REGULATORY MATTERS - Schedule of GAS Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 3,838	$ 720
Other cost of removal obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(2,084)	(2,297)
Deferred income tax credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(6,301)	(6,455)
Over recovered regulatory clause revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(205)	(47)
Other regulatory liabilities
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(86)	(132)
Retiree benefit plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 4,423	3,658
Retiree benefit plans | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	15 years
Long-term debt fair value adjustment
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 107	121
Under recovered regulatory clause revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	254	407
Other regulatory assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 492	581
Other regulatory assets | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	8 years
Vacation pay
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 186	182
Regulatory amortization period	1 year
Southern Company Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ (1,874)	(1,872)
Southern Company Gas | Other cost of removal obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(1,606)	(1,585)
Southern Company Gas | Deferred income tax credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(874)	(940)
Southern Company Gas | Over recovered regulatory clause revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	(82)	(43)
Southern Company Gas | Other regulatory liabilities
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ (22)	(46)
Southern Company Gas | Other regulatory liabilities | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	20 years
Southern Company Gas | Excess Deferred Income Tax Liabilities Not Subject To Normalization
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 12
Southern Company Gas | Environmental remediation
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	296	311
Southern Company Gas | Retiree benefit plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 167	161
Southern Company Gas | Retiree benefit plans | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	15 years
Southern Company Gas | Long-term debt fair value adjustment
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 107	121
Southern Company Gas | Long-term debt fair value adjustment | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	19 years
Southern Company Gas | Under recovered regulatory clause revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 72	90
Regulatory amortization period	6 years
Southern Company Gas | Other regulatory assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 68	59
Southern Company Gas | Other regulatory assets | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	8 years
Southern Company Gas | Financial Instrument Hedging
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 11	8
Southern Company Gas | Financial Instrument Hedging | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	2 years
Southern Company Gas | Vacation pay
Schedule of Regulatory Assets and Liabilities [Line Items]
Net regulatory assets	$ 11	$ 11
Regulatory amortization period	1 year
Southern Company Gas | Deferred Income Tax Charges and Other Cost of Removal Obligations | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period	80 years