Discontinued Operations (Net Outstanding Commodity Forward Contracts That Did Not Qualify For Hedge Accounting) (Detail) (Discontinued Operations [Member], Natural Gas (MMBtu)) [Member])	3 Months Ended	12 Months Ended
Mar. 31, 2013 MMBTU	Dec. 31, 2012 MMBTU
Discontinued Operations [Member] \| Natural Gas (MMBtu)) [Member]
Derivative [Line Items]
Commodity forward contracts	2,047,500	2,867,500
Net Position	Long	Long

Information that would enable users to understand the volume of the entity's derivative activity.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44

-Subparagraph 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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