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STOCKHOLDERS' EQUITY: - Treasury Stock (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Activity in treasury stock    
Balance at the beginning of the period $ 3,001.1  
Balance at the end of the period $ 3,009.1 $ 2,991.6
Southern copper common shares    
Activity in treasury stock    
Treasury stock balance at the end of the period (in shares) 111,551,617 111,568,817
Southern copper common shares | Directors' Stock Award Plan    
Share Based Compensation Plan, share distributions    
Share Based Compensation Plan, share distributions during the period 16,000,000,000 10,800,000,000
Parent Company (Grupo Mexico) common shares    
Activity in treasury stock    
Purchase of shares $ 2,900.0  
Parent Company (Grupo Mexico) common shares | Employee Stock Purchase Plan    
Share Based Compensation Plan, share distributions    
Share Based Compensation Plan, share distributions during the period 1,300,000 300,000
TREASURY STOCK: | Southern copper common shares    
Activity in treasury stock    
Balance at the beginning of the period $ 2,768.7 $ 2,769.0
Used for corporate purposes (0.4) (0.3)
Balance at the end of the period 2,768.3 2,768.7
TREASURY STOCK: | Parent Company (Grupo Mexico) common shares    
Activity in treasury stock    
Balance at the beginning of the period 232.4 218.6
Other activity, including dividend, interest and foreign currency transaction effect 8.4 4.3
Balance at the end of the period $ 240.8 $ 222.9