Dispositions (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended
Nov. 14, 2011 jointventures	Nov. 14, 2011 Wyoming jointventures	Nov. 14, 2011 Montana jointventures	Dec. 31, 2011 Ownership Interest in Joint Venture Surface Mines
Consolidated Statement of Operations, Including Discontinued Operations
Number of joint-venture surface mines	2	1	1
Investments in Joint Venture, Percentage		50.00%	50.00%
Discontinued Operation, Gain on Disposal of Discontinued Operation, Net of Tax				$ 72

Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 45

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 43

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

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