Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19190-110258

Name:	us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair Value Measurements (Details 2-Nonrecurring) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013 Fair Value, Inputs, Level 3 [Member] Assets Held and Used Powder River Basin [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 3 [Member] Assets Held and Used Ozona [Member]	Mar. 31, 2012 Fair Value, Inputs, Level 3 [Member] Assets Held and Used Ozona [Member]	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 2012 Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Financial Statement [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure								$ 5	$ 2
Fair value of long-lived assets held for use, year-to-date								5	77
Assets, Fair Value Adjustment	$ 11	$ 8	$ 49	$ 271	$ 15	$ 21	$ 261