Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) state segment	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) state segment	Sep. 30, 2016 USD ($)
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 states in which entity operates | state	7		7
Number of natural gas distribution utilities | segment	7		7
Number of reportable segments | segment			4
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Wholesale revenues, affiliates | $	$ 105	$ 110	$ 295	$ 313