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Income and Social Contribution Taxes - Summary of Deferred Income and Social Contribution Taxes (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities R$ (1,916) R$ (1,738)
Total, net 1,136 1,215
Total assets 1,871 1,797
Total liabilities (735) (582)
Deferred tax assets 3,052 2,953
Tax loss carry forwards [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 524 290
Provisions [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 1,093 1,027
Post-retirement obligations [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 1,179 1,175
Estimated provision for doubtful receivables [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 207 229
Taxes with suspended liability [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 14 202
Paid concession [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 8 8
Adjustment to fair value: Swap/loss [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 13  
Other [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax assets 14 22
Funding cost [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities (31) (45)
Deemed cost [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities (276) (268)
Cost of acquisition of equity interests [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities (464) (481)
Borrowing costs capitalized [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities (166) (149)
Taxes on revenues not redeemed - Presumed Profit accounting method [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities (1) (2)
Transmission assets: Indemnity gain [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities (937) (517)
Adjustment of expectation of cash flow from the indemnifiable Financial asset of distribution concession [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities   (271)
Other Deferred Income And Social Contribution Taxes Liabilities [member]    
Disclosure of income and social contribution taxes recoverable [Line Items]    
Deferred tax liabilities R$ (41) R$ (5)