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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