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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions							1 Months Ended			3 Months Ended			4 Months Ended	7 Months Ended	9 Months Ended
	Oct. 26, 2016 USD ($) MW	Sep. 01, 2016 USD ($)	Aug. 26, 2016 MW	Jul. 01, 2016 USD ($) MW	May 09, 2016 USD ($) $ / shares	Feb. 11, 2016 MW	Jan. 31, 2017 MW	Nov. 30, 2016 MW	Nov. 04, 2016 USD ($)	Dec. 31, 2016 entities MW	Sep. 30, 2016 USD ($) MW	Sep. 30, 2015 USD ($)	Jan. 31, 2017 USD ($) MW	Jul. 31, 2016 MW	Sep. 30, 2016 USD ($) MW	Sep. 30, 2015 USD ($)	Oct. 24, 2016 MW	Jul. 10, 2016 mi	Mar. 29, 2016	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill											$ 6,223.0				$ 6,223.0					$ 2.0
Revenues											6,264.0	$ 5,401.0			14,715.0	$ 13,921.0
Consolidated net income attributable to Southern Company											1,130.0	959.0			2,226.0	2,096.0
Construction work in progress											10,069.0				10,069.0					9,082.0
Property, plant, and equipment											94,174.0				94,174.0					75,118.0
Intangible assets, net											942.0				942.0					317.0
Other current assets											168.0				168.0					71.0
Southern Company Gas [Member]
Business Acquisition [Line Items]
Revenues											543.0
Consolidated net income attributable to Southern Company											4.0
Southern Power and Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Aggregate purchase price															923.0
Construction debt assumed											217.0				217.0
Southern Power [Member]
Business Acquisition [Line Items]
Aggregate purchase price															830.0
Goodwill											2.0				2.0					2.0
Revenues											500.0	$ 401.0			1,189.0	$ 1,086.0
Business Combination, Contingent Consideration, Liability											30.0				30.0					36.0
Construction work in progress											1,652.0				1,652.0					1,137.0
Property, plant, and equipment											9,491.0				9,491.0					7,275.0
Intangible assets, net											389.0				389.0					317.0
Other current assets											32.0				32.0					30.0
Other accounts payable											$ 218.0				218.0					$ 327.0
Total costs of construction															$ 3,000.0
Subsequent Event [Member]
Business Acquisition [Line Items]
Capacity Of Fuel Cell Systems \| MW																	50
Subsequent Event [Member] \| Southern Power and Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 924.0
Subsequent Event [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 873.0
East Pecos [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent			100.00%
Life output of plant			12 years
Approx. nameplate capacity, wind (MW) \| MW			147
Desert Stateline Holdings, LLC [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent																			15.00%
Life output of plant														20 years
Approx. nameplate capacity, solar (MW) \| MW											299			299	299
Southern Company Gas [Member]
Business Acquisition [Line Items]
Aggregate purchase price				$ 8,000.0
Goodwill											$ 5,937.0				$ 5,937.0
Construction debt assumed											2,201.0				$ 2,201.0
Southern Company Gas [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets															1 year
Southern Company Gas [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets															28 years
Southern Company Gas [Member] \| Bridge Agreement [Member]
Business Acquisition [Line Items]
Acquisition-related costs											40.8				$ 104.1
Southern Company Gas [Member] \| Bridge Agreement [Member] \| Operating Expense [Member]
Business Acquisition [Line Items]
Acquisition-related costs											40.6				73.5
Southern Company Gas [Member] \| Bridge Agreement [Member] \| Other Income Expense Net [Member]
Business Acquisition [Line Items]
Acquisition-related costs											0.2				30.6
PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price					$ 429.0
Goodwill											284.0				284.0
Acquisition, share price (in dollars per share) \| $ / shares					$ 18.75
Construction debt assumed											$ 145.0				$ 145.0
PowerSecure International, Inc. [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets															1 year
PowerSecure International, Inc. [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets															26 years
Southern Natural Gas Company, LLC [Member]
Business Acquisition [Line Items]
Aggregate purchase price		$ 1,400.0
Noncontrolling interest, ownership percentage by parent																		50.00%
Pipeline Infrastructure (in miles) \| mi																		7,000
Piedmont [Member] \| Southern Company Gas [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent											15.00%				15.00%
Piedmont [Member] \| Subsequent Event [Member] \| Southern Company Gas [Member]
Business Acquisition [Line Items]
Aggregate purchase price									$ 160.0
Southstar [Member] \| Southern Company Gas [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent											85.00%				85.00%
