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Supplemental pension plans (Details 5) - BRL (R$)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Sensitivity analysis of the benefit plan obligations    
Discount rate [1] 8.5% - 10% p.a. 11.1% p.a.
Sensitivity analysis: Increase of 1 b.p.    
Sensitivity analysis of the benefit plan obligations    
Discount rate 9.5% - 11.0% p.a.  
Effect on actuarial liabilities reduction  
Effect on the present value of the obligations R$ (256,532)  
Sensitivity analysis: Decrease of 1 b.p.    
Sensitivity analysis of the benefit plan obligations    
Discount rate 9.5% - 11.0% p.a.  
Effect on actuarial liabilities increase  
Effect on the present value of the obligations R$ 303,154  
[1] In 2017, considering an inflation rate of 4.3% p.a. and a real discount rate of 4.0% - 5.5% p.a. (2016 - 4.8% and 6.0% p.a., respectively).