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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
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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%

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