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FINANCIAL INSTRUMENTS - Net investment hedge (Details) R$ in Thousands, $ in Billions	12 Months Ended
	Dec. 31, 2018 BRL (R$)	Dec. 31, 2017 BRL (R$)	Dec. 31, 2016 BRL (R$)	Dec. 31, 2018 USD ($)
Net investment hedge
Unrealized gain (loss) on net investment hedge, net of tax | R$	R$ (1,491,534)	R$ (148,560)	R$ 1,679,312
Ten/Thirty Years Bonds
Net investment hedge
Liabilities (not designated as hedges) at fair value through income statement				$ 0.3
Ten/Thirty Years Bonds | Hedges of net investment in foreign operations
Net investment hedge
Hedging instrument, liabilities				1.8
Unrealized gain (loss) on net investment hedge, net of tax | R$	R$ 1,491,534	R$ 148,560
Ten/Thirty Years Bonds | Gerdau Acominas S.A. and subsidiary | Hedges of net investment in foreign operations
Net investment hedge
Hedging instrument, liabilities				$ 0.1

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No definition available.

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The amount of financial liabilities that meet either of the following conditions: (a) they meet the definition of held for trading; or (b) upon initial recognition they are designated by the entity as at fair value through profit or loss. An entity may use this designation only when permitted by paragraph 4.3.5 of IFRS 9 (embedded derivatives) or when doing so results in more relevant information, because either: (a) it eliminates or significantly reduces a measurement or recognition inconsistency (sometimes referred to as ‘an accounting mismatch’) that would otherwise arise from measuring assets or liabilities or recognising the gains and losses on them on different bases; or (b) a group of financial liabilities or financial assets and financial liabilities is managed and its performance is evaluated on a fair value basis, in accordance with a documented risk management or investment strategy, and information about the group is provided internally on that basis to the entity’s key management personnel (as defined in IAS 24). [Refer: At fair value [member]; Key management personnel of entity or parent [member]; Derivatives [member]; Financial assets; Financial liabilities]

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The gains (losses) recognised in other comprehensive income on hedges of net investments in foreign operations, net of tax. [Refer: Other comprehensive income]

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The amount of a hedging instrument, recognised as a liability. [Refer: Hedging instruments [member]]

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