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Document and Entity Information
 12 Months Ended
Dec. 31, 2018
shares
Document - Document and Entity Information  
Entity Registrant Name GERDAU S.A.
Entity Central Index Key 0001073404
Document Type 20-F
Document Period End Date Dec. 31, 2018
Amendment Flag false
Current Fiscal Year End Date --12-31
Entity Well-known Seasoned Issuer Yes
Entity Voluntary Filers No
Entity Current Reporting Status Yes
Entity Filer Category Large Accelerated Filer
Entity Emerging Growth Company false
Entity Shell Company false
Entity Common Stock, Shares Outstanding 573,627,483
Document Fiscal Year Focus 2018
Document Fiscal Period Focus FY