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Loans, Financings and Debentures - Summary of Loans Financing and Debentures (Detail)
€ in Millions, R$ in Millions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
Dec. 31, 2017
EUR (€)
Dec. 31, 2016
BRL (R$)
Dec. 31, 2016
EUR (€)
Disclosure of detailed information about borrowings [line items]        
Current   € 2,371    
Current | R$ R$ 2,371      
2017 Non-current   12,027    
Non-current | R$ 12,027   R$ 10,342  
Total   14,398   € 15,179
Total | R$ 14,398   15,179  
Interest paid in advance [member]        
Disclosure of detailed information about borrowings [line items]        
2017 Non-current (48) (48)    
Total (48) (48)    
Debt in Foreign Currency [member]        
Disclosure of detailed information about borrowings [line items]        
Current 31 31    
2017 Non-current 3,267 3,267    
Total R$ 3,298 3,298 30 30
TJLP Plus Two Point Three Four [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost TJLP+2.34%      
Currency R$      
Total     74 74
Debt in Brazilian currency [member]        
Disclosure of detailed information about borrowings [line items]        
Current R$ 151 151    
2017 Non-current 1,920 1,920    
Total 2,071 2,071 3,729 3,729
Loans and Financings [member]        
Disclosure of detailed information about borrowings [line items]        
Current 182 182    
2017 Non-current 5,187 5,187    
Total R$ 5,369 5,369 3,759 3,759
Transaction costs [member]        
Disclosure of detailed information about borrowings [line items]        
Current   (2)    
2017 Non-current   (48)    
Total   (50)   (79)
KfW (Cemig GT) [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2019      
Annual financial cost 178      
Currency Euros      
Current   4    
Total   4   7
Eurobonds [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2024      
Annual financial cost 9.25      
Currency USD      
Current R$ 25 25    
2017 Non-current 3,308 3,308    
Total R$ 3,333 3,333    
Banco do Brasil (Cemig GT) [member] | One Zero Eight Point Three Three Percent of Interbank Rate for Certificates of Deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost 108.33% of the CDI Rate      
Currency R$      
Total     73 73
Banco do Brasil (Cemig GT) [member] | One Zero Eight Point Zero Zero Percent of Interbank Rate for Certificates of Deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost 108.00% of CDI      
Currency R$      
Total     151 151
Banco do Brasil (Cemig GT) [member] | One One One Point Zero Zero Percent of Interbank Rate for Certificates of Deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost 111.00% of CDI Rate      
Currency R$      
Total     51 51
Banco do Brasil (Cemig GT) [member] | One four zero point zero percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2021      
Annual financial cost 140.00% of CDI Rate      
Currency R$      
Current R$ 1 1    
2017 Non-current 741 741    
Total R$ 742 742 1,156 1,156
Banco do Brasil (Cemig GT) [member] | One four six point five zero percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2022      
Annual financial cost 146.50% of CDI      
Currency R$      
2017 Non-current R$ 500 500    
Total R$ 500 500 510 510
CEF [member] | One four six point five zero percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2022      
Annual financial cost 146.50% of CDI      
Currency R$      
Current R$ 1 1    
2017 Non-current 626 626    
Total R$ 627 627 698 698
CEF [member] | One One Nine Point Zero Zero Percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost 119.00% of CDI      
Currency R$      
Current R$ 8 8    
Total R$ 8 8 109 109
Eletrobras [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2023      
Annual financial cost UFIR; RGR + 6.00 to 8.00%      
Currency R$      
Current R$ 17 17    
2017 Non-current 33 33    
Total R$ 50 50 68 68
Large Consumers [member] | Various [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2024      
Annual financial cost Various      
Currency R$      
Current R$ 2 2    
2017 Non-current 2 2    
Total R$ 4 4 6 6
FINEP [member] | Tjlp plus five and tjlp plus two point five [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost TJLP + 5% and TJLP + 8%      
Currency R$      
Current R$ 2 2    
Total R$ 2 2 6 6
Promissory Notes - Cemig GT - 7th Issue [member] | One two eight point zero zero of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost 128.00% of CDI      
Currency R$      
Total     674 674
Banco da amazonia [member] | Interbank rate for certificates of deposit plus one point nine zero [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 1.90%      
Currency R$      
Current R$ 122 122    
Total R$ 122 122 123 123
Sonda (Cemig Telecom) [member] | One one zero percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2021      
Annual financial cost 110.00% of CDI      
Currency R$      
2017 Non-current R$ 42 42    
Total 42 42 83 83
Transaction costs [member]        
Disclosure of detailed information about borrowings [line items]        
Current (2) (2)    
2017 Non-current (24) (24)    
Total (26) (26) (53) (53)
Total | R$ (50)   (79)  
Transaction costs [member] | Transaction costs [member]        
Disclosure of detailed information about borrowings [line items]        
Current | R$ (2)      
Non-current | R$ (48)      
Total | R$ R$ (50)   (79)  
Debentures-3rd Issue, 1st Series [member] | Interbank rate for certificates of deposit plus zero point nine zero [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost CDI + 0.90%      
Currency R$      
Total | R$     543  
Total       543
Debentures-3rd Issue, 1st Series [member] | Interbank rate for certificates of deposit plus zero point six nine [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 0.69%      
Currency R$      
Current | R$ R$ 447      
Current   447    
Total | R$ R$ 447   464  
Total   447   464
Debentures-3rd Issue, 2nd Series [member] | Ipca plus six point zero zero [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2019      
Annual financial cost IPCA + 6.00%      
