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28 Operating revenue (Details) - BRL (R$)
R$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Summary of operating revenue [Line Items]      
Net operating revenue R$ 17,797,541 R$ 17,983,654 R$ 16,085,094
Metropolitan region of sao paulo [member]      
Summary of operating revenue [Line Items]      
Net operating revenue 10,722,384 11,849,776 10,295,509
Regional systems [member]      
Summary of operating revenue [Line Items]      
Net operating revenue 4,435,396 4,284,256 3,958,100
Sanitation services [member]      
Summary of operating revenue [Line Items]      
Net operating revenue R$ 15,157,780 R$ 16,134,032 R$ 14,253,609