Joint Ownership Agreements (Tables)

12 Months Ended

Dec. 31, 2017

Jointly Owned Utility Plant Interests [Line Items]

Schedule of percentage ownership and investment in jointly-owned facilities

At December 31, 2017, Alabama Power's, Georgia Power's, Southern Power's, and Southern Company Gas' percentage ownership and investment (exclusive of nuclear fuel) in jointly-owned facilities in commercial operation with the above entities were as follows:

Facility (Type)	Percent Ownership		Plant in Service		Accumulated Depreciation		CWIP
			(in millions)
Plant Vogtle (nuclear) Units 1 and 2	45.7	%	$	3,564	$	2,141	$	70
Plant Hatch (nuclear)	50.1		1,321		595		87
Plant Miller (coal) Units 1 and 2	91.8		1,717		619		54
Plant Scherer (coal) Units 1 and 2	8.4		261		93		8
Plant Wansley (coal)	53.5		1,053		335		72
Rocky Mountain (pumped storage)	25.4		182		132		—
Plant Stanton (combined cycle) Unit A	65.0		155		55		—
Dalton Pipeline (natural gas pipeline)	50.0		241		2		13

Temporary Equity

The following table presents changes during the year in redeemable preferred stock of subsidiaries for Southern Company:

	Redeemable Preferred Stock of Subsidiaries
	(in millions)
Balance at December 31, 2014	$	375
Issued	—
Redeemed	(262		)
Issuance costs	5
Balance at December 31, 2015:	118
Issued	—
Redeemed	—
Balance at December 31, 2016:	118
Issued	250
Redeemed	(38		)
Issuance costs	(6		)
Balance at December 31, 2017:	$	324

ALABAMA POWER CO

Jointly Owned Utility Plant Interests [Line Items]

Schedule of percentage ownership and investment in jointly-owned facilities

In addition to the Company's ownership of SEGCO and joint ownership of an associated gas pipeline, the Company's percentage ownership and investment in jointly-owned generating plants at December 31, 2017 were as follows:

Facility	Total MW Capacity	Company Ownership			Plant in Service		Accumulated Depreciation		Construction Work in Progress
					(in millions)
Greene County	500	60.00	%	(1)	$	172	$	65	$	2
Plant Miller
Units 1 and 2	1,320	91.84	%	(2)	1,717		619		54

(1) Jointly owned with an affiliate, Mississippi Power.

(2) Jointly owned with PowerSouth Energy Cooperative, Inc.

GEORGIA POWER CO

Jointly Owned Utility Plant Interests [Line Items]

Schedule of percentage ownership and investment in jointly-owned facilities

At December 31, 2017, the Company's percentage ownership and investment (exclusive of nuclear fuel) in jointly-owned facilities in commercial operation with the above entities were as follows:

Facility (Type)	Company Ownership		Plant in Service		Accumulated Depreciation		CWIP
			(in millions)
Plant Vogtle (nuclear)
Units 1 and 2	45.7	%	$	3,564	$	2,141	$	70
Plant Hatch (nuclear)	50.1		1,321		595		87
Plant Wansley (coal)	53.5		1,053		335		72
Plant Scherer (coal)
Units 1 and 2	8.4		261		93		8
Unit 3	75.0		1,232		468		26
Rocky Mountain (pumped storage)	25.4		182		132		—

GULF POWER CO

Jointly Owned Utility Plant Interests [Line Items]

Schedule of percentage ownership and investment in jointly-owned facilities

At December 31, 2017, the Company's percentage ownership and investment in these jointly-owned facilities were as follows:

	Plant Scherer Unit 3 (coal)			Plant Daniel Units 1 & 2 (coal)
	(in millions)
Plant in service	$	374		$	696
Accumulated depreciation	147			225
Construction work in progress	9			4
Company ownership	25		%	50		%

MISSISSIPPI POWER CO

Jointly Owned Utility Plant Interests [Line Items]

Schedule of percentage ownership and investment in jointly-owned facilities

At December 31, 2017, the Company's percentage ownership and investment in these jointly-owned facilities in commercial operation were as follows:

Generating Plant	Company Ownership		Plant in Service		Accumulated Depreciation		CWIP
			(in millions)
Greene County
Units 1 and 2	40	%	$	164	$	55	$	1
Daniel
Units 1 and 2	50	%	$	713	$	189	$	4

SOUTHERN Co GAS

Jointly Owned Utility Plant Interests [Line Items]

Temporary Equity

The roll-forwards of the redeemable noncontrolling interest for the successor period of July 1, 2016 through December 31, 2016 and the predecessor period of January 1, 2016 through June 30, 2016 are detailed below:

Predecessor –	(in millions)
Balance at December 31, 2015	$	—
Reclassification of noncontrolling interest to contingently redeemable noncontrolling interest	46
Net income attributable to noncontrolling interest	14
Distribution to noncontrolling interest	(19		)
Balance at June 30, 2016	$	41
Successor –	(in millions)
Balance at July 1, 2016	$	174
Reclassification of contingently redeemable noncontrolling interest to mandatorily redeemable noncontrolling interest	(174		)
Balance at December 31, 2016	$	—

Equity Method Investments

The carrying amounts of the Company's equity method investments as of December 31, 2017 and 2016 and related income from those investments for the successor periods of the year ended December 31, 2017 and July 1, 2016 through December 31, 2016 and predecessor periods of January 1, 2016 through June 30, 2016 and the year ended December 31, 2015 were as follows:

Balance Sheet Information	December 31, 2017		December 31, 2016
	(in millions)
SNG(*)	$	1,262	$	1,394
Triton	42		44
Horizon Pipeline	30		30
PennEast Pipeline	57		22
Atlantic Coast Pipeline	41		33
Pivotal JAX LNG, LLC	44		16
Other	1		2
Total	$	1,477	$	1,541

(*) Includes a $104 million decrease at December 31, 2017 related to the impact of the Tax Reform Legislation and new income tax apportionment factors in several states resulting from the Company's inclusion in the consolidated Southern Company state tax filings.

	Successor				Predecessor
Income Statement Information	Year ended December 31, 2017		July 1, 2016 through December 31, 2016		January 1, 2016 through June 30, 2016		Year ended December 31, 2015
	(in millions)				(in millions)
SNG	$	88	$	56	$	—	$	—
Triton	4		2		1		4
Horizon Pipeline	2		1		1		2
Atlantic Coast Pipeline	6		1		—		—
PennEast Pipeline	6		—		—		—
Total	$	106	$	60	$	2	$	6

Selected financial information of SNG as of December 31, 2017 and 2016 and for the year ended December 31, 2017 and for the period September 1, 2016 through December 31, 2016 is as follows:

	As of December 31,
Balance Sheet Information	2017		2016
	(in millions)
Current assets	$	82	$	95
Property, plant, and equipment	2,439		2,451
Deferred charges and other assets	121		129
Total Assets	$	2,642	$	2,675
Current liabilities	$	110	$	588
Long-term debt	1,102		706
Other deferred charges and other liabilities	76		22
Total Liabilities	$	1,288	$	1,316
Total Stockholders' Equity	1,354		1,359
Total Liabilities and Stockholders' Equity	$	2,642	$	2,675

Income Statement Information	Year ended December 31, 2017		September 1, 2016 through December 31, 2016
	(in millions)
Revenues	$	544	$	230
Operating income	246		138
Net income	$	175	$	115