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Concessions and Authorizations - Schedule of Concession and Amount to Be Paid (Detail)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
Irape enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Percentage interest 100.00%
Nominal value in 2017 R$ 33
Present value in 2017 R$ 14
Inflation index to be used to adjust historical amounts IGP-M
Queimadao Consortium Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Percentage interest 82.50%
Nominal value in 2017 R$ 8
Present value in 2017 R$ 4
Inflation index to be used to adjust historical amounts IGP-M
Salto morais small hydro plant enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Percentage interest 100.00%
Inflation index to be used to adjust historical amounts IPCA
Rio de Pedras Small Hydro Plant Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Percentage interest 100.00%
Nominal value in 2017 R$ 1
Inflation index to be used to adjust historical amounts IPCA
Various Small Hydro Plants Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Percentage interest 100.00%
Nominal value in 2017 R$ 3
Present value in 2017 R$ 3
Inflation index to be used to adjust historical amounts IPCA
Bottom of range [member] | Irape enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 03/2006
Bottom of range [member] | Queimadao Consortium Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 01/2004
Bottom of range [member] | Salto morais small hydro plant enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 06/2013
Bottom of range [member] | Rio de Pedras Small Hydro Plant Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 06/2013
Bottom of range [member] | Various Small Hydro Plants Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 06/2013
Top of range [member] | Irape enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 02/2035
Top of range [member] | Queimadao Consortium Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 12/2032
Top of range [member] | Salto morais small hydro plant enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 07/2020
Top of range [member] | Rio de Pedras Small Hydro Plant Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 09/2024
Top of range [member] | Various Small Hydro Plants Enterprise [member]  
Disclosure of concession and amount to be paid [Line Items]  
Amortization period 08/2025