The amount of assets representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognised. [Refer: Business combinations [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 3
-IssueDate 2020-01-01
-Paragraph B67
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2020-en-r&anchor=para_B67_d&doctype=Appendix&subtype=B
-URIDate 2020-01-01

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2020-01-01
-Paragraph 134
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=36&code=ifrs-tx-2020-en-r&anchor=para_134_a&doctype=Standard
-URIDate 2020-01-01

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2020-01-01
-Paragraph 135
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=36&code=ifrs-tx-2020-en-r&anchor=para_135_a&doctype=Standard
-URIDate 2020-01-01

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 54
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_54_c&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_Goodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The total of income less expenses from continuing and discontinued operations, excluding the components of other comprehensive income. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2020-01-01
-Paragraph 18
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2020-en-r&anchor=para_18_b&doctype=Standard
-URIDate 2020-01-01

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2020-01-01
-Paragraph 24
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=1&code=ifrs-tx-2020-en-r&anchor=para_24_b&doctype=Standard
-URIDate 2020-01-01

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2020-01-01
-Paragraph 32
-Subparagraph a
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=1&code=ifrs-tx-2020-en-r&anchor=para_32_a_ii&doctype=Standard
-URIDate 2020-01-01

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2020-01-01
-Paragraph 28
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2020-en-r&anchor=para_28_b&doctype=Standard
-URIDate 2020-01-01

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Note Effective on first application of IFRS 9
-Name IFRS
-Number 4
-IssueDate 2020-01-01
-Paragraph 39L
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=4&code=ifrs-tx-2020-en-b&anchor=para_39L_e&doctype=Standard
-URIDate 2020-01-01

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2020-01-01
-Paragraph 23
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2020-en-r&anchor=para_23&doctype=Standard
-URIDate 2020-01-01

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 12
-IssueDate 2020-01-01
-Paragraph B10
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2020-en-r&anchor=para_B10_b&doctype=Appendix&subtype=B
-URIDate 2020-01-01

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 106
-Subparagraph d
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_106_d_i&doctype=Standard
-URIDate 2020-01-01

Reference 9: http://www.xbrl.org/2003/role/exampleRef
-Note Effective 2021-01-01
-Name IFRS
-Number 17
-IssueDate 2020-01-01
-Paragraph 113
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=17&code=ifrs-tx-2020-en-r&anchor=para_113_b&doctype=Standard
-URIDate 2020-01-01

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 81A
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_81A_a&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_ProfitLoss
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of tangible assets that: (a) are held for use in the production or supply of goods or services, for rental to others, or for administrative purposes; and (b) are expected to be used during more than one period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 16
-IssueDate 2020-01-01
-Paragraph 73
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=16&code=ifrs-tx-2020-en-r&anchor=para_73_e&doctype=Standard
-URIDate 2020-01-01

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 54
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_54_a&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_PropertyPlantAndEquipment
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of cost related to the issuance of shares.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 106
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_106_d&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_ShareIssueRelatedCost
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of acquisition shares amount.

+ References

No definition available.

+ Details

Name:	sid_AcquisitionSharesAmount
Namespace Prefix:	sid_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	sid_BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems
Namespace Prefix:	sid_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represent information about the concession term.

+ References

No definition available.

+ Details

Name:	sid_ConcessionTerm
Namespace Prefix:	sid_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represent information about the description of holding in other entity.

+ References

No definition available.

+ Details

Name:	sid_DescriptionOfHoldingInOtherEntity
Namespace Prefix:	sid_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of fair value of investment properties.

+ References

No definition available.

+ Details

Name:	sid_FairValueOfInvestmentProperties
Namespace Prefix:	sid_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represent information about the number of shares transfer.

+ References

No definition available.

+ Details

Name:	sid_NumberOfSharesTransfer
Namespace Prefix:	sid_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Represent information about the number of shares transfer one.

+ References

No definition available.

+ Details

Name:	sid_NumberOfSharesTransferOne
Namespace Prefix:	sid_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of other expense.

+ References

No definition available.

+ Details

Name:	sid_OtherExpense
Namespace Prefix:	sid_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represent information about the percentage of interest.

+ References

No definition available.

+ Details

Name:	sid_PercentageOfInterest
Namespace Prefix:	sid_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represent information about the percentage of stakes.

+ References

No definition available.

+ Details

Name:	sid_PercentageOfStakes
Namespace Prefix:	sid_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represent information about the percentage of voting capital.

+ References

No definition available.

+ Details

Name:	sid_PercentageOfVotingCapital
Namespace Prefix:	sid_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	ifrs-full_ClassesOfShareCapitalAxis=sid_ClassBPreferredSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfShareCapitalAxis=sid_CommonSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfShareCapitalAxis=sid_PreferredSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=sid_COMPANHIABRASILEIRADEINFRASTRUCTURESERVICESMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=sid_TransLogisticaSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=sid_EquimacSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=sid_PANATLANTICASAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=sid_ArvediMetalferDoBrasilSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_FTLFerroviaTransnordestinaLogisticaSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_FtlFerroviaTransnordestionaLogisticAsaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_MRSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_CSNMineracaoSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_CKTRBrasilServicosLtdaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_ENGIEBrasilEnergiaSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_MRSLogisticaSAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	sid_LegalEntityOneAxis=sid_ConsorcioDaUsinaHidreletricaDeIgarapavaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

10. BASIS OF CONSOLIDATION AND INVESTMENTS IN SUBSIDIARIES AND AFFILIATES (Details Narrative) - BRL (R$) R$ in Thousands				1 Months Ended	12 Months Ended
	Nov. 29, 2019	Nov. 13, 2019	Nov. 30, 2015	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Net loss					R$ 4,292,618	R$ 2,244,511	R$ 5,200,583
Property, plant and equipment, net of depreciation					19,716,223	19,700,944	R$ 18,046,864
Fair value of investment properties					R$ 1,863,563	1,781,019
Class B preferred shares [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Total number of shares transfer				1,677,816
Number of shares transfer to CSN				501,789
Common shares [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of stake					15.19%
Preferred shares [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of stake					20.29%
COMPANHIA BRASILEIRA DE INFRASTRUCTURE SERVICES [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest	50.00%
Acquisition shares amount	R$ 24,000
Goodwill	R$ 15,225
Transnordestina logistica s.a [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest					47.26%
Percentage of voting capital					92.60%
Equimac S.A [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of stake					50.00%
PANATLANTICA SA [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest					11.31%
Arvedi Metalfer do Brasil S.A. [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of stake					20.00%
FTL - FERROVIA TRANSNORDESTINA LOGISTICA SA ("FTL") [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Subscribed shares amount		R$ 27,670
Net loss		R$ 293
FTL - FERROVIA TRANSNORDESTINA LOGISTICA SA ("FTL") [member] \| Bottom of range [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest		91.69%
FTL - FERROVIA TRANSNORDESTINA LOGISTICA SA ("FTL") [member] \| Top of range [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest		92.38%
MRS. [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of stake			18.63%
CSN Mineracao SA [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest			87.52%
CKTR Brasil Servicos Ltda [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of stake					50.00%
ENGIE Brasil Energia S.A [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest					48.75%
MRS Logistica S.A. [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Concession term					30 years
Description of holding in other entity					The Company directly holds an 18.64% interest in the capital of MRS and indirectly, through its subsidiary CSN Mineracao SA, a 16.30% interest in the capital of MRS, totaling a 34.94% interest.
CONSORCIO DA USINA HIDRELETRICA DE IGARAPAVA [member]
BasisOfConsolidationAndInvestmentsInSubsidiariesAndAffiliatesLineItems [Line Items]
Percentage of interest					17.92%
Property, plant and equipment, net of depreciation					R$ 21,287	22,441
Expense					R$ 6,611	R$ 6,497