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Intangible Assets - Summary of Composition of Intangible Assets (Detail) - BRL (R$) R$ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | R$ 11,156 | R$ 10,820 | R$ 10,275 | R$ 3,379
Temporary Easements Intangible Assets [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 10 | 10 | 10 | 12
Service concession rights [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 8 | 9 | 9 | 24
Assets of Concession [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 10,435 | 9,248 | 8,967 | 2,223
Other intangible assets [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 17 | 18 | 14 | 17
Useful life defined intangible asset [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 10,470 | 9,285 | R$ 9,000 | R$ 2,276
Intangible assets in progress [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 686 | 1,535 |  |
Gross Carrying Amount [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 18,636 | 17,931 |  |
Gross Carrying Amount [member] | Temporary Easements Intangible Assets [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 12 | 12 |  |
Gross Carrying Amount [member] | Service concession rights [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 19 | 19 |  |
Gross Carrying Amount [member] | Assets of Concession [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 17,837 | 16,288 |  |
Gross Carrying Amount [member] | Other intangible assets [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 82 | 77 |  |
Gross Carrying Amount [member] | Useful life defined intangible asset [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 17,950 | 16,396 |  |
Gross Carrying Amount [member] | Intangible assets in progress [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 686 | 1,535 |  |
Accumulated Depreciation and Amortisation [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 7,480 | 7,111 |  |
Accumulated Depreciation and Amortisation [member] | Temporary Easements Intangible Assets [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 2 | 2 |  |
Accumulated Depreciation and Amortisation [member] | Service concession rights [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 11 | 10 |  |
Accumulated Depreciation and Amortisation [member] | Assets of Concession [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 7,402 | 7,040 |  |
Accumulated Depreciation and Amortisation [member] | Other intangible assets [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | 65 | 59 |  |
Accumulated Depreciation and Amortisation [member] | Useful life defined intangible asset [member] |  |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |  |
Accumulated amortization | R$ 7,480 | R$ 7,111 |  |

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