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LEASE OBLIGATIONS (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Operating and Capital Leased Assets [Line Items]      
Operating lease expense $ 25.8 $ 26.3 $ 24.2
Capital lease assets 471.7 406.0  
Accumulated amortization of capital leases 184.5 110.6  
Total minimum capital lease payments 371.7    
Total minimum operating lease payments 95.5    
North American Coal [Member]
     
Operating and Capital Leased Assets [Line Items]      
Capital lease obligation $ 75.9