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Operating Costs And Expenses - Summary of Electricity Purchased for Resale (Detail) - BRL (R$) R$ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | R$ 10,919 | R$ 8,273 | R$ 9,542
Supply From Itaipu Binacional [member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 1,243 | 1,144 | 1,734
Physical guarantee quota contracts [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 461 | 537 | 252
Quotas for Angra I and II Nuclear Plants [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 244 | 217 | 200
Spot market [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 1,498 | 761 | 935
Proinfa Program [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 303 | 323 | 253
'bilateral contracts' [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 385 | 292 | 326
Electricity acquired in Regulated Market auctions [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 3,555 | 2,540 | 3,978
Electricity acquired in the Free Market [member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | 4,283 | 3,279 | 2,762
Credits of Pasep and Cofins taxes [Member] |  |  |
Disclosure of operating costs and expenses [Line Items] |  |  |
Energy purchased for resale | R$ (1,053) | R$ (820) | R$ (898)

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