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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
Jun. 30, 2012
Dec. 31, 2011
Property, Plant and Equipment, Gross $ 12,477.8 $ 11,881.5
Allowance for depreciation and depletion (1,595.7) (1,356.9)
Total depreciable assets 10,882.1 10,524.6
Land Rights And Mineral Rights [Member]
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Property, Plant and Equipment, Gross 7,964.1 7,918.9
Office And Information Technology [Member]
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Property, Plant and Equipment, Gross 63.9 67.0
Buildings [Member]
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Property, Plant and Equipment, Gross 144.7 132.2
Mining Equipment [Member]
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Property, Plant and Equipment, Gross 1,123.2 1,323.8
Processing Equipment [Member]
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Property, Plant and Equipment, Gross 1,998.3 1,441.8
Railroad Equipment [Member]
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Property, Plant and Equipment, Gross 241.5 164.3
Electric Power Facilities [Member]
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Property, Plant and Equipment, Gross 58.2 57.9
Port Facilities [Member]
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Property, Plant and Equipment, Gross 125.0 64.1
Interest Capitalized During Construction [Member]
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Property, Plant and Equipment, Gross 27.6 22.5
Land Improvements [Member]
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Property, Plant and Equipment, Gross 31.8 30.4
Other [Member]
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Property, Plant and Equipment, Gross 32.5 43.2
Construction In Progress [Member]
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Property, Plant and Equipment, Gross $ 667.0 $ 615.4