Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($) state pipeline segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	3
Total operating revenues	$ 5,412	$ 6,372
Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	1,474	1,607
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Total operating revenues	443	509
SOUTHERN POWER CO \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Total operating revenues	87	83
SOUTHERN POWER CO \| Traditional Electric Operating Companies \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Total operating revenues	$ 87	83
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	4
Total operating revenues	$ 1,474	1,639
Number of reportable segments \| segment	4
SOUTHERN Co GAS \| Pipeline
Segment Reporting Information [Line Items]
Number of pipeline construction projects \| pipeline	2
SOUTHERN Co GAS \| SNG
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%
SOUTHERN Co GAS \| Dalton Pipeline
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%
SOUTHERN Co GAS \| Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	$ 1,476	1,631
SOUTHERN Co GAS \| Traditional Electric Operating Companies \| Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	0	0
SOUTHERN Co GAS \| Southern Company \| Natural gas
Segment Reporting Information [Line Items]
Total operating revenues	$ 17	$ 36

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