Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013 mi Bcf	Dec. 31, 2012 Bcf	Jun. 30, 2012 Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline	86
Cost of removal obligation	$ 54.7	$ 51.4	$ 48.6
Number of Natural Gas Storage Outstanding	1,800,000,000	1,900,000,000	1,800,000,000
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3
Other Subsidiaries
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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