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20. Employees benefits (Details 4) - BRL (R$) R$ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Employees Benefits Details 4 |  |  |
Current service expenses | R$ (35,093) | R$ 13,562 | R$ 23,303
Interest cost rate | 260,409 | 285,227 | 287,334
Expected return on the plan assets | (183,689) | (201,779) | (205,981)
Total expenses | R$ 41,627 | R$ 97,011 | R$ 104,656

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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)] + References No definition available. + Details
Name: | SBS_CurrentServiceCostEmployeeBenefits
Namespace Prefix: | SBS_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)] + References No definition available. + Details
Name: | SBS_InterestCostRateEmployeeBenefits
Namespace Prefix: | SBS_
Data Type: | xbrli:monetaryItemType
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 141 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_f&doctype=Standard -URIDate 2017-03-09 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_i&doctype=Standard -URIDate 2017-03-09 + Details
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