Note 4 - Fair Value Measurements (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Weather Derivative Premium	$ 1	$ 3	$ 1
Long-term Debt and Capital Lease Obligations, Current	226	17	17
Nicor Gas [Member]
First Mortgage Bonds Noncurrent	$ 592	$ 599

Obligation related to long-term debt (excluding convertible debt) and capital leases, the portion which is due in one year or less in the future.

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