Note 5 - Derivative Instruments (Detail) - Gains and Losses on Derivative Instruments (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Mar. 31, 2011
Nicor Gas [Member]
Accrued Natural Gas Costs	$ 1
Elizabethtown Gas [Member]
Accrued Natural Gas Costs	$ 9	$ 8

Current liabilities arising from deferral of gas to be purchased.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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