Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 7

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Commitments and Contingencies (Details) (USD $) In Thousands, unless otherwise specified			0 Months Ended
	Mar. 31, 2013	Dec. 05, 2012	Aug. 23, 2012 Western Minerals LLC item	Mar. 31, 2013 Equipment	Dec. 31, 2012 Equipment	Mar. 31, 2013 Land	Dec. 31, 2012 Land	Mar. 31, 2013 Coal purchase commitments	Dec. 31, 2012 Coal purchase commitments	Mar. 31, 2013 Transportation agreements	Dec. 31, 2012 Transportation agreements	Mar. 31, 2013 Materials and supplies	Dec. 31, 2012 Materials and supplies
Contingencies
Outstanding amount of long-term purchase commitment				$ 21,294	$ 20,317	$ 23,700	$ 23,700	$ 31,685	$ 28,633	$ 241,285	$ 159,398	$ 21,698	$ 24,552
Number of counterclaims asserted			6
Ownership interest in Decker (as a percent)	50.00%	50.00%