CONSOLIDATED FINANCIAL STATEMENTS - Joint ventures (Details) - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Joint ventures
Net income (loss)	R$ 1,216,887	R$ 2,326,382	R$ (338,667)
Total comprehensive income (loss)	1,589,369	2,713,308	(298,366)
Joint ventures
Joint ventures
Net income (loss)	(21,548)	(16,403)	(91,553)
Total comprehensive income (loss)	R$ (21,548)	R$ (16,403)	R$ (91,553)
Bradley Steel Processors
Joint ventures
Equity interest in joint venture	50.00%	50.00%	50.00%
MRM Guide Rail
Joint ventures
Equity interest in joint venture	50.00%	50.00%	50.00%
Gerdau Corsa S.A.P.I de C.V.
Joint ventures
Equity interest in joint venture	70.00%	50.00%	50.00%
Capital increase	R$ 463,990
Gerdau Metaldom Corp.
Joint ventures
Equity interest in joint venture	50.00%	50.00%	45.00%
Gerdau Summit Aos Fundidos e Forjados S.A.
Joint ventures
Equity interest in joint venture	58.73%	58.73%	58.73%
Diaco S.A.
Joint ventures
Equity interest in joint venture	49.87%	49.87%	49.87%
Juntos Somos Mais Fidelizao S.A.
Joint ventures
Equity interest in joint venture	27.50%	27.50%

The amount of change in equity resulting from transactions and other events, other than those changes resulting from transactions with owners in their capacity as owners.

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