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LOANS AND FINANCING - Additional Disclosures (Details) R$ in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 BRL (R$)	Apr. 30, 2018 BRL (R$)	Mar. 31, 2018 BRL (R$)	Dec. 31, 2017 BRL (R$)	Oct. 31, 2017 USD ($) tranche	Oct. 31, 2017 BRL (R$) tranche	Jun. 30, 2009 BRL (R$)
LOANS AND FINANCING
Notional value		R$ 13,178,457			R$ 16,207,120
Loans and financing		13,367,841			R$ 16,461,656
Banco do Brasil | Gerdau Aominas
LOANS AND FINANCING
Notional value				R$ 400,000
Citibank S.A. | Gerdau Aominas S.A. and Gerdau Aos Longos S.A
LOANS AND FINANCING
Notional value			R$ 400,000
FINAME/BNDES program
LOANS AND FINANCING
Loan amount guaranteed by asset being financed		R$ 90,500
Credit line with BNDES
LOANS AND FINANCING
Maximum borrowing capacity								R$ 1,500,000
Interest rate basis	TJLP
Adjustment to interest rate basis (as a percent)	2.16%	2.16%
Loans and financing		R$ 253,700
Senior Unsecured Global Working Capital Credit Agreement
LOANS AND FINANCING
Maximum borrowing capacity						$ 800	R$ 3,100,000
Number of tranches | tranche						2	2
Loans and financing	$ 50	R$ 193,700
Senior Unsecured Global Working Capital Credit Agreement | Gerdau's North American subsidiaries
LOANS AND FINANCING
Maximum borrowing capacity						$ 200	R$ 775,000
Senior Unsecured Global Working Capital Credit Agreement | Gerdau's Latin American subsidiaries
LOANS AND FINANCING
Maximum borrowing capacity						$ 600	R$ 2,325,000

X

- Definition

The amount of outstanding funds that the entity is obligated to repay, excluding notes and debentures.

+ References

No definition available.

+ Details

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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

No definition available.

+ Details

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X

- Definition

The number of tranches under the credit agreement.

+ References

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X

- Definition

The adjustment to the basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
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X

- Definition

The basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
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-IssueDate 2018-01-01
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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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- Definition

The amount of financial assets that the entity has pledged as collateral for liabilities or contingent liabilities, including amounts that have been reclassified in accordance with paragraph 3.2.23(a) of IFRS 9. [Refer: Contingent liabilities [member]; Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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