Summary of Significant Accounting Policies Discussion (Details Textuals) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Marketable securities	$ 35,500,000	$ 0	$ 0
Net book value of coal reserves	6,400,000,000	7,600,000,000
Coal reserves not subject to depletion	2,200,000,000	2,700,000,000
Summary of Significant Accounting Policies Discussion (Textuals) [Abstract]
Period considered to treat short-term, highly liquid investments as cash equivalents	three months or less
Recognition of tax benefits from uncertain tax positions	greater than fifty percent likelihood of being realized upon ultimate settlement
Impairment losses recorded		0	0
Foreign currency remeasurement (gains) losses	(4,800,000)	900,000	38,500,000
Foreign currency translation adjustment	19,100,000	0	0
Coal Reserves Held By Fee Ownership [Member]
Property, Plant and Equipment [Line Items]
Net book value of coal reserves	1,200,000,000	1,300,000,000
Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, estimated useful lives, minimum (in years)	3
Property, Plant and Equipment, estimated useful lives, maximum (in years)	43
Mining Properties and Mineral Rights [Member]
Property, Plant and Equipment [Line Items]
Net book value of coal reserves	$ 5,200,000,000	$ 6,300,000,000
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, estimated useful lives, minimum (in years)	4
Property, Plant and Equipment, estimated useful lives, maximum (in years)	29
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, estimated useful lives	Shorter of Useful Life or Remaining Life of Lease
Interest In Middlemount Coal Pty Limited [Member]
Property, Plant and Equipment [Line Items]
Equity Method Investment, Ownership Percentage	50.00%