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INVESTMENTS IN NON-CONSOLIDATED COMPANIES (Narrative) (Details)
$ / shares in Units, $ in Thousands, shares in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
$ / shares
MW
shares
Dec. 31, 2019
USD ($)
$ / shares
Dec. 31, 2018
USD ($)
Dec. 31, 2020
R$ / shares
Dec. 31, 2019
R$ / shares
Dec. 31, 2017
USD ($)
Disclosure of unconsolidated structured entities [line items]            
Fair value of investments in associates for which there are quoted market prices $ 756,300 $ 614,100        
Investments in associates 422,900 486,600        
Revenue 8,735,435 10,192,818 $ 11,454,807      
Net sales 8,735,435 10,192,818 11,454,807      
Non-current assets 8,289,460 8,757,320        
Current assets 4,566,775 4,178,213        
Non-current liabilities 2,559,485 3,452,535        
Current liabilities 1,853,597 1,768,125        
Equity 8,443,153 7,714,873 $ 7,484,576     $ 5,852,771
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.            
Disclosure of unconsolidated structured entities [line items]            
Revenue 3,133,000 3,790,000        
Net sales 3,133,000 3,790,000        
Equity $ 2,860,944 3,484,000        
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium Investments S.a r.l., Ternium Argentina and Prosid Investments S.A.I.C.            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 20.40%          
Usiminas Control Group            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 68.60%          
Usiminas Control Group | T/T Group            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 47.10%          
Usiminas Control Group | Ternium S.A.            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 39.50%          
Usiminas Control Group | TenarisConfab            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 7.60%          
Usiminas Control Group | NSC Group            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 45.90%          
Usiminas Control Group | Previdencia Usiminas            
Disclosure of unconsolidated structured entities [line items]            
Percentage of voting equity interests acquired 7.00%          
Techgen S.A. de C.V.            
Disclosure of unconsolidated structured entities [line items]            
Revenue $ 314,000 344,000        
Number of megawatts (MW) | MW 900          
Net sales $ 314,000 344,000        
Profit (loss) from continuing operations 44,000 24,000        
Non-current assets 833,000 875,000        
Current assets 59,000 48,000        
Non-current liabilities 709,000 791,000        
Current liabilities 95,000 87,000        
Equity 89,000 $ 45,000        
Subordinated liabilities $ 127,400          
Techgen S.A. de C.V. | Ternium S.A.            
Disclosure of unconsolidated structured entities [line items]            
Proportion of ownership interest in associate (percent) 48.00%          
Proportion of ownership interest in associate's power supply (percent) 78.00%          
Techgen S.A. de C.V. | Tecpetrol International S.A.            
Disclosure of unconsolidated structured entities [line items]            
Proportion of ownership interest in associate (percent) 30.00%          
Techgen S.A. de C.V. | Ternium Investments S.a.r.l.            
Disclosure of unconsolidated structured entities [line items]            
Proportion of ownership interest in associate (percent) 22.00%          
Proportion of ownership interest in associate's power supply (percent) 22.00%          
Ordinary shares            
Disclosure of unconsolidated structured entities [line items]            
Share price (per share) | (per share) $ 3.02 $ 2.45   R$ 15.69 R$ 9.87  
Ordinary shares | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium Investments S.a r.l., Ternium Argentina and Prosid Investments S.A.I.C.            
Disclosure of unconsolidated structured entities [line items]            
Number of shares outstanding (shares) | shares 242.6          
Ordinary shares | Usiminas Control Group            
Disclosure of unconsolidated structured entities [line items]            
Number of shares outstanding (shares) | shares 483.6          
Preferred shares            
Disclosure of unconsolidated structured entities [line items]            
Share price (per share) | (per share) $ 2.81 $ 2.36   R$ 14.61 R$ 9.51  
Preferred shares | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium Investments S.a r.l., Ternium Argentina and Prosid Investments S.A.I.C.            
Disclosure of unconsolidated structured entities [line items]            
Number of shares outstanding (shares) | shares 8.5