
Non-trade receivables and others 146,416 12,125  
Non-trade payables and others 152 218  
Accrued interest income 1,630 1,147  
Derivative assets 41,368 27,306  
Derivative liabilities ₩ 6,531 ₩ 18,095