Series of Business Acquisitions [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability											$ 145.0				$ 145.0
Construction work in progress											1,000.0				1,000.0
Property, plant, and equipment											58.0				58.0
Intangible assets, net											77.0				77.0
Amortization expense remainder of fiscal year											1.0				1.0
Amortization expense next twelve months											4.0				4.0
Amortization expense year two											4.0				4.0
Amortization expense year three											4.0				4.0
Amortization expense year four											4.0				4.0
Amortization expense year five											4.0				4.0
Other current assets											24.0				24.0
Other accounts payable											5.0				$ 5.0
Life output of plant															20 years
Revenue of acquirees, actual															$ 14.0
Series of Business Acquisitions [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments											708.0				708.0
Series of Business Acquisitions [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments											$ 775.0				$ 775.0
Calipatria [Member] \| Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent											10.00%				10.00%
Calipatria [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent						90.00%
Life output of plant						20 years
Approx. nameplate capacity, solar (MW) \| MW						20
Henrietta [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent				51.00%
Life output of plant				20 years
Approx. nameplate capacity, solar (MW) \| MW				102
Henrietta [Member] \| SunPower AssetCo, LLC [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent											49.00%				49.00%
Rutherford [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent				90.00%
Life output of plant				15 years
Approx. nameplate capacity, solar (MW) \| MW				74
Rutherford [Member] \| Subsequent Event [Member] \| Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent	9.90%
Mankato [Member] \| Subsequent Event [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Capacity of Natural Gas Facility \| MW	375
Noncontrolling interest, ownership percentage by parent	100.00%
Life output of plant	10 years
Mankato [Member] \| Subsequent Event [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments	$ 170.0
Mankato [Member] \| Subsequent Event [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments	$ 190.0
Mankato Expansion [Member] \| Subsequent Event [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Capacity of Natural Gas Facility \| MW	345
Life output of plant	20 years
Series of Construction Projects [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Construction work in progress											$ 1,200.0				$ 1,200.0
Series of Construction Projects [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments											3,100.0				3,100.0
Series of Construction Projects [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments											$ 3,200.0				$ 3,200.0
Scenario, Forecast [Member] \| Series of Business Acquisitions [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Aggregate purchase price													$ 1,200.0
Scenario, Forecast [Member] \| Business Acquisition, Solar [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent								51.00%
Life output of plant								20 years
Approx. nameplate capacity, solar (MW) \| MW								100
Scenario, Forecast [Member] \| Series of Business Acquisitions, Wind [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent										100.00%
Number of Businesses Acquired \| entities										2
Approx. nameplate capacity, wind (MW) \| MW										299
Scenario, Forecast [Member] \| Series of Business Acquisitions, Wind [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Life output of plant										12 years
Scenario, Forecast [Member] \| Series of Business Acquisitions, Wind [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Life output of plant										14 years
Scenario, Forecast [Member] \| Business Acquisition, Wind [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent							100.00%						100.00%
Life output of plant							12 years
Approx. nameplate capacity, wind (MW) \| MW							275						275
Class A Membership Interest [Member] \| Series of Business Acquisitions [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent											100.00%				100.00%
Class A Membership Interest [Member] \| Scenario, Forecast [Member] \| Business Acquisition, Solar [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage by parent								100.00%
Oklahoma Municipal Power Authority [Member] \| East Pecos [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Life output of plant			20 years
Steelcase, Inc. [Member] \| East Pecos [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Life output of plant			12 years