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Financial Instruments and Risk Management - Additional Information (Detail)
$ in Millions
 1 Months Ended 12 Months Ended
Dec. 31, 2019
May 07, 2019
Nov. 30, 2018
BRL (R$)
Nov. 30, 2017
BRL (R$)
Jul. 31, 2018
BRL (R$)
Jul. 31, 2018
USD ($)
Dec. 31, 2019
Dec. 31, 2018
BRL (R$)
Agency
Dec. 31, 2017
BRL (R$)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
BRL (R$)
Dec. 31, 2015
BRL (R$)
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Description of annual rates loans and debentures               IPCA + 4.70% to 8.07% and CDI + 0.64% to 3.26%.        
Liability               R$ 419,000,000 R$ 336,000,000      
Put option               R$ 415,000,000 815,000,000      
Exercise price of put option as number of times adjusted net income               15        
Exercise price of call option as number of times adjusted net income               17        
Option vesting date               Jan. 01, 2021        
Non cash positive (negative) adjustment from derivative transaction               R$ 893,000,000 (32,000,000)      
Swap operation floor rate         R$ 3.85       3.25      
Swap ceiling rate         R$ 5.00       R$ 5.00      
FX Variation Interest rate         9.25% 9.25%     9.25% 9.25%    
Value of principal contracted | $           $ 500       $ 1,000    
Cemig's obligation         125.52% of the CDI rate 125.52% of the CDI rate     150.49% of the CDI 150.49% of the CDI    
Financial assets, fair value               16,279,000,000 R$ 16,866,000,000      
Maturity   April 15, 2022 to April 15, 2025                    
Derivative hedge instrument, Assets               16,279,000,000 16,866,000,000      
Derivative hedge instrument, Liabilities               R$ (18,120,000,000) (19,556,000,000)      
Percentage of decrease in US dollars               1.92%        
Percentage of decrease in euro               1.55%        
Cash flow projection period               12 months        
Daily liquidity projection period               180 days        
Allowance for doubtful debtors               R$ 751,000,000 568,000,000   R$ 660,000,000 R$ 625,000,000
Equity               R$ 15,939,000,000 14,330,000,000   R$ 12,934,000,000 R$ 12,988,000,000
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%        
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%        
Events After Reporting Period [member] | IPCA [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Actuarial assumption of expected rates of inflation 4.0881%           4.0881%          
Increase in inflation rate 25.00%                      
Events After Reporting Period [member] | IGPM [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Actuarial assumption of expected rates of inflation 4.35%           4.35%          
Increase in inflation rate 50.00%                      
Certificado De Deposito Interbancario [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Calculation of percentage of loans and debentures               141.90%        
Put option - SAAG [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Exercise price of financial remuneration percentage               7.00%        
Liability               R$ 419,000,000 312,000,000      
Put option for shares in RME [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Put option     R$ 659,000,000                  
Equity interest     100.00%                  
Put option [Member] | Bottom of range [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Risk-free interest rate               4.86%        
Volatility               23.00%        
Put option, price               R$ 396,000,000        
Put option [Member] | Top of range [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Risk-free interest rate               8.86%        
Volatility               83.00%        
Put option, price               R$ 449,000,000        
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$ 751,000,000 R$ 546,000,000      
Proportion of portfolio percentage               100.00%        
Credit Risk [Member] | Bottom of range [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Number of risk rating agencies used | Agency               3        
Equity               R$ 400,000,000        
Basel ratio               1.00%        
Portfolio percentage of individual               95.00%        
Credit Risk [Member] | Top of range [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Portfolio percentage of individual               105.00%        
Ceming GT [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Derivative hedge instrument               R$ 1,150,000,000        
Ceming GT [member] | Option / Call spread [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Derivative hedge instrument, Assets               1,200,000,000        
Ceming GT [member] | Swap [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Derivative hedge instrument, Liabilities               43,000,000        
Selic rate [member] | Events After Reporting Period [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Borrowings interest rate 6.50%           6.50%          
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 6.4757%           6.4757%          
Increase in interest rate             50.00%          
Hedge [member] | Ceming GT [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Financial assets, fair value               813,000,000        
Derivative hedge instrument, Assets               R$ 712,000,000        
Bonds [member] | Ceming GT [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Maturity               2024        
US dollar [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Exchange rate               3.80        
Euro [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Exchange rate               4.37        
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%        
Cemig GT [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Exercise price of option at fair value       R$ 185,000,000                
Cemig GT [member] | Put option for shares in Lepsa [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Exercise price of option at fair value       147,000,000                
Cemig GT [member] | Put option for shares in RME [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Exercise price of option at fair value       R$ 38,000,000                
Cemig GT [member] | RME [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Voting shares percentage       50.00%                
Cemig GT [member] | Bottom of range [member] | RME [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Percentage of total capital holding       75.00%                
Cemig GT [member] | Bottom of range [member] | Lepsa [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Percentage of total capital holding       100.00%                
Cemig GT [member] | Top of range [member] | RME [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Percentage of total capital holding       66.27%                
Cemig GT [member] | Top of range [member] | Lepsa [member]                        
Disclosure Of Financial Instruments And Financial Risk Managements [line items]                        
Percentage of total capital holding       66.62%