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Principles of Consolidation - Schedule of Companies Direct Equity Investments (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Cemig Geracao e Transmissao [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Cemig Distribuicao [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Gasmig [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 99.57% | 99.57% | 99.57%
Cemig Telecom [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Rosal Energia [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Sa Carvalho S A [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Horizontes Energia [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Usina Termica Ipatinga [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Cemig PCH [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Cemig Trading [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Efficientia [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Cemig Comercializadora de Energia Incentivada [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
UTE Barreiro (Barreiro Thermal Plant) [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%
Empresade Serviose Comercializaode Energia Eltrica [member] |  |  |
Disclosure of subsidiaries [line items] |  |  |
Direct interest | 100.00% | 100.00% | 100.00%

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