SEGMENT AND RELATED INFORMATION - Narrative (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) state segment	Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	3
Total operating revenues	$ 5,018	$ 5,412
Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	1,249	1,474
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Total operating revenues	375	443
SOUTHERN POWER CO \| Traditional Electric Operating Companies \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Total operating revenues	$ 86	87
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	4
Total operating revenues	$ 1,249	1,474
Number of reportable segments \| segment	4
SOUTHERN Co GAS \| Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	$ 1,240	1,476
SOUTHERN Co GAS \| SOUTHERN POWER CO \| Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	$ 10	$ 17
Southern Natural Gas Company, LLC \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%
PennEast Pipelines \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	20.00%
Dalton Pipeline \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%
Atlantic Coast Pipeline \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	5.00%

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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