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INVESTMENTS IN NON-CONSOLIDATED COMPANIES (Narrative) (Details)
$ / shares in Units, $ in Thousands, shares in Millions
 12 Months Ended
Apr. 10, 2018
term
member
vice_president
Oct. 30, 2014
shares
Jan. 15, 2012
shares
Dec. 31, 2018
USD ($)
$ / shares
MW
shares
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
Dec. 31, 2018
R$ / shares
Jan. 01, 2018
USD ($)
Dec. 31, 2017
R$ / shares
Dec. 31, 2015
USD ($)
Disclosure of unconsolidated structured entities [line items]                    
Fair value of investments in associates for which there are quoted market prices       $ 736,500            
Investments in associates       480,100            
Revenue       11,454,807 $ 9,700,296 $ 7,223,975        
Equity       7,484,576 5,852,771 $ 5,166,593   $ 6,543,604   $ 4,802,997
Nippon Steel & Sumitomo Metal Corporation Group                    
Disclosure of unconsolidated structured entities [line items]                    
Number of shares acquired (shares) | shares     6.7              
Caixa de Previdência dos Funcionários                    
Disclosure of unconsolidated structured entities [line items]                    
Number of shares acquired (shares) | shares   51.4                
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                    
Disclosure of unconsolidated structured entities [line items]                    
Revenue       3,766,000 3,368,000          
Profit from continuing operations       193,000            
Equity       $ 3,681,815 $ 4,164,000          
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium Investments S.à r.l., Ternium Argentina S.A. and Prosid Investments S.A.                    
Disclosure of unconsolidated structured entities [line items]                    
Percentage of voting equity interests acquired       20.50%            
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Previdência Usiminas                    
Disclosure of unconsolidated structured entities [line items]                    
Percentage of voting equity interests acquired       68.60%            
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | T/T Group                    
Disclosure of unconsolidated structured entities [line items]                    
Percentage of voting equity interests acquired       47.10%            
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium S.A.                    
Disclosure of unconsolidated structured entities [line items]                    
Percentage of voting equity interests acquired       39.50%            
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Tenaris                    
Disclosure of unconsolidated structured entities [line items]                    
Percentage of voting equity interests acquired       7.50%            
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Nippon Steel & Sumitomo Metal Corporation Group                    
Disclosure of unconsolidated structured entities [line items]                    
Percentage of voting equity interests acquired       45.90%            
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Control Group                    
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]                    
Number of megawatts (MW) | MW       900            
Subordinated liabilities       $ 151,600            
Techgen S.A. de C.V. | Ternium S.A.                    
Disclosure of unconsolidated structured entities [line items]                    
Proportion of ownership interest in associate       48.00%            
Proportion of ownership interest in associate's power supply       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       30.00%            
Techgen S.A. de C.V. | Tenaris Investments S.à.r.l.                    
Disclosure of unconsolidated structured entities [line items]                    
Proportion of ownership interest in associate       22.00%            
Proportion of ownership interest in associate's power supply       22.00%            
Ordinary shares                    
Disclosure of unconsolidated structured entities [line items]                    
Share price (per share) | (per share)       $ 2.95 $ 3.27   R$ 11.44   R$ 10.83  
Ordinary shares | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium Investments S.à r.l., Ternium Argentina S.A. and Prosid Investments S.A.                    
Disclosure of unconsolidated structured entities [line items]                    
Number of shares outstanding (shares) | shares       242.6            
Ordinary shares | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Previdência Usiminas                    
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)       $ 2.38 $ 2.75   R$ 9.22   R$ 9.10  
Preferred shares | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A. | Ternium Investments S.à r.l., Ternium Argentina S.A. and Prosid Investments S.A.                    
Disclosure of unconsolidated structured entities [line items]                    
Number of shares outstanding (shares) | shares       8.5            
Previdência Usiminas | Tenrium Investments S.a r.l. & Nippon Steel & Sumitomo Metal Corporation                    
Disclosure of unconsolidated structured entities [line items]                    
Alternating time interval to nominate CEO 4 years                  
Number of consecutive terms (terms) | term 2                  
Alternating term to nominate CEO (terms) 2 years                  
Number of members on the board (members) | member 6                  
Number of vice presidents on the board (vice presidents) | vice_president 5                  
Number of members nominated (members) | member 3