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Investments - Allocation of Acquisition Price of Jointly-Controlled Subsidiaries, A Valuation was Made of Intangible Assets Relating to Right to Operate Regulated Activity (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about investment property [line items]      
Beginning balance R$ 801 R$ 861 R$ 906
Addition   99  
Amortization (54) (50) (45)
Written off (129) (109)  
Ending balance 618 801 861
Taesa [member]      
Disclosure of detailed information about investment property [line items]      
Beginning balance 288 415 433
Amortization (13) (18) (18)
Written off (86) (109)  
Ending balance 189 288 415
Light [member]      
Disclosure of detailed information about investment property [line items]      
Beginning balance 209 231 253
Amortization (23) (22) (22)
Ending balance 186 209 231
Gasmig [member]      
Disclosure of detailed information about investment property [line items]      
Beginning balance 207 215 220
Amortization (7) (8) (5)
Ending balance 200 207 R$ 215
Lepsa [member]      
Disclosure of detailed information about investment property [line items]      
Beginning balance 49    
Addition   50  
Amortization (6) (1)  
Written off (43)    
Ending balance   49  
RME [member]      
Disclosure of detailed information about investment property [line items]      
Beginning balance 48    
Addition   49  
Amortization (5) (1)  
Ending balance R$ 43 R$ 48