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Operating Context - Additional Information (Detail) R$ in Millions, $ in Billions | 12 Months Ended
--- | ---
Dec. 31, 2017 BRL (R$) Plant MW | Dec. 31, 2017 USD ($) Plant MW | Dec. 31, 2016 BRL (R$) | Dec. 31, 2015 BRL (R$)
Disclosure of operating context [line items] |  |  |  |
Current liabilities exceeded current assets amount | R$ | R$ (126) |  |  |
Short-term loans, financings and debentures | R$ | 2,371 |  | R$ 4,837 |
Long-term loans, financings and debentures | R$ | 12,027 |  | 10,342 |
Positive operating cash flows | R$ | 580 |  | 1,213 | R$ 3,008
Indebtedness from loans, financings and debentures | R$ | 14,398 |  | R$ 15,179 |
Reprofiling of debt | R$ | R$ 3,400 |  |  |
Baguari Energia [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 49.00% | 49.00% |  |
Neoenergia [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 51.00% | 51.00% |  |
Alianca Norte [member] | Vale [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 50.10% | 50.10% |  |
Alianca Norte [member] | Norte Energia [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Interest acquisition percentage | 9.00% |  |  |
Amazonia energia participacoes S A [member] | Light SA [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 25.50% | 25.50% |  |
Amazonia energia participacoes S A [member] | Norte Energia S.A [Member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 9.77% | 9.77% |  |
Alianca [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 1,158 | 1,158 |  |
Assured energy offtake level of plant | 652 | 652 |  |
Alianca [member] | Vale [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Percentage of total capital holding | 55.00% | 55.00% |  |
Transmissora Alianca de Energia Eletrica S.A. [member] | Brazil [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Principal place of business of joint operation | 17 states | 17 states |  |
Axxiom solucoesTecnologicas S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 49.00% | 49.00% |  |
Axxiom solucoesTecnologicas S.A. [member] | Light [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 51.00% | 51.00% |  |
Cemig GT [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Debt | $ |  | $ 1 |  |
Number of power plants | Plant | 60 | 60 |  |
Cemig GT [member] | Alianca Norte [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 49.90% | 49.90% |  |
Cemig GT [member] | Alianca [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Percentage of total capital holding | 45.00% | 45.00% |  |
Cemig GT [member] | Hydroelectrical power plants [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Number of power plants | Plant | 56 | 56 |  |
Cemig GT [member] | Wind power plants [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Number of power plants | Plant | 3 | 3 |  |
Cemig GT [member] | Thermal power plant [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Number of power plants | Plant | 1 | 1 |  |
Cemig Gerao Trs Marias S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 396 | 396 |  |
Assured energy offtake level of plant | 239 | 239 |  |
Cemig Gerao Salto Grande S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 102 | 102 |  |
Assured energy offtake level of plant | 75 | 75 |  |
Cemig Gerao Camargos S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 46 | 46 |  |
Assured energy offtake level of plant | 21 | 21 |  |
Cemig Geracao Itutinga S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 28 | 28 |  |
Assured energy offtake level of plant | 52 | 52 |  |
Cemig Geracao Lest S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 35.16 | 35.16 |  |
Assured energy offtake level of plant | 18.64 | 18.64 |  |
Cemig Geracao Oeste S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 28.90 | 28.90 |  |
Assured energy offtake level of plant | 11.21 | 11.21 |  |
Cemig Geracao Sul S.A. [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Plant capacity | 39.53 | 39.53 |  |
Assured energy offtake level of plant | 27.42 | 27.42 |  |
Cemig Telecomunicacoes S.A. [member] | Ativas Data Center [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 19.60% | 19.60% |  |
Ceming GT [Member] | Amazonia energia participacoes S A [member] |  |  |  |
Disclosure of operating context [line items] |  |  |  |
Ownership interest | 74.50% | 74.50% |  |

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