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Intangible Assets - Summary of Changes in Intangible Assets (Detail) - BRL (R$) R$ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | R$ 10,820 | R$ 10,275 | R$ 3,379
Additions | 1,105 | 1,163 | 1,116
Transfer of assets to Alianca Geracao de Energia |  |  | (14)
Special obligations write-down |  | 98 |
Disposals | (19) | (40) | (38)
Amortization | (646) | (599) | (519)
Financial to Intangible on renewal of concessions |  |  | 7,162
Transfer | (104) | (77) | (811)
Ending balance | 11,156 | 10,820 | 10,275
Intangible assets in progress [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | 1,535 | 1,275 | 1,103
Additions | 1,105 | 1,157 | 1,108
Disposals | (8) | (8) | (17)
Transfer | (1,946) | (889) | (919)
Ending balance | 686 | 1,535 | 1,275
Temporary Easements Intangible Assets [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | 10 | 10 | 12
Transfer of assets to Alianca Geracao de Energia |  |  | (2)
Ending balance | 10 | 10 | 10
Service concession rights [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | 9 | 9 | 24
Transfer of assets to Alianca Geracao de Energia |  |  | (13)
Amortization | (1) |  | (2)
Ending balance | 8 | 9 | 9
Assets of Concession [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | 9,248 | 8,967 | 2,223
Additions |  | 6 | 8
Special obligations write-down |  | 98 |
Disposals | (11) | (32) | (21)
Amortization | (638) | (595) | (512)
Financial to Intangible on renewal of concessions |  |  | 7,162
Transfer | 1,836 | 804 | 107
Ending balance | 10,435 | 9,248 | 8,967
Other intangible assets [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | 18 | 14 | 17
Transfer of assets to Alianca Geracao de Energia |  |  | 1
Amortization | (7) | (4) | (5)
Transfer | 6 | 8 | 1
Ending balance | 17 | 18 | 14
Useful life defined intangible asset [member] |  |  |
Disclosure of detailed information about intangible assets [line items] |  |  |
Beginning balance | 9,285 | 9,000 | 2,276
Additions |  | 6 | 8
Transfer of assets to Alianca Geracao de Energia |  |  | (14)
Special obligations write-down |  | 98 |
Disposals | (11) | (32) | (21)
Amortization | (646) | (599) | (519)
Financial to Intangible on renewal of concessions |  |  | 7,162
Transfer | 1,842 | 812 | 108
Ending balance | R$ 10,470 | R$ 9,285 | R$ 9,000

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- Definition The increase in intangible assets other than goodwill resulting from acquisitions through business combinations. [Refer: Business combinations [member]; Intangible assets other than goodwill] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 38 -IssueDate 2017-01-01 -Paragraph 118 -Subparagraph e -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=38&date=2017-03-01&anchor=para_118_e_i&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The amount of additions to intangible assets other than goodwill, other than those acquired through business combinations. [Refer: Business combinations [member]; Intangible assets other than goodwill] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 38 -IssueDate 2017-01-01 -Paragraph 118 -Subparagraph e -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=38&date=2017-03-01&anchor=para_118_e_i&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The decrease in intangible assets other than goodwill resulting from disposals. [Refer: Intangible assets other than goodwill] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 38 -IssueDate 2017-01-01 -Paragraph 118 -Subparagraph e -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=38&date=2017-03-01&anchor=para_118_e_ii&doctype=Standard -URIDate 2017-03-09 + Details
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