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Operating Costs And Expenses - Summary of Construction Cost (Detail) - BRL (R$) R$ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of construction cost [Abstract] |  |  |
Personnel and managers | R$ 36 | R$ 58 | R$ 65
Materials | 550 | 534 | 521
Outsourced services | 406 | 448 | 504
Others | 127 | 153 | 162
Construction cost | R$ 1,119 | R$ 1,193 | R$ 1,252

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- Definition Construction contracts costs. + References No definition available. + Details
Name: | xcmig_ConstructionContractsCosts
Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Disclosure of construction cost. + References No definition available. + Details
Name: | xcmig_DisclosureOfConstructionCostAbstract
Namespace Prefix: | xcmig_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Material construction cost. + References No definition available. + Details
Name: | xcmig_MaterialConstructionCost
Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Other construction cost. + References No definition available. + Details
Name: | xcmig_OtherConstructionCost
Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Outsourced service cost. + References No definition available. + Details
Name: | xcmig_OutsourcedServiceCost
Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Personnel and managers cost. + References No definition available. + Details
Name: | xcmig_PersonnelAndManagersCost
Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration