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Financial Instruments and Risk Management - Additional Information (Detail)
€ in Millions, R$ in Millions, $ in Millions
 1 Months Ended 12 Months Ended
Dec. 31, 2018
Nov. 30, 2016
BRL (R$)
Oct. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
BRL (R$)
shares
Dec. 31, 2017
EUR (€)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
BRL (R$)
shares
Sep. 23, 2016
shares
Sep. 06, 2016
shares
May 31, 2016
shares
Dec. 31, 2015
BRL (R$)
shares
Dec. 31, 2014
BRL (R$)
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Securities held for trading         R$ 1,044     R$ 995          
Financial instruments held to maturity         R$ 45     50          
Description of annual rates loans and debentures         IPCA + 4.70% and 8.07%, and CDI + 0.69% to 5.62%                
Loans, financings and debentures         R$ 14,398     R$ 15,179          
Preferred shares | shares         1,258,841,654 1,258,841,654 1,258,841,654 1,258,841,654       1,258,841,654  
Exercise price of put option as number of times adjusted net profit         15                
Exercise price of call option as number of times adjusted net profit         16                
Option vesting date         Jan. 01, 2021                
Net effect of call and put options         R$ 1                
Non cash negative adjustment from derivative transaction         R$ (32)                
Percentage of increase in US dollars         3.11% 3.11% 3.11%            
Percentage of increase in euro         1.13%                
Actuarial assumption of expected rates of inflation         4.00% 4.00% 4.00% 4.50%       5.50%  
Cash flow projection period         12 months                
Daily liquidity projection period         180 days                
Allowance for doubtful debtors         R$ 568     R$ 660       R$ 625 R$ 650
Equity         R$ 14,330     12,934       R$ 12,988 R$ 11,285
Events After Reporting Period [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Increase in inflation rate 50.00%                        
Events After Reporting Period [Member] | IPCA [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Actuarial assumption of expected rates of inflation 4.43%     4.43%                  
Increase in inflation rate 25.00%                        
Probable scenario 2 [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Percentage of depreciation from real exchange rate 25.00%     25.00% 25.00% 25.00% 25.00%            
Scenario 3 [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Percentage of depreciation from real exchange rate 50.00%     50.00% 50.00% 50.00% 50.00%            
Bottom of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Risk-free interest rate         3.50%                
Volatility         20.00%                
Put option, price         R$ 487                
Top of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Risk-free interest rate         11.50%                
Volatility         90.00%                
Put option, price         R$ 532                
Certificado De Deposito Interbancario [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Calculation of percentage of loans and debentures         147.50%                
Put option - SAAG [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Exercise price of financial remuneration percentage         7.00%                
Liability         R$ 312     196          
Put option [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Description of terms of shares reserved for issue under options and contracts for sale of shares         1) The maturity of the Put Option granted in 2011 by Cemig in favor of the unit holders of FIP Redentor, initially specified to be May 31, 2016, was postponed, to two separate exercise dates: a) First option exercise window: The intention to exercise may be stated by any direct Shareholder/s who decide to do so, independently of the exercise of the Put Option by the other direct Shareholders, up to September 23, 2016, inclusive, and shall cover only preferred shares in Parati, up to a limit of 153,634,195 shares, representing 14.30% of the total shares in Parati held by the other direct Shareholders. b) Second option exercise window: The intention to exercise may be stated by any direct Shareholder/s who decide to do so, independently of the exercise of the Put Option by the other direct Shareholders, up to September 23, 2017, inclusive, and may cover the totality of the shares in Parati, being independent of any exercise, or not, of the Put Option in the first payment window. Cemig made the payment on November 30, 2017.                
Put option [Member] | Bottom of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Risk-free interest rate         6.05%                
Volatility         23.00%                
Put option, price         R$ 276                
Put option [Member] | Top of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Risk-free interest rate         10.05%                
Volatility         83.00%                
Put option, price         R$ 377                
Put options for shares in Parati [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Exercise price of financial remuneration percentage         0.90%                
Credit Risk [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Allowance for doubtful debtors         R$ 546     R$ 660          
Credit Risk [Member] | Bottom of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Number of risk rating agencies used         3                
Equity         R$ 400                
Basel ratio         12.00%                
Selic rate [member] | Events After Reporting Period [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Borrowings interest rate 6.75%     6.75%                  
Increase in interest rate       25.00%                  
TJLP [member] | Events After Reporting Period [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Borrowings interest rate 7.00%     7.00%                  
Increase in interest rate       50.00%                  
US dollar [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Loans, financings and debentures         R$ 3,357   $ 3,357            
Exchange rate         3.411                
Euro [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Loans, financings and debentures         R$ 4 € 4              
Exchange rate         3.922                
Ordinary shares [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Number of shares reserved for issue under options | shares                     55,234,637    
Ordinary shares [member] | Light SA [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Number of shares reserved for issue under options | shares                     53,152,298    
Preference shares [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Preferred shares | shares         838,076,946 838,076,946 838,076,946 838,076,946       838,076,946  
Preference shares [member] | Put option [Member] | Top of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Number of shares held by associates | shares                 153,634,195        
Percentage of preferred shares                 14.30%        
Preference shares [member] | Transmissora Alianca de Energia Eletrica S.A. [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Number of shares reserved for issue under options | shares                     110,469,274    
Lepsa [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Percentage of share of put option divided among partners     50.00%                    
RME [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Percentage of share of put option divided among partners     50.00%                    
Cemig Telecom [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Cash paid for exercise of put option   R$ 222                      
Cemig Telecom [member] | Preference shares [member] | Put option [Member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Preferred shares | shares                   153,634,195      
Level 2 [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Financial instruments held to maturity         R$ 45     R$ 50          
Loans, financings and debentures               R$ 14,711          
Parent [member] | Credit Risk [Member] | Bottom of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Portfolio percentage of individual         30.00%                
Individual company [Member] | Credit Risk [Member] | Bottom of range [member]                          
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items]                          
Portfolio percentage of individual         50.00%