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Basis of Preparation - Additional Information (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Summary Of Significant Accounting Policies [Line Items]    
Extension of concession agreement period 30 years  
Increase (decrease) due to application of IFRS 15 [member]    
Disclosure Of Summary Of Significant Accounting Policies [Line Items]    
Revenue reclassified amount from adoption of pronouncement R$ 41 R$ 48
Residential consumers [member]    
Disclosure Of Summary Of Significant Accounting Policies [Line Items]    
Provision for doubtful receivables 90 days  
Commercial consumers [member]    
Disclosure Of Summary Of Significant Accounting Policies [Line Items]    
Provision for doubtful receivables 180 days  
Other consumers [member]    
Disclosure Of Summary Of Significant Accounting Policies [Line Items]    
Provision for doubtful receivables 360 days