Acquisitions - Textual (Details) $ / shares in Units, mi in Thousands, $ in Millions									3 Months Ended								6 Months Ended		8 Months Ended	12 Months Ended			36 Months Ended
	Oct. 26, 2016 MW	Sep. 01, 2016 USD ($) mi	Aug. 26, 2016 MW	Jul. 01, 2016 USD ($) $ / shares MW shares	May 09, 2016 USD ($) $ / shares	Feb. 11, 2016 MW	Oct. 22, 2015 MW	Aug. 31, 2015 MW	Dec. 31, 2016 USD ($) $ / shares MW shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares MW shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) $ / shares MW shares	Jun. 30, 2016 USD ($)	Jul. 31, 2016 MW	Dec. 31, 2016 USD ($) $ / shares MW shares	Dec. 31, 2015 USD ($) $ / shares MW shares	Dec. 31, 2014 USD ($)	Dec. 31, 2020 Facility	Oct. 24, 2016 MW	Oct. 03, 2016 USD ($)	Oct. 02, 2016	Mar. 29, 2016	Feb. 12, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill									$ 6,251.0				$ 2.0				$ 6,251.0			$ 6,251.0	$ 2.0
Revenues									5,181.0	$ 6,264.0	$ 4,459.0	$ 3,992.0	3,568.0	$ 5,401.0	$ 4,337.0	$ 4,183.0				19,896.0	17,489.0	$ 18,467.0
Net income									197.0	1,139.0	623.0	489.0	271.0	959.0	629.0	508.0				2,448.0	2,367.0	1,963.0
Capacity Of Fuel Cell Systems | MW																								50
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months									108.0								108.0			108.0
Acquisitions Payable									$ 489.0				$ 0.0				$ 489.0			489.0	0.0
Noncontrolling Interest, Increase from Contributions from Noncontrolling Interest Holders																				$ 618.0	$ 567.0	221.0
Common Stock, Shares Authorized | shares									1,500,000,000				1,500,000,000				1,500,000,000			1,500,000,000	1,500,000,000
Common stock, par value per share (in dollars per share) | $ / shares									$ 5				$ 5				$ 5			$ 5	$ 5
Construction in Progress, Gross									$ 8,977.0				$ 9,082.0				$ 8,977.0			$ 8,977.0	$ 9,082.0
PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets									282.0								282.0			282.0
Goodwill									101.0								101.0			101.0
Business acquisition share price (in dollars per share) | $ / shares					$ 18.75
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable					$ 429.0
Current liabilities									114.0								114.0			114.0
Southern Company Gas [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross				$ 8,000.0
Common Stock, Shares Authorized | shares				100,000,000
Preferred Stock, Shares Authorized | shares				10,000,000
Common stock, par value per share (in dollars per share) | $ / shares				$ 0.01
Southern Power [Member]
Business Acquisition [Line Items]
Total Cost Of Construction																				3,200.0
Payments to Acquire Businesses, Gross																				$ 2,300.0	1,400.0
Finite-Lived Intangible Asset, Useful Life																				19 years
Revenues									389.0	500.0	373.0	315.0	304.0	401.0	337.0	348.0				$ 1,577.0	$ 1,390.0	1,501.0
Net income									23.0	176.0	89.0	50.0	34.0	102.0	46.0	33.0
Life Output Of Plant																					20 years
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months									25.0								25.0			25.0
Acquisitions Payable									461.0				0.0				461.0			461.0	$ 0.0
Business Combination, Contingent Consideration, Liability									$ 46.0				$ 36.0				$ 46.0			46.0	36.0
Noncontrolling Interest, Increase from Contributions from Noncontrolling Interest Holders																				$ 618.0	$ 567.0	221.0
Common Stock, Shares Authorized | shares									1,000,000				1,000,000				1,000,000			1,000,000	1,000,000
Common stock, par value per share (in dollars per share) | $ / shares									$ 0.01				$ 0.01				$ 0.01			$ 0.01	$ 0.01
Construction in Progress, Gross									$ 398.0				$ 1,137.0				$ 398.0			$ 398.0	$ 1,137.0
Capital Contributions from Parent Company																				$ 1,850.0	$ 646.0	147.0
Southern Power [Member] | Desert Stateline Holdings, LLC [Member]
Business Acquisition [Line Items]
Power of Solar Polycrystalline Silicon Facility | MW								299	299								299		299	299
Capacity Of Small Power Production Facility | MW													110								110
Noncontrolling Interest, Ownership Percentage by Parent								51.00%																			15.00%
Life Output Of Plant								20 years											20 years
Southern Power [Member] | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Increase from Contributions from Noncontrolling Interest Holders																					$ 227.0
Southern Power [Member] | 70SM1 8ME, LCC (Calipatria) [Member]
Business Acquisition [Line Items]
Power of Solar Polycrystalline Silicon Facility | MW						20
Noncontrolling Interest, Ownership Percentage by Parent						90.00%
Life Output Of Plant						20 years
Southern Power [Member] | Rutherford [Member]
Business Acquisition [Line Items]
Power of Solar Polycrystalline Silicon Facility | MW				74
Noncontrolling Interest, Ownership Percentage by Parent				90.00%
Life Output Of Plant				15 years
Southern Power [Member] | GASNA 31P, LLC (Morelos) [Member]
Business Acquisition [Line Items]
Power of Solar Polycrystalline Silicon Facility | MW							15
Noncontrolling Interest, Ownership Percentage by Parent							90.00%
Life Output Of Plant							20 years
Southern Power [Member] | Grant Plains [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent			100.00%
Life Output Of Plant			12 years
Energy From Wind-Powered Generating Facilities | MW			147
Southern Power [Member] | Mankato [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	100.00%
Life Output Of Plant	10 years																			10 years
Capacity of Natural Gas Facility | MW	375
Southern Power [Member] | Wake Wind [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	90.10%
Life Output Of Plant	12 years
Energy From Wind-Powered Generating Facilities | MW	257
Southern Power [Member] | Mankato Expansion [Member]
