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Intangible Assets - Additional Information (Detail) - BRL (R$) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about intangible assets [line items] |  |  |
Additions of intangible asset | R$ 1,143,000,000 |  |
Financing costs on Intangible assets | 71,000,000 | R$ 142,000,000 | R$ 159,000,000
Intangible assets with non-defined useful life | R$ 0 |  |
Annual average amortization rate | 3.85 percentage |  |
Cemig D [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Intangible assets | R$ 2,368,000,000 | R$ 2,243,000,000 |

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- Details
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