Southern Power 10-Q 1 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2017shares
Document Information [Line Items]
Entity Registrant Name	SOUTHERN CO
Entity Central Index Key	92,122
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Amendment Flag	false
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	1,003,627,691
ALABAMA POWER CO
Document Information [Line Items]
Entity Registrant Name	ALABAMA POWER CO
Entity Central Index Key	3,153
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	30,537,500
GEORGIA POWER CO
Document Information [Line Items]
Entity Registrant Name	GEORGIA POWER CO
Entity Central Index Key	41,091
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	9,261,500
GULF POWER CO
Document Information [Line Items]
Entity Registrant Name	GULF POWER CO
Entity Central Index Key	44,545
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	7,392,717
MISSISSIPPI POWER CO
Document Information [Line Items]
Entity Registrant Name	MISSISSIPPI POWER CO
Entity Central Index Key	66,904
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	1,121,000
SOUTHERN POWER CO
Document Information [Line Items]
Entity Registrant Name	SOUTHERN POWER CO
Entity Central Index Key	1,160,661
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	1,000
SOUTHERN Co GAS
Document Information [Line Items]
Entity Registrant Name	SOUTHERN Co GAS
Entity Central Index Key	1,004,155
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)	100

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating Revenues:
Retail electric revenues	$ 4,615	$ 4,808		$ 11,786	$ 11,932
Wholesale electric revenues	718	613		1,867	1,455
Other electric revenues	168	181		510	529
Natural gas revenues	532	518		2,746	518
Other revenues	168	144		494	281
Total operating revenues	6,201	6,264		17,403	14,715
Operating Expenses:
Fuel	1,285	1,400		3,372	3,334
Purchased power	256	227		646	581
Cost of natural gas	134	133		1,085	133
Cost of other sales	90	84		293	161
Other operations and maintenance	1,287	1,411		3,918	3,616
Depreciation and amortization	767	695		2,236	1,805
Taxes other than income taxes	303	309		941	821
Estimated loss on Kemper IGCC	34	88		3,155	222
Total operating expenses	4,156	4,347		15,646	10,673
Operating Income	2,045	1,917		1,757	4,042
Other Income and (Expense):
Allowance for equity funds used during construction	18	52		133	150
Earnings from equity method investments	32	29		100	28
Interest expense, net of amounts capitalized	(407)	(374)		(1,248)	(913)
Other income (expense), net	11	(8)		2	(66)
Total other income and (expense)	(346)	(301)		(1,013)	(801)
Earnings Before Income Taxes	1,699	1,616		744	3,241
Income taxes	590	439		317	917
Consolidated Net Income	1,109	1,177		427	2,324
Less:
Dividends on preferred and preference stock of subsidiaries	10	11		32	34
Net income attributable to noncontrolling interests	30	27		48	39
Net Income	$ 1,069	$ 1,139		$ 347	$ 2,251
Earnings per share —
Basic (in dollars per share)	$ 1.07	$ 1.18		$ 0.35	$ 2.40
Diluted (in dollars per share)	$ 1.06	$ 1.17		$ 0.35	$ 2.38
Average number of shares of common stock outstanding (in millions)
Basic (in shares)	1,003	968		998	940
Diluted (in shares)	1,010	975		1,005	945
Cash dividends paid per share of common stock (in dollars per share)	$ 0.58	$ 0.56		$ 1.7200	$ 1.6625
ALABAMA POWER CO
Operating Revenues:
Retail electric revenues	$ 1,595	$ 1,629		$ 4,155	$ 4,139
Wholesale revenues, non-affiliates	77	82		210	211
Wholesale revenues, affiliates	18	18		83	49
Other revenues	50	56		158	162
Total operating revenues	1,740	1,785		4,606	4,561
Operating Expenses:
Fuel	343	410		944	973
Purchased power, non-affiliates	57	63		132	139
Purchased power, affiliates	55	41		117	129
Other operations and maintenance	391	348		1,134	1,097
Depreciation and amortization	185	177		549	524
Taxes other than income taxes	93	96		284	286
Total operating expenses	1,124	1,135		3,160	3,148
Operating Income	616	650		1,446	1,413
Other Income and (Expense):
Allowance for equity funds used during construction	11	7		27	23
Interest expense, net of amounts capitalized	(76)	(77)		(229)	(224)
Other income (expense), net	(5)	(5)		(8)	(16)
Total other income and (expense)	(70)	(75)		(210)	(217)
Earnings Before Income Taxes	546	575		1,236	1,196
Income taxes	216	219		493	462
Consolidated Net Income	330	356		743	734
Less:
Dividends on preferred and preference stock of subsidiaries	5	4		14	13
Net Income	325	352		729	721
GEORGIA POWER CO
Operating Revenues:
Retail electric revenues	2,402	2,540		5,995	6,164
Wholesale revenues, non-affiliates	45	49		124	131
Wholesale revenues, affiliates	6	9		23	24
Other revenues	93	100		284	302
Total operating revenues	2,546	2,698		6,426	6,621
Operating Expenses:
Fuel	482	575		1,297	1,390
Purchased power, non-affiliates	119	102		310	277
Purchased power, affiliates	161	142		470	392
Other operations and maintenance	413	496		1,194	1,393
Depreciation and amortization	225	215		669	639
Taxes other than income taxes	112	114		311	311
Total operating expenses	1,512	1,644		4,251	4,402
Operating Income	1,034	1,054		2,175	2,219
Other Income and (Expense):
Allowance for equity funds used during construction				29	36
