Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) state segment	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) state segment	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates | state	4		4
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities	7		7
Number of reportable segments			4
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Wholesale revenues, affiliates | $	$ 90	$ 107	$ 190	$ 204