Business Acquisition [Line Items]
Life Output Of Plant																				20 years
Capacity of Natural Gas Facility | MW									345								345			345
Southern Power [Member] | Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Intangible assets									$ 128.0				$ 274.0				$ 128.0			$ 128.0	274.0
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months									9.0				14.0				9.0			9.0	14.0
Business Combination, Contingent Consideration, Liability									281.0				195.0				281.0			281.0	195.0
Construction in Progress, Gross									386.0								386.0			386.0
Southern Power [Member] | Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Revenues																				37.0
Noncontrolling Interests [Member] | Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability									$ 142.0				$ 76.0				$ 142.0			$ 142.0	$ 76.0
Invenergy [Member] | Rutherford [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	9.90%
Turner Renewable Energy [Member] | 70SM1 8ME, LCC (Calipatria) [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent						10.00%
Turner Renewable Energy [Member] | Rutherford [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent				10.00%
Turner Renewable Energy [Member] | GASNA 31P, LLC (Morelos) [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent							10.00%						10.00%								10.00%
Turner Renewable Energy [Member] | 70SM1 8ME, LCC (Calipatria) and Rutherford [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent									10.00%								10.00%			10.00%
First Solar [Member] | Desert Stateline, Lost Hills Blackwell, and North Star [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent													49.00%								49.00%
Southern Company Gas [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months									$ 73.0								$ 73.0			$ 73.0
Southern Company Gas [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Business acquisition share price (in dollars per share) | $ / shares				$ 66
Southern Company Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Shares, Authorized | shares				110,000,000
Southern Company Gas [Member] | Southern Natural Gas Company, LLC [Member]
Business Acquisition [Line Items]
Pipeline infrastructure | mi		7
SunPower AssetCo, LLC [Member] | Boulder 1 and Henrietta [Member] [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent									49.00%								49.00%			49.00%
Southern Power and Turner Renewable Energy [Member]
Business Acquisition [Line Items]
Current liabilities									$ 217.0								$ 217.0			$ 217.0
Southern Power, Turner Renewable Energy, SunPower, and Invenergy [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross																				$ 2,600.0
Recurrent Energy [Member] | Garland, Garland A, Roserock, and Tranquility [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent													49.00%								49.00%
Southern Power, Turner Renewable Energy, First Solar, and Recurrent [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross																					$ 1,900.0
Elizabethtown Gas [Member]
Business Acquisition [Line Items]
Business Combination, Regulatory Approval Requirements, Required Rate Credit Payments to Customers									$ 0.4	17.5
Elizabethtown Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Business Combination, Regulatory Approval Requirements, Base Rate Case Filing Period																				3 years
Elkton Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Business Combination, Regulatory Approval Requirements, Base Rate Case Filing Period																				2 years
Class A Membership Interest [Member] | Southern Power [Member] | Desert Stateline Holdings, LLC [Member]
Business Acquisition [Line Items]
Percentage Of Entitled Cash Distributions									66.00%								66.00%			66.00%
Class A Membership Interest [Member] | Southern Power [Member] | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent									100.00%								100.00%			100.00%
Percentage Of Entitled Cash Distributions									51.00%								51.00%			51.00%
Class B Membership Interest [Member] | First Solar and Recurrent Energy [Member] | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent									100.00%								100.00%			100.00%
Class B Membership Interest [Member] | First Solar [Member] | Desert Stateline Holdings, LLC [Member]
Business Acquisition [Line Items]
Percentage Of Entitled Cash Distributions									34.00%								34.00%			34.00%
Class B Membership Interest [Member] | First Solar [Member] | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A [Member]
Business Acquisition [Line Items]
Percentage Of Entitled Cash Distributions									49.00%								49.00%			49.00%
Minimum [Member] | PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life																				1 year
Minimum [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life																				1 year
Minimum [Member] | Southern Power [Member] | Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments									$ 530.0								$ 530.0			$ 530.0
Maximum [Member] | PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life																				26 years
Maximum [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life																				28 years
Maximum [Member] | Southern Power [Member] | Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Estimated Future Construction Payments									590.0								590.0			$ 590.0
Bridge Agreement [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																				111.0	41.0
Successor [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Intangible assets									400.0								400.0			400.0
Goodwill									5,967.0								5,967.0			5,967.0
Current liabilities									2,201.0								2,201.0			2,201.0
Successor [Member] | Elizabethtown Gas and Elkton Gas [Member]
Business Acquisition [Line Items]
Business Combination, Regulatory Approval Requirements, Required Rate Credit Payments to Customers																	18.0
Successor [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Goodwill									30.0
Merger-related expenses																	41.0
Goodwill									5,967.0								5,967.0			$ 5,967.0
Revenues									1,109.0	543.0							1,652.0
Net income									$ 110.0	$ 4.0							$ 114.0
Common Stock, Shares Authorized | shares									100,000,000								100,000,000			100,000,000
Common stock, par value per share (in dollars per share) | $ / shares									$ 0.01								$ 0.01			$ 0.01
Construction in Progress, Gross									$ 496.0								$ 496.0			$ 496.0
Capital Contributions from Parent Company																	1,085.0
Successor [Member] | Southern Company Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Business Combination, Integration Related Costs																	20.0
Goodwill									$ 6,000.0								6,000.0			6,000.0
Business Combination, Provisional Information, Pushdown Accounting, Adjustment, Income (Loss) before Income Taxes, Extraordinary Items, Noncontrolling Interest																	(20.0)
Business Combination, Provisional Information, Pushdown Accounting, Adjustment, Revenue																	(17.0)
Business Combination, Provisional Information, Pushdown Accounting, Adjustment, Amortization Expense																	22.0
Business Combination, Provisional Information, Pushdown Accounting, Adjustment, Interest Expense																	(19.0)
Predecessor [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Goodwill																	4,154.0
Intangible assets											101.0							$ 101.0
Goodwill											1,813.0							1,813.0
Current liabilities											2,205.0							2,205.0
Predecessor [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																		56.0			44.0	0.0
Goodwill													$ 1,813.0								1,813.0
Revenues											571.0	1,334.0	962.0	584.0	674.0	1,721.0		1,905.0			3,941.0	5,385.0
Net income											$ (51.0)	$ 182.0	$ 107.0	$ 11.0	$ 42.0	$ 193.0		131.0			$ 353.0	482.0
Common Stock, Shares Authorized | shares													750,000,000								750,000,000
Common stock, par value per share (in dollars per share) | $ / shares													$ 5								$ 5
Construction in Progress, Gross													$ 414.0								$ 414.0
Predecessor [Member] | Southern Company Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																		56.0			44.0
Business Combination, Integration Related Costs																		25.0
Merger-Related Expenses [Member] | Successor [Member] | Southern Company Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																	$ 3.0
Merger-Related Expenses [Member] | Predecessor [Member] | Southern Company Gas [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																		$ 31.0
Operating Expense [Member] | Bridge Agreement [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																				80.0	27.0
Other Income Expense Net [Member] | Bridge Agreement [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Merger-related expenses																				$ 31.0	14.0
SNG [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Capital Contributions from Parent Company		$ 1,050.0
Proceeds from Contributions from Affiliates		360.0
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		$ 700.0
Southstar [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent																										85.00%
Southern Natural Gas Company, LLC [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Ownership percentage, equity method investment		50.00%
Equity Method Investment, Aggregate Cost		$ 1,400.0
Southstar [Member] | Piedmont [Member]
Business Acquisition [Line Items]
Ownership percentage of noncontrolling interest																										15.00%
Southstar [Member] | Southern Company Gas [Member]
Business Acquisition [Line Items]
Ownership percentage of noncontrolling interest																										85.00%
Agreement to purchase remaining interest																									$ 160.0			$ 160.0
Allianz Risk Transfer (Bermuda) Ltd. [Member] | Southern Power [Member] | Grant Plains [Member]
Business Acquisition [Line Items]
Life Output Of Plant			10 years
Renewable Energy Systems Americas, Inc. [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW									3,000								3,000			3,000
Wind Generating Facility [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Project Qualification for Production Tax Credits, Percentage																				100.00%
Wind Generating Facility [Member] | Scenario, Forecast [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Number of Projects to be Placed In Service | Facility																							10
Grant County [Member] | Southern Power [Member] | Grant Plains [Member]
Business Acquisition [Line Items]
Life Output Of Plant			20 years
Intangible Assets 1 [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Life Output Of Plant																				10 years
Intangible Assets 2 [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Life Output Of Plant																				20 years
Noncontrolling Interest [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Increase from Contributions from Noncontrolling Interest Holders																				$ 618.0	567.0	221.0
Noncontrolling Interest [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Increase from Contributions from Noncontrolling Interest Holders																				618.0	$ 567.0	$ 221.0
Noncontrolling Interest [Member] | Southern Power [Member] | Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Increase from Contributions from Noncontrolling Interest Holders																				$ 51.0