Currency R$      
Current | R$ R$ 158      
Current   158    
Non-current | R$ 143      
2017 Non-current   143    
Total | R$ R$ 301   293  
Total   301   293
Debentures-3rd Issue, 2nd Series [member] | Ipca plus four point seven zero [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2021      
Annual financial cost IPCA + 4.70%      
Currency R$      
Current | R$ R$ 59      
Current   59    
Non-current | R$ 1,478      
2017 Non-current   1,478    
Total | R$ R$ 1,537   1,496  
Total   1,537   1,496
Debentures-3rd Issue, 3rd Series [member] | Ipca plus six point two zero [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2022      
Annual financial cost IPCA + 6.20%      
Currency R$      
Current | R$ R$ 51      
Current   51    
Non-current | R$ 960      
2017 Non-current   960    
Total | R$ R$ 1,011   984  
Total   1,011   984
Debentures-3rd Issue, 3rd Series [member] | Ipca plus five point one zero [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2025      
Annual financial cost IPCA + 5.10%      
Currency R$      
Current | R$ R$ 39      
Current   39    
Non-current | R$ 882      
2017 Non-current   882    
Total | R$ R$ 921   896  
Total   921   896
Debentures-5th Issue, 1st Series [member] | Interbank rate for certificates of deposit plus one point seven zero [Member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 1.70%      
Currency R$      
Current | R$ R$ 703      
Current   703    
Total | R$ R$ 703   1,411  
Total   703   1,411
Debentures sixth issue first series [member] | Interbank rate for certificates of deposit plus one point six zero [Member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 1.60%      
Currency R$      
Current | R$ R$ 508      
Current   508    
Total | R$ R$ 508   1,041  
Total   508   1,041
Debentures sixth issue first series [member] | Ipca plus eight point zero seven [Member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2020      
Annual financial cost IPCA +8.07%      
Currency R$      
Current | R$ R$ 1      
Current   1    
Non-current | R$ 31      
2017 Non-current   31    
Total | R$ R$ 32   31  
Total   32   31
Debentures-7th Issue, 1st Series [member] | One four zero point zero percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2021      
Annual financial cost 140.00% of CDI      
Currency R$      
Current | R$ R$ 47      
Current   47    
Non-current | R$ 1,636      
2017 Non-current   1,636    
Total | R$ R$ 1,683   2,242  
Total   1,683   2,242
Debentures, 2nd Issue [member] | Ipca plus seven point nine six [Member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2017      
Annual financial cost IPCA + 7.96%      
Currency R$      
Total | R$     235  
Total       235
Debentures fourth issue first series [member] | One four six point five zero percent of interbank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2022      
Annual financial cost 146.50% of CDI      
Currency R$      
Current | R$ R$ 1      
Current   1    
Non-current | R$ 1,575      
2017 Non-current   1,575    
Total | R$ R$ 1,576      
Total   1,576    
Debentures fourth issue first series [member] | Interbank rate for certificates of deposit plus four point zero five percentage one [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 4.05%      
Currency R$      
Current | R$ R$ 20      
Current   20    
Total | R$ 20   1,627  
Total   20   1,627
Debentures [member]        
Disclosure of detailed information about borrowings [line items]        
Total | R$ R$ 9,029   11,420  
Debentures [member] | CDI + 1.60 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 1.60%      
Currency R$      
Current | R$ R$ 100      
Current   100    
Total | R$ R$ 100   101  
Total   100   101
Debentures [member] | Interbank rate for certificates of deposit plus zero point seven four [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2018      
Annual financial cost CDI + 0.74%      
Currency R$      
Current | R$ R$ 34      
Current   34    
Total | R$ R$ 34   67  
Total   34   67
Debentures [member] | Tjlp plus one point eight two percentage selic plus one point eight two percentage [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2022      
Annual financial cost TJLP+1.82% (75%); Selic+1.82% (25%)      
Currency R$      
Current | R$ R$ 33      
Current   33    
Non-current | R$ 122      
2017 Non-current   122    
Total | R$ R$ 155   133  
Total   155   133
Debentures [member] | One one six point five zero percent of inter bank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2019      
Annual financial cost 116.50% of CDI      
Currency R$      
Non-current | R$ R$ 50      
2017 Non-current   50    
Total | R$ R$ 50      
Total   50    
Debentures [member] | One two eight point five zero percent of inter bank rate for certificates of deposit [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2019      
Annual financial cost 128.50% of CDI      
Currency R$      
Current | R$ R$ 15      
Current   15    
Non-current | R$ 11      
2017 Non-current   11    
Total | R$ 26      
Total   26    
Subsidiary Securities [member]        
Disclosure of detailed information about borrowings [line items]        
Current | R$ (25)      
Current   (25)    
Total | R$ (25)   (65)  
Total   (25)   (65)
Total | R$ (25)   (65)  
Net debentures [member]        
Disclosure of detailed information about borrowings [line items]        
Current | R$ 2,189      
Current   2,189    
Non-current | R$ 6,840      
2017 Non-current   6,840    
Total | R$ R$ 9,029   R$ 11,420  
Total   9,029   11,420
Debt in Foreign Currency [member] | Banco do Brasil (Cemig GT) [member] | Various [member]        
Disclosure of detailed information about borrowings [line items]        
Principal maturity 2024      
Annual financial cost Various      
Currency US$      
Current   2    
2017 Non-current   22    
Total   24   € 23
Transaction costs [member]        
Disclosure of detailed information about borrowings [line items]        
2017 Non-current R$ (15) (15)    
Total R$ (15) € (15)