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Subsequent Events - Additional Information (Detail) - BRL (R$)
 1 Months Ended 3 Months Ended
Apr. 23, 2018
Mar. 31, 2018
Mar. 21, 2018
Oct. 26, 2017
Jan. 05, 2018
Mar. 31, 2018
May 03, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of non-adjusting events after reporting period [line items]                    
Number of shares issued               1,258,841,654 1,258,841,654 1,258,841,654
Capital increase R$ 1,000,000,000     R$ 1,000,000,000            
New shares authorized 199,910,947     199,910,947            
Par value per share R$ 5.00     R$ 5.00            
Proceeds from issuance capital R$ 1,324,000,000                  
Capital reserve R$ 324,000,000             R$ 1,925,000,000 R$ 1,925,000,000 R$ 1,925,000,000
Preference shares [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Number of shares issued               838,076,946 838,076,946 838,076,946
New shares authorized 133,061,442     133,061,442            
Par value per share       R$ 6.57            
Common shares [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Number of shares issued               420,764,708 420,764,708 420,764,708
New shares authorized 66,849,505     66,849,505            
Par value per share       R$ 6.57            
Chipley [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Percentage of shares to be acquired             100.00%      
Renova [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Percentage of shares to be acquired             51.00%      
Bottom of range [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Capital increase R$ 6,294,000,000                  
Top of range [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Capital increase R$ 7,294,000,000                  
Promissory Notes [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Borrowings         R$ 400,000,000          
Debt instrument interest rate         151.00%          
Entering into significant commitments or contingent liabilities [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Financial volume from stock issuance     R$ 111,000,000              
Entering into significant commitments or contingent liabilities [member] | Ordinary shares [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Number of shares issued     13,139,799              
Average price per share     R$ 7.30              
Share price     R$ 96,000,000              
Entering into significant commitments or contingent liabilities [member] | Preference shares [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Number of shares issued     1,815,750              
Average price per share     R$ 8.22              
Share price     R$ 15,000,000              
Entering into significant commitments or contingent liabilities [member] | Ceming GT [Member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Advanced payment for power supply           R$ 71,000,000        
Entering into significant commitments or contingent liabilities [member] | Ceming GT [Member] | Interbank deposit certificate CDI rate [member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Advanced discount percentage           155.00%        
Entering into significant commitments or contingent liabilities [member] | Two Thousand Eighteen Employee Voluntary Severance Program [Member]                    
Disclosure of non-adjusting events after reporting period [line items]                    
Description of type of retirement benefit plan   In March 2018, the Company approved the Programmed Voluntary Retirement Program (‘the 2018 PDVP’). Employees eligible to be part of the program are those will have worked for Cemig for 25 years or more by December 31, 2018. The period for joining the program was April 2 through 30, 2018. The program offers payment of the severance amounts specified by law, including payment for notice period, and especial deposit of the ‘penalty’ amount of 40% of the FGTS (Labour Guarantee Fund) Base Value, and other charges specified by the legislation, with no provision for the payment of any additional amounts.                
Description of eligible employees to take part of plan   Employees eligible to be part of the program are those will have worked for Cemig for 25 years or more by December 31, 2018.                
Description of benefits to participants   The program offers payment of the severance amounts specified by law, including payment for notice period, and especial deposit of the 'penalty' amount of 40% of the FGTS (Labour Guarantee Fund) Base Value, and other charges specified by the legislation                
Percentage of penalty payment on base value of the employee's FGTS fund   40.00%