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DERIVATIVE FINANCIAL INSTRUMENTS (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2013
Dec. 31, 2012
Disclosure of detailed information about financial instruments [line items]          
Reserve of cash flow hedges, net of tax $ 0.5 $ (0.6) $ (0.4)    
Interest rate swap contracts          
Disclosure of detailed information about financial instruments [line items]          
Other comprehensive income that will be reclassified to profit or loss, before tax $ 0.7        
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 $ 100.0
Derivative, average fixed interest rate 1.92%        
Reserve of cash flow hedges, net of tax $ 0.5        
Non-deliverable forward contract | Ternium Colombia S.A.S (formerly Ferrasa S.A.S)          
Disclosure of detailed information about financial instruments [line items]          
Notional amount 30.0        
Forward contract | Ternium Mexico S.A. de C.V.          
Disclosure of detailed information about financial instruments [line items]          
Notional amount 228.2        
Forward contract | Ternium Investments S.à.r.l.          
Disclosure of detailed information about financial instruments [line items]          
Notional amount $ 28.7