26 EMPLOYEE BENEFITS (Details 18) - Post-employment Healthcare Benefits [Member] R$ in Thousands	Dec. 31, 2019 BRL (R$)
EmployeeBenefitsLineItems [Line Items]
Year 1	R$ 83,290
Year 2	80,574
Year 3	77,649
Year 4	74,529
Year 5	71,218
Next 5 years	301,853
Total forecast payments	R$ 689,113

Amount of benefits for defined benefit plan Forecast to be paid.

No definition available.

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Amount of benefits for defined benefit plan Forecast to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

No definition available.

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Amount of benefits for defined benefit plan Forecast to be paid in next fiscal year following latest fiscal year.

No definition available.

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Amount of benefits for defined benefit plan Forecast to be paid in fifth fiscal year following latest fiscal year.

No definition available.

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Amount of benefits for defined benefit plan Forecast to be paid in fourth fiscal year following latest fiscal year.

No definition available.

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Amount of benefits for defined benefit plan Forecast to be paid in third fiscal year following latest fiscal year.

No definition available.

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Amount of benefits for defined benefit plan Forecast to be paid in second fiscal year following latest fiscal year.

No definition available.

Name:	sid_DefinedBenefitPlanForecastFutureBenefitPaymentsYearTwo
Namespace Prefix:	sid_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

No definition available.

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