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