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PROPERTY: (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Property      
Total property $ 14,473.3 $ 13,456.3  
Accumulated depreciation, amortization and depletion (5,706.8) (5,193.5)  
Total property and mine development, net 8,766.5 8,262.8 $ 7,436.4
Depreciation and depletion expense 639.1 503.6 $ 440.1
Buildings and equipment      
Property      
Total property 12,177.1 11,529.4  
Construction in progress      
Property      
Total property 1,811.8 1,449.6  
Mine development      
Property      
Total property 259.1 265.9  
Mineral assets      
Property      
Total property 83.2 93.0  
Land, other than mineral      
Property      
Total property $ 142.1 $ 118.4