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Provisions - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
BRL (R$)
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Oct. 25, 2017
BRL (R$)
Dec. 31, 2016
USD ($)
Disclosure of provisions and contingencies [Line Items]              
Description of contingencies Cemig and Cemig D are defendants in several public civil claims (class actions) requesting nullity of the clause in the Electricity Supply Contracts for public illumination signed between the Company and the various municipalities of its concession area, and restitution by the Company of the difference representing the amounts charged in the last 20 years, in the event that the courts recognize that these amounts were unduly charged. Cemig and Cemig D are defendants in several public civil claims (class actions) requesting nullity of the clause in the Electricity Supply Contracts for public illumination signed between the Company and the various municipalities of its concession area, and restitution by the Company of the difference representing the amounts charged in the last 20 years, in the event that the courts recognize that these amounts were unduly charged.          
Estimated legal and other disputes based on non-compliance of contracts R$ 80,000,000   R$ 71,000,000        
Estimated dispute based on irregularities of tender proceedings 26,000,000   26,000,000        
Provision for dispute based on irregularities in tender proceedings 0            
Minas Gerias Brazil [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount R$ 79,000,000   71,000,000        
Percentage of annual gross operating revenue to be invested 0.50% 0.50%          
Description of municipalities involved Emborcação, Pissarrão, Funil, Volta Grande, Poquim, Paraúna, Miranda, Nova Ponte, Rio de Pedras and Peti plants in environmental protection and preservation of the water tables of the counties where these power plants are located, Emborcação, Pissarrão, Funil, Volta Grande, Poquim, Paraúna, Miranda, Nova Ponte, Rio de Pedras and Peti plants in environmental protection and preservation of the water tables of the counties where these power plants are located,          
Higher Appeal Court And Federal Supreme Court [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount R$ 127,000,000   113,000,000        
Contingent provision 0            
Early settlement of earnings compensation account [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 398,000,000   390,000,000        
Employees [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 311,000,000   290,000,000        
Indemnity Liabilities     178,000,000        
Indemnity liabilities court permitted payment into court 122,000,000            
Indemnity liabilities court permitted payment into court updated represents amount 267,000,000   255,000,000        
Social Security Contributions [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 1,647,000,000   1,510,000,000        
Non homologation of offsetting of tax credit [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 275,000,000   317,000,000        
Corporate tax return restitution and offsetting [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount R$ 576,000,000   535,000,000        
Parati [member]              
Disclosure of provisions and contingencies [Line Items]              
Equity interest 100.00% 100.00%          
Luce brasil equity investment fund [member]              
Disclosure of provisions and contingencies [Line Items]              
Equity interest 75.00% 75.00%          
Light SA [member]              
Disclosure of provisions and contingencies [Line Items]              
voting shares percentage 13.03%       13.03%    
Income tax withheld at sourced on capital gain in stockholding transaction [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount R$ 212,000,000   198,000,000        
Social contribution tax on net income [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 322,000,000   280,000,000        
Icms tax [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 82,000,000            
Aneel Normative Resolution 456/2000 [member]              
Disclosure of provisions and contingencies [Line Items]              
Contingent provision 1,224,000,000   1,305,000,000        
Accounting of electricity sale transactions in electricity trading chamber [member]              
Disclosure of provisions and contingencies [Line Items]              
Expense on purchase of energy 288,000,000   264,000,000        
System service charges resolution of national energy policy council [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 202,000,000   182,000,000        
Tariff increases [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 275,000,000   254,000,000        
Periodic tariff adjustment neutrality of portio A [Member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount     317,000,000        
Luz Para Todos [member]              
Disclosure of provisions and contingencies [Line Items]              
Losses on breach of contract 261,000,000   237,000,000        
Provision on losses 0            
Alteration of monetary updating index of employment law cases [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 220,000,000   176,000,000        
Additional provision for estimated financial effect of contingent liabilities 0            
Labor claims [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 1,854,000,000   1,544,000,000        
Contingent provision 474,000,000   349,000,000        
Corporate [Member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount           R$ 239,000,000  
Customers claims [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 56,000,000   33,000,000        
Contingent provision 19,000,000   15,000,000        
Tax [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 159,000,000   200,000,000        
Contingent provision 44,000,000   57,000,000        
IPTU Urban Land Tax to real estate properties [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount | $         $ 122   $ 95
Contingent provision | $   $ 13   $ 13      
Environment [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 68,000,000   34,000,000        
Regulatory [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 222,000,000   236,000,000        
Contingent provision 40,000,000   43,000,000        
Other civil actions [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 218,000,000   227,000,000        
Contingent provision 43,000,000   40,000,000        
Contingent provision 32,000,000   28,000,000        
Other [member]              
Disclosure of provisions and contingencies [Line Items]              
Estimated contingent amount 170,000,000   158,000,000        
Contingent provision R$ 14,000,000   9,000,000        
Contingent provision     R$ 21,000,000