Interest expense, net of amounts capitalized	(105)	(98)		(310)	(290)
Other income (expense), net	5	11		41	35
Total other income and (expense)	(100)	(87)		(269)	(255)
Earnings Before Income Taxes	934	967		1,906	1,964
Income taxes	350	363		705	734
Consolidated Net Income	584	604		1,201	1,230
Less:
Dividends on preferred and preference stock of subsidiaries	4	4		13	13
Net Income	580	600		1,188	1,217
GULF POWER CO
Operating Revenues:
Retail electric revenues	375	377		972	978
Wholesale revenues, non-affiliates	14	17		44	48
Wholesale revenues, affiliates	28	23		75	59
Other revenues	20	19		53	51
Total operating revenues	437	436		1,144	1,136
Operating Expenses:
Fuel	127	141		323	342
Purchased power, non-affiliates	37	33		104	95
Purchased power, affiliates	2	3		13	9
Other operations and maintenance	81	86		252	239
Depreciation and amortization	42	49		95	129
Taxes other than income taxes	33	34		88	93
Loss on Plant Scherer Unit 3	0	0		33	0
Total operating expenses	322	346		908	907
Operating Income	115	90		236	229
Other Income and (Expense):
Interest expense, net of amounts capitalized	(13)	(11)		(37)	(36)
Other income (expense), net	1	(2)		0	(4)
Total other income and (expense)	(12)	(13)		(37)	(40)
Earnings Before Income Taxes	103	77		199	189
Income taxes	40	30		78	74
Consolidated Net Income	63	47		121	115
Less:
Dividends on preferred and preference stock of subsidiaries	0	2		4	7
Net Income	63	45		117	108
MISSISSIPPI POWER CO
Operating Revenues:
Retail electric revenues	243	263		665	652
Wholesale revenues, non-affiliates	72	78		196	198
Wholesale revenues, affiliates	21	7		40	23
Other revenues	5	4		14	12
Total operating revenues	341	352		915	885
Operating Expenses:
Fuel	120	112		301	268
Purchased power, non-affiliates	4	3		7	4
Purchased power, affiliates	2	5		13	14
Other operations and maintenance	66	74		206	211
Depreciation and amortization	39	30		120	114
Taxes other than income taxes	25	31		77	81
Estimated loss on Kemper IGCC	34	88		3,155	222
Total operating expenses	290	343		3,879	914
Operating Income	51	9		(2,964)	(29)
Other Income and (Expense):
Allowance for equity funds used during construction	1	31		72	90
Interest expense, net of amounts capitalized	13	(15)		(23)	(46)
Other income (expense), net	(1)	(1)		(3)	(4)
Total other income and (expense)	13	15		46	40
Earnings Before Income Taxes	64	24		(2,918)	11
Income taxes	24	(2)		(885)	(29)
Consolidated Net Income	40	26		(2,033)	40
Less:
Dividends on preferred and preference stock of subsidiaries	0	0		1	1
Net Income	40	26		(2,034)	39
SOUTHERN POWER CO
Operating Revenues:
Wholesale revenues, non-affiliates	510	387		1,293	866
Wholesale revenues, affiliates	105	110		295	313
Other revenues	3	3		9	10
Total operating revenues	618	500		1,597	1,189
Operating Expenses:
Fuel	189	154		460	341
Purchased power, non-affiliates	36	25		90	60
Purchased power, affiliates	7	8		23	16
Other operations and maintenance	83	81		272	246
Depreciation and amortization	131	93		379	247
Taxes other than income taxes	13	5		37	17
Total operating expenses	459	366		1,261	927
Operating Income	159	134		336	262
Other Income and (Expense):
Interest expense, net of amounts capitalized	(47)	(35)		(144)	(78)
Other income (expense), net	3	2		3	3
Total other income and (expense)	(44)	(33)		(141)	(75)
Earnings Before Income Taxes	115	101		195	187
Income taxes	(39)	(102)		(129)	(167)
Consolidated Net Income	154	203		324	354
Less:
Net income attributable to noncontrolling interests	30	27		48	39
Net Income	124	176		276	$ 315
SOUTHERN Co GAS
Operating Revenues:
Natural gas revenues	532	518		2,746
Other revenues	33	25		95
Total operating revenues	565	543		2,841
Operating Expenses:
Cost of natural gas	134	133		1,085
Cost of other sales	7	2		20
Other operations and maintenance	205	216		671
Depreciation and amortization	125	116		370
Merger-related expenses	0	35		0
Taxes other than income taxes	26	29		140
Total operating expenses	497	531		2,286
Operating Income	68	12		555
Other Income and (Expense):
Earnings from equity method investments	32	29		100
Interest expense, net of amounts capitalized	(51)	(39)		(145)
Other income (expense), net	18	9		26
Total other income and (expense)	(1)	(1)		(19)
Earnings Before Income Taxes	67	11		536
Income taxes	52	7		233
Consolidated Net Income	15	4		303
Less:
Net income attributable to noncontrolling interests	0	0		0
Net Income	$ 15	$ 4		$ 303
Predecessor | SOUTHERN Co GAS
Operating Revenues:
Natural gas revenues			$ 1,841
Other revenues			64
Total operating revenues			1,905
Operating Expenses:
Cost of natural gas			755
Cost of other sales			14
Other operations and maintenance			454
Depreciation and amortization			206
Merger-related expenses			56
Taxes other than income taxes			99
Total operating expenses			1,584
Operating Income			321
Other Income and (Expense):
Earnings from equity method investments			2
Interest expense, net of amounts capitalized			(96)
Other income (expense), net			5
Total other income and (expense)			(89)
Earnings Before Income Taxes			232
Income taxes			87
Consolidated Net Income			145
Less:
Net income attributable to noncontrolling interests			14
Net Income			$ 131

