10. Related-Party Balances and Transactions (Details 1) - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SummaryOfRelatedPartyBalancesAndTransactionsLineItems [Line Items]
Revenues	R$ 14,608,233	R$ 14,098,208	R$ 11,711,569
Sanitation services
SummaryOfRelatedPartyBalancesAndTransactionsLineItems [Line Items]
Revenues	462,989	445,726	357,512
Payments received from related parties
SummaryOfRelatedPartyBalancesAndTransactionsLineItems [Line Items]
Revenues	(471,081)	(424,549)	(338,471)
Receipt of GESP reimbursement referring to Law 4819/58
SummaryOfRelatedPartyBalancesAndTransactionsLineItems [Line Items]
Receipt of GESP reimbursement	R$ (192,889)	R$ (139,472)	R$ (121,709)

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