TRADE ACCOUNTS RECEIVABLE - Changes in impairment of financial assets (Details) - Trade accounts receivable - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Changes in the impairment
Impairment at beginning of period	R$ (158,157)	R$ (131,639)	R$ (199,306)
Effects of IFRS 9 adoption				R$ (48,063)
Adjusted impairment balance on financial assets				R$ (179,702)
Impairment on financial assets during the year	(81,243)	(111,900)	(98,476)
Recoveries in the year	60,199	101,986	80,134
Write-offs	78,635	39,943	73,591
Loss of control by joint venture creation			1,487
Assets held for sale		(3,348)	11,207
Exchange variation	2,487	(5,136)	(276)
Impairment at end of period	R$ (98,079)	R$ (158,157)	R$ (131,639)

The adjusted amount of an allowance account used to record impairments to financial assets due to credit losses.

No definition available.

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Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

The decrease in an allowance account for credit losses of financial assets resulting from the loss of control in a joint venture.

+ References

No definition available.

+ Details

Name:	ggb_DecreaseThroughLossOfControlInJointVentureAllowanceAccountForCreditLossesOfFinancialAssets
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Period Type:	duration

- Definition

The increase (decrease) in an allowance account for credit losses of financial assets resulting from effects of IFRS 9 adoption.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The increase (decrease) in allowance for trade receivables through transfer to assets held for sale and related adjustments.

+ References

No definition available.

+ Details

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Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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- Definition

The amount of additional allowance for credit losses of financial assets recognised in profit or loss. [Refer: Allowance account for credit losses of financial assets]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2021-01-01
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph 16
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2017-en-b&anchor=para_16&doctype=Standard&book=b
-URIDate 2019-03-27

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of an allowance account used to record impairments to financial assets due to credit losses. [Refer: Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2021-01-01
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph 16
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2017-en-b&anchor=para_16&doctype=Standard&book=b
-URIDate 2019-03-27

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The increase (decrease) in an allowance account for credit losses of financial assets resulting from the net exchange differences arising when the financial statements are translated from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Allowance account for credit losses of financial assets]

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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Balance Type:	na
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- Definition

The decrease in an allowance account for credit losses of financial assets resulting from the reversal of impairment. [Refer: Allowance account for credit losses of financial assets]

+ References

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

The decrease in an allowance account for credit losses of financial assets resulting from the utilisation of the allowance. [Refer: Allowance account for credit losses of financial assets]

+ References

+ Details

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

- Details

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