Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Fair Value Disclosures [Abstract]
Weather derivative premium	$ 6	$ 3	$ 3

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