Condensed Consolidated Stateme3

Condensed Consolidated Statements of Income (Unaudited) - Parenthetical - SOUTHERN Co GAS - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017
Excise taxes collected	$ 9	$ 9		$ 75
Predecessor
Excise taxes collected			$ 57

Condensed Consolidated Stateme4

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income	$ 1,109	$ 1,177		$ 427	$ 2,324
Qualifying hedges:
Changes in fair value, net of tax	25	19		54	(118)
Reclassification adjustment for amounts included in net income, net of tax	(17)	2		(59)	20
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, net of tax	1	1		3	3
Total other comprehensive income (loss)	9	22		(2)	(95)
Comprehensive Income	1,118	1,199		425	2,229
Less:
Dividends on preferred and preference stock of subsidiaries	10	11		32	34
Comprehensive income attributable to noncontrolling interests	30	27		48	39
Comprehensive Income (Loss) Attributable to Parent	1,078	1,161		345	2,156
ALABAMA POWER CO
Net Income	330	356		743	734
Qualifying hedges:
Changes in fair value, net of tax	0	0		0	(2)
Reclassification adjustment for amounts included in net income, net of tax	1	1		3	3
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	1	1		3	1
Comprehensive Income	331	357		746	735
Less:
Dividends on preferred and preference stock of subsidiaries	5	4		14	13
GEORGIA POWER CO
Net Income	584	604		1,201	1,230
Qualifying hedges:
Reclassification adjustment for amounts included in net income, net of tax	1	1		2	2
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	1	1		2	2
Comprehensive Income	585	605		1,203	1,232
Less:
Dividends on preferred and preference stock of subsidiaries	4	4		13	13
GULF POWER CO
Net Income	63	47		121	115
Qualifying hedges:
Changes in fair value, net of tax	0	0		(1)	(4)
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	0	0		(1)	(4)
Comprehensive Income	63	47		120	111
Less:
Dividends on preferred and preference stock of subsidiaries	0	2		4	7
MISSISSIPPI POWER CO
Net Income	40	26		(2,033)	40
Qualifying hedges:
Changes in fair value, net of tax	(1)	0		0	(1)
Reclassification adjustment for amounts included in net income, net of tax	0	0		1	1
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	(1)	0		1	0
Comprehensive Income	39	26		(2,032)	40
Less:
Dividends on preferred and preference stock of subsidiaries	0	0		1	1
SOUTHERN POWER CO
Net Income	154	203		324	354
Qualifying hedges:
Changes in fair value, net of tax	25	23		58	(1)
Reclassification adjustment for amounts included in net income, net of tax	(20)	(1)		(68)	13
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	5	22		(10)	12
Comprehensive Income	159	225		314	366
Less:
Comprehensive income attributable to noncontrolling interests	30	27		48	39
Comprehensive Income (Loss) Attributable to Parent	129	198		266	$ 327
SOUTHERN Co GAS
Net Income	15	4		303
Qualifying hedges:
Changes in fair value, net of tax	0	(3)		(3)
Reclassification adjustment for amounts included in net income, net of tax	0	0		0
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, net of tax	0	0		0
Total other comprehensive income (loss)	0	(3)		(3)
Comprehensive Income	15	1		300
Less:
Comprehensive income attributable to noncontrolling interests	0	0		0
Comprehensive Income (Loss) Attributable to Parent	$ 15	$ 1		$ 300
Predecessor | SOUTHERN Co GAS
Net Income			$ 145
Qualifying hedges:
Changes in fair value, net of tax			(41)
Reclassification adjustment for amounts included in net income, net of tax			1
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, net of tax			5
Total other comprehensive income (loss)			(35)
Comprehensive Income			110
Less:
Comprehensive income attributable to noncontrolling interests			14
Comprehensive Income (Loss) Attributable to Parent			$ 96

