Stockholders' Equity - Changes in Stockholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	938	658
Other comprehensive income	30	(9)
Stock issued	113	186
Stock-based compensation	36	57
Cash dividends on common stock	(586)	(556)
Contributions from noncontrolling interests	9	71
Distributions to noncontrolling interests	(13)	(18)
Net income (loss) attributable to noncontrolling interests	(6)	(4)
Other	$ (24)	(1)
Ending balance, treasury shares (in shares)	(1,000)
Ending balance	$ 26,025	$ 26,996
Number of Common Shares Issued
Changes in Stockholders' Equity
Beginning balance (in shares)	1,008,532	991,213
Stock issued (in shares)	4,055	4,240
Other (in shares)	0
Ending balance (in shares)	1,012,587	995,453
Number of Common Shares Treasury Issued
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)	(929)	(819)
Stock issued (in shares)	0	0
Other (in shares)	(33)	(35)
Ending balance, treasury shares (in shares)	(962)	(854)
Common Stockholders' Equity
Changes in Stockholders' Equity
Beginning balance	$ 24,167	$ 24,758
Consolidated net income attributable to Southern Company	938	658
Other comprehensive income	30	(9)
Stock issued	113	186
Stock-based compensation	36	57
Cash dividends on common stock	(586)	(556)
Other	(22)	0
Ending balance	24,676	25,094
Preferred and Preference Stock of Subsidiaries
Changes in Stockholders' Equity
Beginning balance	0	609
Other	0
Ending balance	0	609
Noncontrolling Interest
Changes in Stockholders' Equity
Beginning balance	1,361	1,245
Contributions from noncontrolling interests	9	71
Distributions to noncontrolling interests	(13)	(18)
Net income (loss) attributable to noncontrolling interests	(6)	(4)
Other	(2)
Ending balance	$ 1,349	$ 1,293