Summary of Significant Accounting Policies - General (Details) $ in Millions	3 Months Ended									6 Months Ended	12 Months Ended			48 Months Ended
	Dec. 31, 2017 USD ($) state segment utility Employee	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) state segment utility Employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) state segment utility Employee
Accounting Policies [Line Items]
Other intangible assets, net of amortization	$ 873				$ 970					$ 970	$ 873	$ 970		$ 873
Number of traditional operating companies | segment	4										4			4
Retail revenues											$ 15,330	15,234	$ 14,987
Net income after dividends on preferred and preference stock	$ 496	$ 1,069	$ (1,381)	$ 658	197	$ 1,139	$ 623	$ 489			842	2,448	2,367
SOUTHERN Co GAS
Accounting Policies [Line Items]
Other intangible assets, net of amortization	280				366					366	280	366		$ 280
Net income after dividends on preferred and preference stock	$ (60)	15	49	239	110	4				114	$ 243
Number of states in which entity operates | state	7										7			7
Number of natural gas distribution utilities | utility	7										7			7
ALABAMA POWER CO
Accounting Policies [Line Items]
Retail revenues											$ 5,458	5,322	5,234
Net income after dividends on preferred and preference stock	$ 119	325	230	174	102	351	213	156			$ 848	822	785
Traditional Operating Companies
Accounting Policies [Line Items]
Number of states in which entity operates | state	4										4			4
GEORGIA POWER CO
Accounting Policies [Line Items]
Environmental exit costs, assets previously disposed, liability for remediation	$ 22				17					17	$ 22	17		$ 22
Retail revenues											7,738	7,772	7,727
Net income after dividends on preferred and preference stock	227	580	347	260	113	599	349	269			$ 1,414	1,330	1,260
SOUTHERN POWER CO
Accounting Policies [Line Items]
Net income after dividends on preferred and preference stock	$ 795	124	82	70	23	176	89	50
Southern Company Services, Inc.
Accounting Policies [Line Items]
Number of employee transfers | Employee	538										538			538
MISSISSIPPI POWER CO
Accounting Policies [Line Items]
Retail revenues											$ 854	859	776
Net income after dividends on preferred and preference stock	$ (556)	$ 40	$ (2,054)	$ (20)	$ (89)	$ 26	$ 2	$ 11			(2,590)	(50)	(8)
Restatement Adjustment | GEORGIA POWER CO
Accounting Policies [Line Items]
Retail revenues									$ (75)
Net income after dividends on preferred and preference stock									$ (47)
Southern Company Services, Inc. | SOUTHERN Co GAS
Accounting Policies [Line Items]
Related party transaction, amount										$ 17	63
Southern Company Services, Inc. | ALABAMA POWER CO
Accounting Policies [Line Items]
Related party transaction, amount											479	460	438
Southern Company Services, Inc. | GEORGIA POWER CO
Accounting Policies [Line Items]
Related party transaction, amount											625	606	585
Southern Company Services, Inc. | SOUTHERN POWER CO
Accounting Policies [Line Items]
Related party transaction, amount											$ 218	193	146
Number of employee transfers | Employee	538										538			538
Southern Company Services, Inc. | MISSISSIPPI POWER CO
Accounting Policies [Line Items]
Related party transaction, amount											$ 140	231	295
Fuel Purchases | ALABAMA POWER CO | MISSISSIPPI POWER CO
Accounting Policies [Line Items]
Related party transaction, amount													8
Power Pool [Member] | ALABAMA POWER CO | MISSISSIPPI POWER CO
Accounting Policies [Line Items]
Related party transaction, amount											16	$ 29	$ 7
Other Regulatory Assets [Member] | GEORGIA POWER CO
Accounting Policies [Line Items]
Amortization of regulatory assets											$ 54
Storm damage reserves | GEORGIA POWER CO
Accounting Policies [Line Items]
Amortization of regulatory assets														$ 319