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DERIVATIVE FINANCIAL INSTRUMENTS (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2013
Disclosure of detailed information about financial instruments [line items]        
Reserve of cash flow hedges, net of tax $ (0.2) $ (0.1) $ 0.5  
Other comprehensive loss that will be reclassified to profit or loss, before tax 0.2      
Interest rate swap contracts | Tenigal S. de R.L. de C.V.        
Disclosure of detailed information about financial instruments [line items]        
Notional amount       $ 100.0
Derivative, average fixed interest rate       1.92%
Reserve of cash flow hedges, net of tax (0.2)      
Non-deliverable forward contract | Ternium Colombia S.A.S        
Disclosure of detailed information about financial instruments [line items]        
Notional amount 75.4      
Non-deliverable forward contract | Ternium Investments S.a.r.l.        
Disclosure of detailed information about financial instruments [line items]        
Notional amount 3.4      
Forward contract | Ternium Mexico S.A. de C.V.        
Disclosure of detailed information about financial instruments [line items]        
Notional amount 59.2      
Forward contract | Ternium Investments S.a.r.l.        
Disclosure of detailed information about financial instruments [line items]        
Notional amount $ 4.9