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Financial Instruments and Risk Management - Additional Information (Detail) € in Millions, R$ in Millions, $ in Millions |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |
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Dec. 31, 2018 | Nov. 30, 2016 BRL (R$) | Oct. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 BRL (R$) shares | Dec. 31, 2017 EUR (€) shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 BRL (R$) shares | Sep. 23, 2016 shares | Sep. 06, 2016 shares | May 31, 2016 shares | Dec. 31, 2015 BRL (R$) shares | Dec. 31, 2014 BRL (R$)
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Securities held for trading |  |  |  |  | R$ 1,044 |  |  | R$ 995 |  |  |  |  |
Financial instruments held to maturity |  |  |  |  | R$ 45 |  |  | 50 |  |  |  |  |
Description of annual rates loans and debentures |  |  |  |  | IPCA + 4.70% and 8.07%, and CDI + 0.69% to 5.62% |  |  |  |  |  |  |  |
Loans, financings and debentures |  |  |  |  | R$ 14,398 |  |  | R$ 15,179 |  |  |  |  |
Preferred shares | shares |  |  |  |  | 1,258,841,654 | 1,258,841,654 | 1,258,841,654 | 1,258,841,654 |  |  |  | 1,258,841,654 |
Exercise price of put option as number of times adjusted net profit |  |  |  |  | 15 |  |  |  |  |  |  |  |
Exercise price of call option as number of times adjusted net profit |  |  |  |  | 16 |  |  |  |  |  |  |  |
Option vesting date |  |  |  |  | Jan. 01, 2021 |  |  |  |  |  |  |  |
Net effect of call and put options |  |  |  |  | R$ 1 |  |  |  |  |  |  |  |
Non cash negative adjustment from derivative transaction |  |  |  |  | R$ (32) |  |  |  |  |  |  |  |
Percentage of increase in US dollars |  |  |  |  | 3.11% | 3.11% | 3.11% |  |  |  |  |  |
Percentage of increase in euro |  |  |  |  | 1.13% |  |  |  |  |  |  |  |
Actuarial assumption of expected rates of inflation |  |  |  |  | 4.00% | 4.00% | 4.00% | 4.50% |  |  |  | 5.50% |
Cash flow projection period |  |  |  |  | 12 months |  |  |  |  |  |  |  |
Daily liquidity projection period |  |  |  |  | 180 days |  |  |  |  |  |  |  |
Allowance for doubtful debtors |  |  |  |  | R$ 568 |  |  | R$ 660 |  |  |  | R$ 625 | R$ 650
Equity |  |  |  |  | R$ 14,330 |  |  | 12,934 |  |  |  | R$ 12,988 | R$ 11,285
Events After Reporting Period [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in inflation rate | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Events After Reporting Period [Member] | IPCA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Actuarial assumption of expected rates of inflation | 4.43% |  |  | 4.43% |  |  |  |  |  |  |  |  |
Increase in inflation rate | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Probable scenario 2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of depreciation from real exchange rate | 25.00% |  |  | 25.00% | 25.00% | 25.00% | 25.00% |  |  |  |  |  |
Scenario 3 [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of depreciation from real exchange rate | 50.00% |  |  | 50.00% | 50.00% | 50.00% | 50.00% |  |  |  |  |  |
Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  |  | 3.50% |  |  |  |  |  |  |  |
Volatility |  |  |  |  | 20.00% |  |  |  |  |  |  |  |
Put option, price |  |  |  |  | R$ 487 |  |  |  |  |  |  |  |
Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  |  | 11.50% |  |  |  |  |  |  |  |
Volatility |  |  |  |  | 90.00% |  |  |  |  |  |  |  |
Put option, price |  |  |  |  | R$ 532 |  |  |  |  |  |  |  |
Certificado De Deposito Interbancario [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Calculation of percentage of loans and debentures |  |  |  |  | 147.50% |  |  |  |  |  |  |  |
Put option - SAAG [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of financial remuneration percentage |  |  |  |  | 7.00% |  |  |  |  |  |  |  |
Liability |  |  |  |  | R$ 312 |  |  | 196 |  |  |  |  |
Put option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Description of terms of shares reserved for issue under options and contracts for sale of shares |  |  |  |  | 1) The maturity of the Put Option granted in 2011 by Cemig in favor of the unit holders of FIP Redentor, initially specified to be May 31, 2016, was postponed, to two separate exercise dates: a) First option exercise window: The intention to exercise may be stated by any direct Shareholder/s who decide to do so, independently of the exercise of the Put Option by the other direct Shareholders, up to September 23, 2016, inclusive, and shall cover only preferred shares in Parati, up to a limit of 153,634,195 shares, representing 14.30% of the total shares in Parati held by the other direct Shareholders. b) Second option exercise window: The intention to exercise may be stated by any direct Shareholder/s who decide to do so, independently of the exercise of the Put Option by the other direct Shareholders, up to September 23, 2017, inclusive, and may cover the totality of the shares in Parati, being independent of any exercise, or not, of the Put Option in the first payment window. Cemig made the payment on November 30, 2017. |  |  |  |  |  |  |  |
