Stockholders' Equity - Changes in Stockholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)			(900)
Beginning balance			$ 25,528	$ 26,612
Consolidated net income attributable to Southern Company	$ 1,164	$ 1,069	1,948	347
Other comprehensive income (loss)			52	(2)
Stock issued			878	613
Stock-based compensation			74	97
Cash dividends on common stock			(1,805)	(1,716)
Preference stock redemption				(150)
Contributions from noncontrolling interests			154	77
Distributions to noncontrolling interests			(87)	(87)
Net income attributable to noncontrolling interests			71	45
Sale of noncontrolling interests			1,280
Reclassification from redeemable noncontrolling interests				114
Other			$ (28)	(11)
Ending balance, treasury shares (in shares)	(1,000)		(1,000)
Ending balance	$ 28,065	$ 25,939	$ 28,065	$ 25,939
Number of Common Shares Issued
Changes in Stockholders' Equity
Beginning balance (in shares)			1,008,532	991,213
Stock issued (in shares)			21,342	13,308
Other (in shares)			0
Ending balance (in shares)	1,029,874	1,004,521	1,029,874	1,004,521
Number of Common Shares Treasury Issued
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)			(929)	(819)
Other (in shares)			(57)	(75)
Ending balance, treasury shares (in shares)	(986)	(894)	(986)	(894)
Common Stockholders' Equity
Changes in Stockholders' Equity
Beginning balance			$ 24,167	$ 24,758
Consolidated net income attributable to Southern Company			1,948	347
Other comprehensive income (loss)			52	(2)
Stock issued			878	613
Stock-based compensation			74	97
Cash dividends on common stock			(1,805)	(1,716)
Sale of noncontrolling interests			(410)
Other			(27)	(15)
Ending balance	$ 24,877	$ 24,082	24,877	24,082
Preferred and Preference Stock of Subsidiaries
Changes in Stockholders' Equity
Beginning balance			0	609
Preference stock redemption				(150)
Other				3
Ending balance	0	462	0	462
Noncontrolling Interests
Changes in Stockholders' Equity
Beginning balance			1,361	1,245
Contributions from noncontrolling interests			154	77
Distributions to noncontrolling interests			(87)	(87)
Net income attributable to noncontrolling interests			71	45
Sale of noncontrolling interests			1,690
Reclassification from redeemable noncontrolling interests				114
Other			(1)	1
Ending balance	$ 3,188	$ 1,395	$ 3,188	$ 1,395