Coal and Other Mineral Rights (Details) (USD $)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Coal and other mineral rights
Coal and other mineral rights	$ 1,645,451,000	$ 1,629,286,000
Less accumulated depletion and amortization	(387,950,000)	(347,650,000)
Net book value	1,257,501,000	1,281,636,000
Total depletion and amortization expense on coal and other mineral interests	47,230,000	38,501,000	48,591,000
Coal and Other Mineral Rights (Textuals) [Abstract]
Terminated lease charges from mine closure			$ 8,200,000

Carrying amount at the balance sheet date of mineral properties, gross of adjustments.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 15

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-Name Accounting Principles Board Opinion (APB)

-Number 12

-Paragraph 5

-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_MineralPropertiesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant