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Financial Instruments and Risk Management - Summary of Derivative Instruments Contracted (Detail) R$ in Millions | 12 Months Ended
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Dec. 31, 2017 BRL (R$) | Dec. 31, 2017 USD ($)
Derivative financial instruments [Abstract] |  |
emig's right | US$: FX variation + ate (9.25% p.a.) | US$: FX variation + ate (9.25% p.a.)
Cemig's obligatio | R$: 150.49% of CDI | R$: 150.49% of CDI
n Maturity period | 12/2017 to 12/2024 | 12/2017 to 12/2024
principal racted | $ |  | $ 1,000,000
Unrealized ga Amount according to contract 2017 | R$ 51 |
in /loss Fair value 2017 | R$ (32) |

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- Definition Derivative financial instruments [Abstract] + References No definition available. + Details
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- Definition Description of obligation on borrowings. + References No definition available. + Details
Name: | xcmig_DescriptionOfObligationOnBorrowings
Namespace Prefix: | xcmig_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Financial instruments maturity period. + References No definition available. + Details
Name: | xcmig_FinancialInstrumentsMaturityPeriod
Namespace Prefix: | xcmig_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Right to call description. + References No definition available. + Details
Name: | xcmig_RightToCallDescription
Namespace Prefix: | xcmig_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Unrealized gain / loss according to contract + References No definition available. + Details
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Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Unrealized gain / loss Fair value + References No definition available. + Details
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Namespace Prefix: | xcmig_
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Balance Type: | debit
Period Type: | duration

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- Definition Value of principal contracted + References No definition available. + Details
Name: | xcmig_ValueOfPrincipalContracted
Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration