Note 9 - Commitments, Guarantees and Contingencies (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012 ERC Liability [Member]	Sep. 30, 2012 Illinois Commission [Member]	Sep. 30, 2012 IAGO [Member]	Sep. 30, 2012 CUB [Member]
Unrecorded Unconditional Purchase Obligation, Description	On September 30, 2011, Nicor Gas signed an agreement to purchase approximately 25 Bcf of SNG annually for a 10-year term beginning as early as 2015. The agreement required, among other things, the developer to begin construction of the SNG plant by July 1, 2012. The developer did not meet this deadline and, as a result, the agreement automatically terminated. Additionally, on October 11, 2011, the Illinois Power Agency (IPA) approved the form of a draft 30-year contract for the purchase by Nicor Gas of approximately 20 Bcf per year of SNG from a second proposed plant beginning as early as 2018. The purchase price of the SNG that may be produced from this proposed coal gasification plant may significantly exceed market prices for natural gas and is expected to be dependent upon a variety of factors, including the developer's financing, plant construction costs and volumes sold, which are currently not determinable. The Illinois law pertaining to this plant provides that the price paid for SNG purchased from the plant is to be considered prudent and not subject to review or disallowance by the Illinois Commission. As such, Illinois law effectively requires our customers to provide subordinated financial support to the developer.
Loss Contingency, Estimate of Possible Loss		$ 103	$ 64
Loss Contingency, Damages Sought, Value			$ 85	$ 255	$ 305