Commitments And Contingent Liabilities (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Sep. 30, 2012 East Texas System [Member]	Jan. 31, 2012 East Texas System [Member]	Jan. 03, 2012 East Texas System [Member]	Dec. 31, 2011 Dart Energy Corporation [Member]	Dec. 31, 2011 Prospect Street Energy, LLC [Member]	Dec. 31, 2011 EE Group, LLC [Member]	Dec. 31, 2011 Marysville Acquisition [Member]
Percentage of Business Acquired		49.90%	49.90%	49.90%	90.00%	5.00%	100.00%	5.00%
Proposed penalty assessment	$ 0.6
Percentage of investment in affiliate				50.10%

Percentage of voting equity interests acquired in the business combination.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7659399&loc=d3e1392-128463

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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