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Cash and cash equivalents (Details 1) (Detail) - BRL (R$)
R$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Cash and cash equivalents [abstract]            
Short-term bank deposits R$ 20,694 R$ 66,767   R$ 79,969    
Short-term investment 2,524,695          
Total R$ 2,545,389 R$ 66,767 R$ 14,744 R$ 79,969 R$ 6,888 R$ 1,199