Put option [Member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  |  | 6.05% |  |  |  |  |  |  |  |
Volatility |  |  |  |  | 23.00% |  |  |  |  |  |  |  |
Put option, price |  |  |  |  | R$ 276 |  |  |  |  |  |  |  |
Put option [Member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  |  | 10.05% |  |  |  |  |  |  |  |
Volatility |  |  |  |  | 83.00% |  |  |  |  |  |  |  |
Put option, price |  |  |  |  | R$ 377 |  |  |  |  |  |  |  |
Put options for shares in Parati [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of financial remuneration percentage |  |  |  |  | 0.90% |  |  |  |  |  |  |  |
Credit Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Allowance for doubtful debtors |  |  |  |  | R$ 546 |  |  | R$ 660 |  |  |  |  |
Credit Risk [Member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of risk rating agencies used |  |  |  |  | 3 |  |  |  |  |  |  |  |
Equity |  |  |  |  | R$ 400 |  |  |  |  |  |  |  |
Basel ratio |  |  |  |  | 12.00% |  |  |  |  |  |  |  |
Selic rate [member] | Events After Reporting Period [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings interest rate | 6.75% |  |  | 6.75% |  |  |  |  |  |  |  |  |
Increase in interest rate |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
TJLP [member] | Events After Reporting Period [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings interest rate | 7.00% |  |  | 7.00% |  |  |  |  |  |  |  |  |
Increase in interest rate |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
US dollar [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Loans, financings and debentures |  |  |  |  | R$ 3,357 |  | $ 3,357 |  |  |  |  |  |
Exchange rate |  |  |  |  | 3.411 |  |  |  |  |  |  |  |
Euro [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Loans, financings and debentures |  |  |  |  | R$ 4 | € 4 |  |  |  |  |  |  |
Exchange rate |  |  |  |  | 3.922 |  |  |  |  |  |  |  |
Ordinary shares [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares reserved for issue under options | shares |  |  |  |  |  |  |  |  |  |  | 55,234,637 |  |
Ordinary shares [member] | Light SA [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares reserved for issue under options | shares |  |  |  |  |  |  |  |  |  |  | 53,152,298 |  |
Preference shares [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred shares | shares |  |  |  |  | 838,076,946 | 838,076,946 | 838,076,946 | 838,076,946 |  |  |  | 838,076,946 |
Preference shares [member] | Put option [Member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares held by associates | shares |  |  |  |  |  |  |  |  | 153,634,195 |  |  |  |
Percentage of preferred shares |  |  |  |  |  |  |  |  | 14.30% |  |  |  |
Preference shares [member] | Transmissora Alianca de Energia Eletrica S.A. [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares reserved for issue under options | shares |  |  |  |  |  |  |  |  |  |  | 110,469,274 |  |
Lepsa [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of share of put option divided among partners |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
RME [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of share of put option divided among partners |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Cemig Telecom [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cash paid for exercise of put option |  | R$ 222 |  |  |  |  |  |  |  |  |  |  |
Cemig Telecom [member] | Preference shares [member] | Put option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred shares | shares |  |  |  |  |  |  |  |  |  | 153,634,195 |  |  |
Level 2 [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Financial instruments held to maturity |  |  |  |  | R$ 45 |  |  | R$ 50 |  |  |  |  |
Loans, financings and debentures |  |  |  |  |  |  |  | R$ 14,711 |  |  |  |  |
Parent [member] | Credit Risk [Member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Portfolio percentage of individual |  |  |  |  | 30.00% |  |  |  |  |  |  |  |
Individual company [Member] | Credit Risk [Member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure Of Financial Instruments And Financial Risk Managements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Portfolio percentage of individual |  |  |  |  | 50.00% |  |  |  |  |  |  |  |

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