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Non-Cash Transactions - Additional Information (Detail) - BRL (R$) R$ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Non Cash Transactions [abstract] |  |  |
Capitalized financial costs | R$ 71 | R$ 142 | R$ 159

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- Definition Financial charges capitalized. + References No definition available. + Details
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Namespace Prefix: | xcmig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Non Cash Transactions [abstract] + References No definition available. + Details
Name: | xcmig_NonCashTransactionsAbstract
Namespace Prefix: | xcmig_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration