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]
Property, Plant and Equipment, Gross	7,964.1	7,918.9
Office And Information Technology [Member]
Property, Plant and Equipment, Gross	63.9	67.0
Buildings [Member]
Property, Plant and Equipment, Gross	144.7	132.2
Mining Equipment [Member]
Property, Plant and Equipment, Gross	1,123.2	1,323.8
Processing Equipment [Member]
Property, Plant and Equipment, Gross	1,998.3	1,441.8
Railroad Equipment [Member]
Property, Plant and Equipment, Gross	241.5	164.3
Electric Power Facilities [Member]
Property, Plant and Equipment, Gross	58.2	57.9
Port Facilities [Member]
Property, Plant and Equipment, Gross	125.0	64.1
Interest Capitalized During Construction [Member]
Property, Plant and Equipment, Gross	27.6	22.5
Land Improvements [Member]
Property, Plant and Equipment, Gross	31.8	30.4
Other [Member]
Property, Plant and Equipment, Gross	32.5	43.2
Construction In Progress [Member]
Property, Plant and Equipment, Gross	$ 667.0	$ 615.4