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Post-Retirement Obligation- Summary of Sensitivity Analysis of the Effects of Changes in the Principal Actuarial Assumptions Used to Determine the Defined-benefit Obligation (Detail)
R$ in Millions
Dec. 31, 2017
BRL (R$)
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Reduction of one year in the mortality table R$ 294
Increase of one year in the mortality table 15
Reduction of 1% in the discount rate 1,395
Pension plans and retirement supplement plans [member]  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Reduction of one year in the mortality table 273
Reduction of 1% in the discount rate 1,103
Health plan [member]  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Reduction of one year in the mortality table 36
Reduction of 1% in the discount rate 244
Dental plan [member]  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Reduction of one year in the mortality table 1
Reduction of 1% in the discount rate 5
Life insurance [member]  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Reduction of one year in the mortality table (16)
Increase of one year in the mortality table 15
Reduction of 1% in the discount rate R$ 43