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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