TAX, CIVIL AND LABOR CLAIMS AND CONTINGENT ASSETS - Contingent Assets (Details) R$ in Millions	12 Months Ended
Dec. 31, 2017 BRL (R$) kW
TAX, CIVIL AND LABOR CLAIMS AND CONTINGENT ASSETS
Industrial consumers monthly electricity consumption limit (in Kilo watt hour) \| kW	2,000
Maximum period to return the compulsory loan to taxpayers	20 years
Claims receivable from monetary correction of principal, interest, moratory and others due to compulsory loans \| R$	R$ 1,260

It represents the claims receivable from monetary correction of principal, interest, moratory and other accessory amounts owed due to compulsory loans.

No definition available.

Name:	ggb_ClaimsReceivableFromMonetaryCorrectionOfPrincipalInterestMoratoryAndOthers
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration