Regulatory Matters - Schedule of GAS Unrecognized Ratemaking Amounts (Details) - Regulatory Asset Off Balance Sheet - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	$ 95	$ 104
Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	11	11
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	4	2
Southern Company Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	$ 110	$ 117