The net amount of deferred income taxes and income tax credits less the tax benefit from exercise of stock options.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 20

-Section 55

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=6576910&loc=d3e11374-113907

Name:	us-gaap_DeferredIncomeTaxesAndTaxCredits
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Summary of Significant Accounting Policies (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended				0 Months Ended
	Mar. 31, 2011	Sep. 30, 2011 jointventures	Sep. 30, 2010	Sep. 30, 2011 Montana jointventures	Sep. 30, 2011 Wyoming jointventures	Oct. 20, 2011 Reverse stock split	Oct. 19, 2011 Reverse stock split
Description of Business
Number of joint-venture surface mines		2		1	1
Percentage of ownership of common stock in a joint-venture (as a percent)				50.00%	50.00%
Reverse stock split ratio (in shares)						15	15
Deferred taxes on certain indefinite-lived intangible assets	$ 26	$ 34	$ 0