28. Operating revenue (Details) - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SummaryOfOperatingRevenueLineItems [Line Items]
Net operating revenue	R$ 16,085,094	R$ 14,608,233	R$ 14,098,208
Metropolitan region of Sao Paulo
SummaryOfOperatingRevenueLineItems [Line Items]
Net operating revenue	10,295,509	8,636,926	7,749,694
Regional Systems
SummaryOfOperatingRevenueLineItems [Line Items]
Net operating revenue	3,958,100	3,586,820	3,372,538
Sanitation services
SummaryOfOperatingRevenueLineItems [Line Items]
Net operating revenue	R$ 14,253,609	R$ 12,223,746	R$ 11,122,232

The income arising in the course of an entity's ordinary activities. Income is increases in economic benefits during the accounting period in the form of inflows or enhancements of assets or decreases of liabilities that result in an increase in equity, other than those relating to contributions from equity participants.

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+ Details