REGULATORY MATTERS - Schedule of GAS Unrecognized Ratemaking Amounts (Details) - Regulatory Asset Off Balance Sheet - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	$ 70	$ 95
Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	10	11
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	2	4
Southern Company Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	$ 82	$ 110