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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions								3 Months Ended	6 Months Ended
	Aug. 08, 2016 USD ($)	Jul. 10, 2016 USD ($) mi	Jul. 01, 2016 USD ($) MW	May 09, 2016 USD ($) $ / shares	Apr. 07, 2016 MW	Mar. 04, 2016 MW	Feb. 11, 2016 MW	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) entities MW	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill								$ 264.0		$ 264.0	$ 2.0
Construction work in progress								9,451.0		9,451.0	9,082.0
Property, Plant and Equipment, Gross								78,112.0		78,112.0	75,118.0
Other Assets, Current								71.0		71.0	71.0
Southern Power [Member]
Business Acquisition [Line Items]
Aggregate purchase price										477.0
Goodwill								2.0		2.0	2.0
Total costs of construction										2,700.0
Construction work in progress								1,852.0		1,852.0	1,137.0
Contingent consideration								23.0		23.0	36.0
Property, Plant and Equipment, Gross								8,348.0		8,348.0	7,275.0
Other Assets, Current								31.0		31.0	30.0
Accounts Payable, Other, Current								175.0		175.0	$ 327.0
Southern Power and Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Aggregate purchase price										483.0
Subsequent Event [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 275.0
Subsequent Event [Member] \| Southern Power and Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Aggregate purchase price	447.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	217.0
Subsequent Event [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments	260.0
Subsequent Event [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments	$ 300.0
Southern Company Gas [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities								2,205.0		2,205.0
Goodwill								6,333.0		6,333.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net								8,002.0		8,002.0
Southern Company Gas [Member] \| Subsequent Event [Member]
Business Acquisition [Line Items]
Aggregate purchase price			$ 8,000.0
Southern Company Gas [Member] \| Bridge Agreement [Member]
Business Acquisition [Line Items]
Acquisition-related costs								43.4		63.3
Southern Company Gas [Member] \| Bridge Agreement [Member] \| Operating Expense [Member]
Business Acquisition [Line Items]
Acquisition-related costs								26.9		32.9
Southern Company Gas [Member] \| Bridge Agreement [Member] \| Other Income Expense Net [Member]
Business Acquisition [Line Items]
Acquisition-related costs								16.5		30.4
PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price				$ 429.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities								111.0		111.0
Acquisition, share price (in dollars per share) \| $ / shares				$ 18.75
Goodwill								262.0		262.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net								429.0		$ 429.0
PowerSecure International, Inc. [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets										3 years
PowerSecure International, Inc. [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets										26 years
Southern Natural Gas Company, LLC [Member] \| Subsequent Event [Member]
Business Acquisition [Line Items]
Aggregate purchase price		$ 1,500.0
Ownership percentage		50.00%
Pipeline Infrastructure (in miles) \| mi		7,600
Grant Wind, LLC [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage					100.00%
Approx. Nameplate Capacity, Wind (MW) \| MW					151
Life Output Of Plant					20 years
East Pecos [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage						100.00%
Approx. Nameplate Capacity, Solar (MW) \| MW						120
Life Output Of Plant						15 years
East Pecos [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments								160.0		$ 160.0
East Pecos [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments								180.0		180.0
Series of Business Acquisitions [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Revenue of acquirees, actual										4.0
Construction work in progress								426.0		426.0
Contingent consideration								6.0		6.0
Property, Plant and Equipment, Gross								58.0		58.0
Other Assets, Current								4.0		4.0
Accounts Payable, Other, Current								7.0		7.0
Series of Construction Projects [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Construction work in progress								1,700.0		1,700.0
Series of Construction Projects [Member] \| Minimum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments								3,000.0		3,000.0
Series of Construction Projects [Member] \| Maximum [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments								$ 3,200.0		$ 3,200.0
Calipatria [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage							90.00%
Approx. Nameplate Capacity, Solar (MW) \| MW							20
Life Output Of Plant							20 years
Calipatria [Member] \| Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Ownership percentage								10.00%		10.00%
Henrietta [Member] \| Subsequent Event [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage			51.00%
Approx. Nameplate Capacity, Solar (MW) \| MW			102
Life Output Of Plant			20 years
Henrietta [Member] \| Subsequent Event [Member] \| SunPower AssetCo, LLC [Member]
Business Acquisition [Line Items]
Ownership percentage	49.00%
Rutherford [Member] \| Subsequent Event [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage			90.00%
Approx. Nameplate Capacity, Solar (MW) \| MW			74
Life Output Of Plant			15 years
Rutherford [Member] \| Subsequent Event [Member] \| Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Ownership percentage	10.00%
Scenario, Forecast [Member] \| Series of Business Acquisitions [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Aggregate purchase price									$ 1,100.0
Scenario, Forecast [Member] \| Series of Business Acquisitions, Wind [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage									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]
Ownership percentage									90.10%
Approx. Nameplate Capacity, Wind (MW) \| MW									257
Life Output Of Plant									12 years
Scenario, Forecast [Member] \| Boulder 1 [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage									51.00%
Approx. Nameplate Capacity, Solar (MW) \| MW									100
Life Output Of Plant									20 years
Class A Membership Interest [Member] \| Series of Business Acquisitions [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage								100.00%		100.00%
Class A Membership Interest [Member] \| Scenario, Forecast [Member] \| Boulder 1 [Member] \| Southern Power [Member]
Business Acquisition [Line Items]
Ownership percentage									100.00%