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Accounting Policies (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Apr. 09, 2012
Statutory Accounting Practices        
Deferred Finance Costs, Net $ 101,500,000 $ 90,500,000    
Deferred Finance Costs, Current, Net 17,300,000 15,800,000    
Interest Costs Capitalized 15,600,000      
Number of mining complexes operated 15      
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners       0.50%
Property, Plant and Equipment, Net 7,337,098,000 7,949,150,000    
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months 216,200,000      
Unrecorded Unconditional Purchase Obligation, Due within Two Years 135,300,000      
Unrecorded Unconditional Purchase Obligation, Due within Three Years 135,100,000      
Unrecorded Unconditional Purchase Obligation, Due within Four Years 102,500,000      
Purchase of noncontrolling interest (17,500,000)        
Exploration and Production Equipment [Member] | Minimum [Member]
       
Statutory Accounting Practices        
Property, Plant and Equipment, Useful Life 5 years      
Exploration and Production Equipment [Member] | Maximum [Member]
       
Statutory Accounting Practices        
Property, Plant and Equipment, Useful Life 32 years      
Building and Building Improvements [Member] | Minimum [Member]
       
Statutory Accounting Practices        
Property, Plant and Equipment, Useful Life 10 years      
Building and Building Improvements [Member] | Maximum [Member]
       
Statutory Accounting Practices        
Property, Plant and Equipment, Useful Life 30 years      
Mining Properties and Mineral Rights [Member]
       
Statutory Accounting Practices        
Property, Plant and Equipment, Net 5,100,000,000 5,600,000,000    
Capital Addition Purchase Commitments [Member]
       
Statutory Accounting Practices        
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months 83,400,000      
Unrecorded Unconditional Purchase Obligation, Due within Two Years 67,300,000      
Unrecorded Unconditional Purchase Obligation, Due within Three Years 60,000,000      
Unrecorded Unconditional Purchase Obligation, Due within Four Years $ 60,000,000