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10. Related-Party Balances and Transactions (Details 2) - BRL (R$) R$ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
SummaryOfRelatedPartyBalancesAndTransactionsLineItems [Line Items] |  |
Principal disbursed amount | R$ 19,000 |
Interest balance | 13,857 |
Loan agreement total | 32,857 | R$ 50,617
Aquapolo Ambiental |  |
SummaryOfRelatedPartyBalancesAndTransactionsLineItems [Line Items] |  |
Principal disbursed amount | 19,000 |
Interest balance | 13,857 |
Loan agreement total | R$ 32,857 |
Interest rate | CDI + 1.2% a.a |
Maturity | The loan agreement originally expired on April 30, 2015, was extended to October 30, 2015 and on November 25, 2015 a new amendment changed the payment schedule for three annual installments, the first of which maturing on December 30, 2021 and the last on December 30, 2023. |

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- References No definition available. + Details
Name: | SBS_InterestBalanceRelatedPartyAgreement
Namespace Prefix: | SBS_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | SBS_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | SBS_RelatedPartyLoanAgreement
Namespace Prefix: | SBS_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | SBS_RelatedPartyLoanAgreementInterestRate
Namespace Prefix: | SBS_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | SBS_RelatedPartyLoanAgreementMaturity
Namespace Prefix: | SBS_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | SBS_SummaryOfRelatedPartyBalancesAndTransactionsLineItems
Namespace Prefix: | SBS_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_CategoriesOfRelatedPartiesAxis=SBS_AquapoloAmbiental1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |