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Equity and Remuneration to Shareholders - Summary of Share Capital for Common Shares and Preferred Shares (Detail) - shares
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of shares issued 1,258,841,654 1,258,841,654 1,258,841,654
Percentage of changes in share capital 100.00% 100.00% 100.00%
Minas Gerais State [Member]      
Number of shares issued 214,414,739 214,414,739 214,414,739
Percentage of changes in share capital 17.00% 17.00% 17.00%
Other entities of M.G. State [Member]      
Number of shares issued 4,916,931 4,916,931 10,475,515
Percentage of changes in share capital 1.00% 1.00% 1.00%
FIA Dinamica Energia S.A. [member]      
Number of shares issued 104,105,344    
Percentage of changes in share capital 8.00%    
Other Shareholders In Brazil [Member]      
Number of shares issued 347,517,216 365,062,766 237,485,208
Percentage of changes in share capital 27.00% 28.00% 18.00%
Foreign shareholders [member]      
Number of shares issued 587,887,424 590,089,362 615,093,581
Percentage of changes in share capital 47.00% 47.00% 49.00%
AGC Energia S.A. [Member]      
Number of shares issued   84,357,856 181,372,611
Percentage of changes in share capital   7.00% 15.00%
Common shares [member]      
Number of shares issued 420,764,708 420,764,708 420,764,708
Percentage of changes in share capital 100.00% 100.00% 100.00%
Common shares [member] | Minas Gerais State [Member]      
Number of shares issued 214,414,739 214,414,739 214,414,739
Percentage of changes in share capital 51.00% 51.00% 51.00%
Common shares [member] | Other entities of M.G. State [Member]      
Number of shares issued 56,703 56,703 56,703
Common shares [member] | FIA Dinamica Energia S.A. [member]      
Number of shares issued 41,635,754    
Percentage of changes in share capital 10.00%    
Common shares [member] | Other Shareholders In Brazil [Member]      
Number of shares issued 110,343,209 112,584,011 58,127,167
Percentage of changes in share capital 26.00% 27.00% 14.00%
Common shares [member] | Foreign shareholders [member]      
Number of shares issued 54,314,303 9,351,399 9,465,251
Percentage of changes in share capital 13.00% 2.00% 2.00%
Common shares [member] | AGC Energia S.A. [Member]      
Number of shares issued   84,357,856 138,700,848
Percentage of changes in share capital   20.00% 33.00%
Preference shares [member]      
Number of shares issued 838,076,946 838,076,946 838,076,946
Percentage of changes in share capital 100.00% 100.00% 100.00%
Preference shares [member] | Other entities of M.G. State [Member]      
Number of shares issued 4,860,228 4,860,228 10,418,812
Percentage of changes in share capital 1.00% 1.00% 1.00%
Preference shares [member] | FIA Dinamica Energia S.A. [member]      
Number of shares issued 62,469,590    
Percentage of changes in share capital 7.00%    
Preference shares [member] | Other Shareholders In Brazil [Member]      
Number of shares issued 237,174,007 252,478,755 179,358,041
Percentage of changes in share capital 28.00% 30.00% 21.00%
Preference shares [member] | Foreign shareholders [member]      
Number of shares issued 533,573,121 580,737,963 605,628,330
Percentage of changes in share capital 64.00% 69.00% 73.00%
Preference shares [member] | AGC Energia S.A. [Member]      
Number of shares issued     42,671,763
Percentage of changes in share capital     5.00%