Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended
Dec. 31, 2011 A	Dec. 31, 2010 Richland County [Member] A	Dec. 31, 2010 Roosevelt County [Member] A	Jun. 15, 2009 Kerogen Acquisition [Member]	Sep. 30, 2009 Fidelity Acquisition [Member]
Business Acquisition [Line Items]
Number of acres acquired	0	10,000	16,700
Payments to acquire land		$ 30.1	$ 52.3
Acquisition cost				27.1	10.7
Estimated fair value of acquisition properties				$ 27.1	$ 10.7

The cash outflow from the acquisition of real estate intended to generate income for the owner; excludes land acquired for use by the owner.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 13

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3213-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration