Joint Ownership Agreements - Narrative (Details) gal in Thousands	6 Months Ended		10 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Oct. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) tranche mi MW gal	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2016	Aug. 31, 2016	Dec. 31, 2014 mi Bcf
Jointly Owned Utility Plant Interests [Line Items]
Construction work in progress	$ 8,977,000,000			$ 6,904,000,000	$ 8,977,000,000
Short-term debt	2,241,000,000			2,439,000,000	2,241,000,000
Earnings from equity method investments			$ 15,000,000	106,000,000	59,000,000	$ 0
Distributions to noncontrolling interests				$ 119,000,000	72,000,000	18,000,000
ALABAMA POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				14.00%
Construction work in progress	491,000,000			$ 908,000,000	491,000,000
Total megawatt capacity | MW				1,000
Jointly owned affiliate equity				$ 95,000,000
Jointly owned affiliate long term debt				125,000,000
Jointly owned affiliate long term debt annual interest requirement				4,000,000
Short-term debt	0			$ 3,000,000	0
Ownership percentage, equity method investment				50.00%
ALABAMA POWER CO | SEGCO
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				86.00%
ALABAMA POWER CO | Southern Electric Generating Company
Jointly Owned Utility Plant Interests [Line Items]
Total megawatt capacity | MW				1,020
Share of purchased power				$ 76,000,000	55,000,000	76,000,000
Unconditional guarantee to pay outstanding pollution control revenue bond principal				25,000,000
ALABAMA POWER CO | SEGCO
Jointly Owned Utility Plant Interests [Line Items]
Short-term debt				14,000,000
Dividends paid by equity method investment				24,000,000
GEORGIA POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Construction work in progress	4,939,000,000			4,613,000,000	4,939,000,000
Short-term debt	391,000,000			$ 150,000,000	391,000,000
GEORGIA POWER CO | Plant Vogtle (nuclear) Units 1 and 2
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				45.70%
Plant in service				$ 3,564,000,000
Accumulated depreciation				$ 2,141,000,000
GEORGIA POWER CO | ALABAMA POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Total megawatt capacity | MW				1,020
GEORGIA POWER CO | Intercession City Combustion Turbine
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership								33.00%
GEORGIA POWER CO | Plant Scherer Unit 3 (coal)
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				75.00%
Plant in service				$ 1,232,000,000
Accumulated depreciation				468,000,000
GEORGIA POWER CO | Southern Electric Generating Company
Jointly Owned Utility Plant Interests [Line Items]
Share of purchased power				78,000,000	57,000,000
GULF POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Construction work in progress	51,000,000			91,000,000	51,000,000
Short-term debt	268,000,000			$ 45,000,000	268,000,000
GULF POWER CO | Plant Daniel Units 1 & 2 (coal)
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				50.00%
Total megawatt capacity | MW				1,000
Plant in service				$ 696,000,000
Accumulated depreciation				$ 225,000,000
GULF POWER CO | Plant Scherer Unit 3 (coal)
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				25.00%
Total megawatt capacity | MW				818
Plant in service				$ 374,000,000
Accumulated depreciation				147,000,000
SOUTHERN POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Construction work in progress	398,000,000			$ 511,000,000	398,000,000
Total megawatt capacity | MW				659
Short-term debt	209,000,000			$ 105,000,000	209,000,000
Distributions to noncontrolling interests				$ 119,000,000	57,000,000	18,000,000
SOUTHERN POWER CO | Plant Stanton (combined cycle) Unit A
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				65.00%
Plant in service				$ 155,000,000
Accumulated depreciation				55,000,000
SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Construction work in progress	496,000,000			491,000,000	496,000,000
Short-term debt	1,257,000,000			1,518,000,000	$ 1,257,000,000
Earnings from equity method investments	60,000,000			106,000,000
Distributions to noncontrolling interests	15,000,000			$ 0
Number of tranches invested in | tranche				7
SOUTHERN Co GAS | Dalton Pipeline
Jointly Owned Utility Plant Interests [Line Items]
Undivided ownership interest to be leased				50.00%	50.00%
Public utilities, equipment				$ 252,000,000
