SUMMARY OF SIGNIFICANT ACCOUNTING PRACTICES (Details)	12 Months Ended
Dec. 31, 2017 shares	Dec. 31, 2016
Dividends
Mandatory Minimum Annual Dividend, Percentage	30.00%
Dilutive effect of instruments on EPS
Antidilutive shares excluded from calculation of diluted EPS	0
Venezuelan bolivar fuerte
Hyperinflation
Foreign Currency Hyperinflation Exchange Rate, Bolivar Forte to Real	1,011.19	206.61

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

No definition available.

Name:	ggb_IfrsAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	ggb_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration