Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) state segment	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates | state	4
Natural gas revenues (includes alternative revenue programs of $(24) and $9, respectively)	$ 1,607	$ 1,530
Number of reportable segments | segment	4
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates | state	7
Number of natural gas distribution utilities | segment	7
Natural gas revenues (includes alternative revenue programs of $(24) and $9, respectively)	$ 1,631	1,521
Number of reportable segments | segment	4
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Wholesale revenues, affiliates	$ 83	100
Southern Company [Member] | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Natural gas revenues (includes alternative revenue programs of $(24) and $9, respectively)	$ 36	$ 23