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Revenues - Summary of Other Operating Revenues (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of revenue [line Items]      
Other revenue R$ 1,585 R$ 1,484 R$ 1,421
Charged service [member]      
Disclosure of revenue [line Items]      
Other revenue 14 10 6
Telecoms services [Member]      
Disclosure of revenue [line Items]      
Other revenue 0 149 137
Services rendered [Member]      
Disclosure of revenue [line Items]      
Other revenue 188 156 167
Subsidies [Member]      
Disclosure of revenue [line Items]      
Other revenue 1,136 1,034 1,001
Rental and leasing [member]      
Disclosure of revenue [line Items]      
Other revenue 90 121 105
Reimbursement for decontracted supply [member]      
Disclosure of revenue [line Items]      
Other revenue 145    
Other [Member]      
Disclosure of revenue [line Items]      
Other revenue R$ 12 R$ 14 R$ 5