Regulated Operations (Details) - Estimated Recognition of Rate Making Assets - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Regulatory Assets [Line Items]
Regulatory Asset	$ 701	$ 714	$ 742
Regulatory Asset Off Balance Sheet [Member]
Regulatory Assets [Line Items]
Regulatory Asset	116	127	118
Atlanta Gas Light [Member] | Regulatory Asset Off Balance Sheet [Member]
Regulatory Assets [Line Items]
Regulatory Asset	99	113	104
Virginia Natural Gas [Member] | Regulatory Asset Off Balance Sheet [Member]
Regulatory Assets [Line Items]
Regulatory Asset	12	12	12
Elizabethtown Gas [Member] | Regulatory Asset Off Balance Sheet [Member]
Regulatory Assets [Line Items]
Regulatory Asset	3	2	2
Nicor Gas [Member] | Regulatory Asset Off Balance Sheet [Member]
Regulatory Assets [Line Items]
Regulatory Asset	$ 2	$ 0	$ 0