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Post-Retirement Obligation - Disclosure Of Consolidated Actuarial Information (Detail) - BRL (R$)
R$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position R$ 4,186 R$ 4,242 R$ 3,253 R$ 2,631
Pension plans and retirement supplement plans [member]        
Disclosure of defined benefit plans [line items]        
Fair value of plan assets (8,546) (8,128) (6,703) (8,051)
Health plan [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 1,809 1,711 1,323 1,120
Dental plan [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 38 38 R$ 30 R$ 33
Actuarial assumptions [Member]        
Disclosure of defined benefit plans [line items]        
Present value of funded obligations 12,663 12,306    
Fair value of plan assets (8,546) (8,128)    
Net liabilities in the statement of financial position 4,186 4,242    
Actuarial assumptions [Member] | Initial net liabilities [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 4,117 4,178    
Actuarial assumptions [Member] | Adjustment to asset ceiling [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 69 64    
Actuarial assumptions [Member] | Pension plans and retirement supplement plans [member]        
Disclosure of defined benefit plans [line items]        
Present value of funded obligations 10,545 9,743    
Fair value of plan assets (8,546) (8,128)    
Net liabilities in the statement of financial position 2,068 1,679    
Actuarial assumptions [Member] | Pension plans and retirement supplement plans [member] | Initial net liabilities [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 1,999 1,615    
Actuarial assumptions [Member] | Pension plans and retirement supplement plans [member] | Adjustment to asset ceiling [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 69 64    
Actuarial assumptions [Member] | Health plan [member]        
Disclosure of defined benefit plans [line items]        
Present value of funded obligations 1,809 1,711    
Net liabilities in the statement of financial position 1,809 1,711    
Actuarial assumptions [Member] | Health plan [member] | Initial net liabilities [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 1,809 1,711    
Actuarial assumptions [Member] | Dental plan [member]        
Disclosure of defined benefit plans [line items]        
Present value of funded obligations 39 38    
Net liabilities in the statement of financial position 39 38    
Actuarial assumptions [Member] | Dental plan [member] | Initial net liabilities [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position 39 38    
Actuarial assumptions [Member] | Life insurance [member]        
Disclosure of defined benefit plans [line items]        
Present value of funded obligations 270 814    
Net liabilities in the statement of financial position 270 814    
Actuarial assumptions [Member] | Life insurance [member] | Initial net liabilities [member]        
Disclosure of defined benefit plans [line items]        
Net liabilities in the statement of financial position R$ 270 R$ 814