Quarterly report pursuant to Section 13 or 15(d)

Property, Plant And Equipment (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Detail)

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Property, Plant And Equipment (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross $ 12,683.7 $ 11,695.6
Allowance for depreciation and depletion (1,653.0) (1,291.5)
Total depreciable assets 11,030.7 10,404.1
Land Rights And Mineral Rights [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 7,952.1 7,868.7
Office And Information Technology [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 83.3 66.8
Buildings [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 143.0 132.2
Mining Equipment [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 1,318.9 1,323.8
Processing Equipment [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 1,839.4 1,311.6
Railroad Equipment [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 224.3 161.6
Electric Power Facilities [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 58.8 57.9
Port Facilities [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 112.1 64.1
Interest Capitalized During Construction [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 32.2 22.5
Land Improvements [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 43.3 30.4
Other [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 31.9 43.2
Construction In Progress [Member]
    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross $ 844.4 $ 612.8