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ACQUISITION OF BUSINESS - USINAS SIDERGICAS DE MINAS GERAIS S.A. - USIMINAS (Narrative) (Details)
R$ / shares in Units, $ / shares in Units, R$ in Millions, $ in Millions
 12 Months Ended
Jul. 19, 2016
USD ($)
shares
Jul. 19, 2016
BRL (R$)
shares
Apr. 18, 2016
BRL (R$)
shares
Oct. 02, 2014
BRL (R$)
R$ / shares
shares
Sep. 30, 2014
USD ($)
Jan. 16, 2012
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2017
R$ / shares
Dec. 31, 2016
R$ / shares
Apr. 20, 2016
USD ($)
shares
Apr. 20, 2016
BRL (R$)
shares
Dec. 31, 2015
shares
Disclosure of detailed information about business combination [line items]                          
Discount rate used in current estimate of value in use               11.49%          
Number of shares issued             2,004,743,442 2,004,743,442         2,004,743,442
Fair value of investments in associates for which there are quoted market prices | $             $ 625.5 $ 817.6          
Investments in associates | $             411.1 $ 466.3          
USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Decrease through write-off, financial assets | $         $ 739.8                
Discount rate used in current estimate of value in use         10.40%                
Impairment loss recognised in profit or loss, investment property | $             $ 191.9            
Preferred stock, shares subscribed                     8,500,000 8,500,000  
Preferred stock, value, subscriptions                     $ 3.1 R$ 10.9  
Number of additional shares authorised     200,000,000                    
Capital shares, value, subscriptions | R$     R$ 1,000.0                    
Proceeds from issuing shares $ 110.9 R$ 382.2                      
Ternium Investments S.à r.l. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares acquired           0              
Prosid Investments S.A. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares acquired           0              
Confab Industrial S.A. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares acquired           25,000,000              
Nippon Steel & Sumitomo Metal Corporation Group | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of voting equity interests acquired           46.10%              
T/T Group | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of voting equity interests acquired           43.30%              
Previdência Usiminas | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of voting equity interests acquired           10.60%              
Caixa de Previdência dos Funcionários | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares acquired       51,400,000                  
Siderar S.A.I.C., Ternium Investments S.à r.l. and Prosid Investments S.A. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of voting equity interests acquired               20.50%          
Ternium Investments S.a r.l., Ternium Argentina and Prosid Investments S.A.I.C. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of voting equity interests acquired               35.60%          
Level of participation of entity compared with other participating entities               20.50%          
Ordinary shares                          
Disclosure of detailed information about business combination [line items]                          
Share price | (per share)             $ 2.53 $ 3.27 R$ 10.83 R$ 8.26      
Ordinary shares | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares issued 76,400,000 76,400,000                      
Ordinary shares | T/T Group | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of shares owned               0.396          
Ordinary shares | Caixa de Previdência dos Funcionários | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Business acquisition, share price | R$ / shares       R$ 12                  
Consideration transferred, acquisition-date fair value | R$       R$ 616.7                  
Ordinary shares | Siderar S.A.I.C., Ternium Investments S.à r.l. and Prosid Investments S.A. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares outstanding               242,600,000          
Preferred shares                          
Disclosure of detailed information about business combination [line items]                          
Share price | (per share)             $ 1.26 $ 2.75 R$ 9.10 R$ 4.10      
Preferred shares | T/T Group | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Percentage of shares owned               0.018          
Preferred shares | Siderar S.A.I.C., Ternium Investments S.à r.l. and Prosid Investments S.A. | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.                          
Disclosure of detailed information about business combination [line items]                          
Number of shares outstanding               8,500,000