Costs included in CWIP	124,000,000				$ 124,000,000
Pipeline infrastructure | mi				115
SOUTHERN Co GAS | Dalton Pipeline Arrangement 2
Jointly Owned Utility Plant Interests [Line Items]
Undivided ownership interest to be leased				50.00%
Future minimum payments receivable				$ 26,000,000
Term of contract				25 years
Maturity December 1, 2018 | ALABAMA POWER CO | Southern Electric Generating Company
Jointly Owned Utility Plant Interests [Line Items]
Guarantee of unsecured senior notes				$ 100,000,000
Natural Gas Processing Plant [Member] | GEORGIA POWER CO | ALABAMA POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Total megawatt capacity | MW				1,000
Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				45.70%
Construction work in progress				$ 3,300,000,000
Predecessor | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Earnings from equity method investments		$ 2,000,000				6,000,000
Distributions to noncontrolling interests		19,000,000				18,000,000
Purchased Power from Affiliates | GEORGIA POWER CO | Southern Electric Generating Company
Jointly Owned Utility Plant Interests [Line Items]
Share of purchased power						$ 78,000,000
Orlando Utilities Commission | SOUTHERN POWER CO | Plant Stanton (combined cycle) Unit A
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				28.00%
Florida Municipal Power Agency | SOUTHERN POWER CO | Plant Stanton (combined cycle) Unit A
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				3.50%
Kissimmee Utility Authority | SOUTHERN POWER CO | Plant Stanton (combined cycle) Unit A
Jointly Owned Utility Plant Interests [Line Items]
Percent ownership				3.50%
Southstar | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentage of noncontrolling interest							85.00%
Agreement to purchase remaining interest			$ 160,000,000
Southstar | Piedmont
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentage of noncontrolling interest							15.00%
Georgia Natural Gas | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Noncontrolling ownership percentage held by parent						100.00%
Piedmont | Southstar
Jointly Owned Utility Plant Interests [Line Items]
Payments of ordinary dividends to noncontrolling interests	15,000,000
Piedmont | Southstar | Predecessor
Jointly Owned Utility Plant Interests [Line Items]
Distributions to noncontrolling interests		19,000,000				$ 18,000,000
Southern Natural Gas Company, LLC | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentage, equity method investment							50.00%
Horizon Pipeline | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Earnings from equity method investments	1,000,000			$ 2,000,000
Pipeline infrastructure | mi				70
Horizon Pipeline | Predecessor | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Earnings from equity method investments		1,000,000				2,000,000
PennEast Pipeline | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentage, equity method investment									20.00%
Pipeline infrastructure | mi									118
Atlantic Coast Pipeline | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentage, equity method investment									5.00%
Earnings from equity method investments	$ 1,000,000			$ 6,000,000
Pipeline infrastructure | mi									594
Atlantic Coast Pipeline | Predecessor | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Earnings from equity method investments		$ 0				$ 0
Pivotal JAX LNG, LLC | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentage, equity method investment				50.00%
Minimum | Horizon Pipeline | Nicor Gas
Jointly Owned Utility Plant Interests [Line Items]
Capacity of natural gas facility, percent				70.00%
Minimum | PennEast Pipeline | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Natural gas pipeline capacity (volume) | Bcf									1.0
Minimum | Atlantic Coast Pipeline | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Natural gas pipeline capacity (volume) | Bcf									1.5
Maximum | Horizon Pipeline | Nicor Gas
Jointly Owned Utility Plant Interests [Line Items]
Capacity of natural gas facility, percent				80.00%
Liquefied Natural Gas (LNG) | Pivotal JAX LNG, LLC | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Storage facility capacity | gal				2,000
Liquefied Natural Gas (LNG) | Minimum | Pivotal JAX LNG, LLC | SOUTHERN Co GAS
Jointly Owned Utility Plant Interests [Line Items]
Storage facility production capacity | gal				120