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27. REVENUE (Details 3) - BRL (R$) R$ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Revenue [abstract] |  |  |
Construction and upgrades revenue | R$ 201 | R$ 312 | R$ 138
Construction and upgrades costs | (147) | (220) | (96)
Margin | R$ 54 | R$ 92 | R$ 42
Mark-up (%) | 36.73% | 41.82% | 43.75%
Operation and maintenance revenue | R$ 279 | R$ 352 | R$ 343
Operation and maintenance cost | (223) | (388) | (215)
Margin | R$ 56 | R$ (36) | R$ 128
Mark-up (%) | 25.11% | (9.28%) | 59.53%

X
- Definition The amount of construction and upgrades costs. + References No definition available. + Details
Name: | cig_ConstructionAndUpgradesCosts
Namespace Prefix: | cig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of construction and upgrades revenue. + References No definition available. + Details
Name: | cig_ConstructionAndUpgradesRevenue
Namespace Prefix: | cig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of margin construction revenue. + References No definition available. + Details
Name: | cig_MarginConstructionRevenue
Namespace Prefix: | cig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of margin operation revenue. + References No definition available. + Details
Name: | cig_MarginOperationRevenue
Namespace Prefix: | cig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of markup construction revenue. + References No definition available. + Details
Name: | cig_MarkupConstructionRevenue
Namespace Prefix: | cig_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of markup operation revenue. + References No definition available. + Details
Name: | cig_MarkupOperationRevenue
Namespace Prefix: | cig_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of operation and maintenance cost. + References No definition available. + Details
Name: | cig_OperationAndMaintenanceCost
Namespace Prefix: | cig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of operation and maintenance revenue. + References No definition available. + Details
Name: | cig_OperationAndMaintenanceRevenue
Namespace Prefix: | cig_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | ifrs-full_RevenueAbstract
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration