Note 3 - Fair Value Measurements (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Weather Derivative Premium	$ 2	$ 4	$ 1

Financial instruments that can be used for risk management purposes to hedge against losses due to adverse, unpredictable weather recorded on the balance sheet based on intrinsic value, not fair value.

No definition available.

Name:	gas_WeatherDerivativePremium
Namespace Prefix:	gas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant