Summary of Significant Accounting Policies - Property, Plant, and Equipment (Details) $ in Millions	1 Months Ended	6 Months Ended		12 Months Ended
	Jul. 31, 2016	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) Property	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation		$ 48,836		$ 51,279	$ 48,836
Transmission		11,156		11,562	11,156
Distribution		18,418		19,239	18,418
General		4,629		4,276	4,629
Plant acquisition adjustment		126		126	126
Electric Utility Plant in Service		83,165		86,482	83,165
Transportation and distribution		11,996		13,078	11,996
Utility plant in service		95,161		99,560	95,161
Information technology equipment and software		544		752	544
Communications equipment		424		456	424
Storage facilities		1,463		1,598	1,463
Other		824		1,176	824
Other plant in service		3,255		3,982	3,255
Total plant in service		98,416		103,542	98,416
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Less: Accumulated amortization		(69)		(72)	(69)
Balance, net of amortization		144		201	144
Accrued property additions at year-end				985	1,300	$ 844
Capital Lease Obligations
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Non-cash property additions recognized				162	18	13
Office building
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Capital leased assets, gross		61		216	61
Nitrogen plant
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Capital leased assets, gross		83		0	83
Computer-related equipment
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Capital leased assets, gross		63		51	63
Gas pipeline
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Capital leased assets, gross		6		6	6
SOUTHERN POWER CO
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end				$ 32	251	257
Alabama Power and Georgia Power
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Refueling cycles for minimum period				18 months
Refueling cycles maximum period				24 months
SOUTHERN Co GAS
Property, Plant and Equipment [Line Items]
Period for collection of revenue prior to billings				24 months
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Utility plant in service		11,996		$ 13,079	11,996
Information technology equipment and software		324		366	324
Storage facilities		1,463		1,599	1,463
Other		725		789	725
Other plant in service		2,512		2,754	2,512
Total plant in service		14,508		15,833	14,508
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end		63		$ 135
SOUTHERN Co GAS | Predecessor
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end			$ 41			48
ALABAMA POWER CO
Property, Plant and Equipment [Line Items]
Number of units for which outage operations and maintenance expenses accrued | Property				2
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation		13,551		$ 14,213	13,551
Transmission		3,921		4,119	3,921
Distribution		6,707		7,034	6,707
General		1,840		1,948	1,840
Plant acquisition adjustment		12		12	12
Total plant in service		26,031		27,326	26,031
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end				$ 245	84	121
Period over which deferred costs are being amortized to nuclear operations and maintenance expenses				18 months
GEORGIA POWER CO
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation		16,668		$ 17,038	16,668
Transmission		5,779		5,947	5,779
Distribution		9,553		9,978	9,553
General		1,813		1,870	1,813
Plant acquisition adjustment		28		28	28
Total plant in service		33,841		$ 34,861	33,841
Refueling cycles for minimum period				18 months
Refueling cycles maximum period				24 months
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end				$ 550	336	387
GULF POWER CO
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation		3,001		3,005	3,001
Transmission		706		720	706
Distribution		1,241		1,282	1,241
General		191		188	191
Plant acquisition adjustment		1		1	1
Total plant in service		5,140		5,196	5,140
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end				31	33	20
MISSISSIPPI POWER CO
Property, Plant and Equipment [Line Items]
Regulatory amortization period	5 years
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation		2,632		2,801	2,632
Transmission		712		737	712
Distribution		916		946	916
General		520		204	520
Plant acquisition adjustment		85		85	85
Total plant in service		$ 4,865		4,773	4,865
Capital Leases, Balance Sheet, Assets by Major Class, Net [Abstract]
Accrued property additions at year-end				$ 32	$ 78	$ 105
Maximum | SOUTHERN POWER CO | Natural Gas Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life				45 years
Maximum | SOUTHERN POWER CO | Biomass Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life				40 years
Maximum | SOUTHERN POWER CO | Solar Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life				35 years
Maximum | SOUTHERN POWER CO | Wind Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life				30 years