Regulated Operations - Estimated Recognition of Rate Making Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 738	$ 714
Regulatory Asset Off Balance Sheet [Member]
Regulatory Assets [Line Items]
Regulatory assets	122	127
Regulatory Asset Off Balance Sheet [Member] | Atlanta Gas Light [Member]
Regulatory Assets [Line Items]
Regulatory assets	103	113
Regulatory Asset Off Balance Sheet [Member] | Virginia Natural Gas [Member]
Regulatory Assets [Line Items]
Regulatory assets	12	12
Regulatory Asset Off Balance Sheet [Member] | Elizabethtown Gas [Member]
Regulatory Assets [Line Items]
Regulatory assets	4	2
Regulatory Asset Off Balance Sheet [Member] | Nicor Gas [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 3	$ 0