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Post-Retirement Obligation- Summary of Independent Actuaries Estimate for the Expense Amount (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [line items]      
Cost of current service R$ 19 R$ 17 R$ 17
Interest on the actuarial obligation 1,246 1,263 1,153
Total expense in 2018 as per actuarial opinion 390 781 545
Pension plans and retirement supplement plans [member]      
Disclosure of defined benefit plans [line items]      
Total expense in 2018 as per actuarial opinion 374 274 535
Health plan [member]      
Disclosure of defined benefit plans [line items]      
Cost of current service 11 9 7
Interest on the actuarial obligation 178 173 135
Total expense in 2018 as per actuarial opinion 22 305 138
Dental plan [member]      
Disclosure of defined benefit plans [line items]      
Cost of current service     1
Interest on the actuarial obligation 3 4 3
Total expense in 2018 as per actuarial opinion (1) 6 (4)
Life insurance [member]      
Disclosure of defined benefit plans [line items]      
Total expense in 2018 as per actuarial opinion (5) R$ 196 R$ (124)
2017 business year [Member]      
Disclosure of defined benefit plans [line items]      
Cost of current service 15    
Interest on the actuarial obligation 1,161    
Expected return on the assets of the Plan (771)    
Total expense in 2018 as per actuarial opinion 405    
2017 business year [Member] | Pension plans and retirement supplement plans [member]      
Disclosure of defined benefit plans [line items]      
Cost of current service 3    
Interest on the actuarial obligation 959    
Expected return on the assets of the Plan (771)    
Total expense in 2018 as per actuarial opinion 191    
2017 business year [Member] | Health plan [member]      
Disclosure of defined benefit plans [line items]      
Cost of current service 10    
Interest on the actuarial obligation 173    
Total expense in 2018 as per actuarial opinion 183    
2017 business year [Member] | Dental plan [member]      
Disclosure of defined benefit plans [line items]      
Interest on the actuarial obligation 4    
Total expense in 2018 as per actuarial opinion 4    
2017 business year [Member] | Life insurance [member]      
Disclosure of defined benefit plans [line items]      
Cost of current service 2    
Interest on the actuarial obligation 25    
Total expense in 2018 as per actuarial opinion R$ 27