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Operating Costs And Expenses - Summary of Operating Costs and Expenses (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Operating Costs And Expenses [Abstract]      
Personnel R$ 1,627 R$ 1,643 R$ 1,435
Employees' and managers' profit sharing 5 7 137
Post-retirement benefits - Note 23 (229) 345 156
Materials 61 58 154
Raw materials and inputs for production of energy 10    
Outsourced services 974 867 899
Energy bought for resale 10,919 8,273 9,542
Depreciation and amortization 850 834 835
Operating provisions 854 704 1,401
Charges for use of the national grid 1,174 947 999
Gas bought for resale 1,071 878 1,051
Construction costs 1,119 1,193 1,252
Other operating expenses, net 383 155 426
Operating costs and expenses R$ 18,818 R$ 15,904 R$ 18,287