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in fair value, tax	$ 15	$ 12		$ 32	$ (74)
Reclassification adjustment for qualified hedges, tax	(10)	2		(36)	13
Reclassification adjustment for pension and other post retirement benefit plans, tax	1	1		2	2
ALABAMA POWER CO
Changes in fair value, tax	0	0		0	(1)
Reclassification adjustment for qualified hedges, tax	1	1		2	2
GEORGIA POWER CO
Reclassification adjustment for qualified hedges, tax	0	0		1	1
GULF POWER CO
Changes in fair value, tax	0	0		(1)	(3)
MISSISSIPPI POWER CO
Changes in fair value, tax	0	0		0	0
Reclassification adjustment for qualified hedges, tax	0	0		0	0
SOUTHERN POWER CO
Changes in fair value, tax	15	14		35	(1)
Reclassification adjustment for qualified hedges, tax	(12)	(1)		(42)	$ 7
SOUTHERN Co GAS
Changes in fair value, tax	0	(2)		(2)
Reclassification adjustment for qualified hedges, tax	0	0		0
Reclassification adjustment for pension and other post retirement benefit plans, tax	$ 0	$ 0		$ (1)
Predecessor | SOUTHERN Co GAS
Changes in fair value, tax			$ (23)
Reclassification adjustment for qualified hedges, tax			0
Reclassification adjustment for pension and other post retirement benefit plans, tax			$ 4

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating Activities:
Net Income	$ 427,000,000	$ 2,324,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	2,564,000,000	2,109,000,000
Deferred income taxes	15,000,000	(22,000,000)
Allowance for equity funds used during construction	(133,000,000)	(150,000,000)
Pension, postretirement, and other employee benefits	(64,000,000)	(158,000,000)
Settlement of asset retirement obligations	(137,000,000)	(117,000,000)
Hedge settlements	0	(236,000,000)
Estimated loss on Kemper IGCC	3,148,000,000	222,000,000
Other, net	(8,000,000)	(1,000,000)
Changes in certain current assets and liabilities —
-Receivables	426,000,000	(458,000,000)
-Fossil fuel stock	59,000,000	204,000,000
-Natural gas for sale, net of temporary LIFO liquidation	0	(222,000,000)
-Other current assets	(164,000,000)	(112,000,000)
-Accounts payable	(467,000,000)	(9,000,000)
-Accrued taxes	157,000,000	1,062,000,000
-Accrued compensation	(230,000,000)	(122,000,000)
-Retail fuel cost over recovery	(211,000,000)	(106,000,000)
-Other current liabilities	(129,000,000)	88,000,000
Net cash provided from operating activities	5,253,000,000	4,296,000,000
Investing Activities:
Business acquisitions, net of cash acquired	(1,032,000,000)	(9,513,000,000)
Property additions	(5,242,000,000)	(5,252,000,000)
Investment in restricted cash	(16,000,000)	(750,000,000)
Distribution of restricted cash	33,000,000	746,000,000
Nuclear decommissioning trust fund purchases	(585,000,000)	(838,000,000)
Nuclear decommissioning trust fund sales	580,000,000	832,000,000
Cost of removal, net of salvage	(208,000,000)	(155,000,000)
Change in construction payables, net	120,000,000	(259,000,000)
Investment in unconsolidated subsidiaries	(134,000,000)	(1,421,000,000)
Payments pursuant to LTSAs	(189,000,000)	(125,000,000)
Other investing activities	(14,000,000)	95,000,000
Net cash used for investing activities	(6,687,000,000)	(16,640,000,000)
Financing Activities:
Increase (decrease) in notes payable, net	(515,000,000)	655,000,000
Proceeds —
Long-term debt	4,068,000,000	14,091,000,000
Common stock	613,000,000	3,265,000,000
Preferred stock	250,000,000	0
Short-term borrowings	1,263,000,000	0
Redemptions and repurchases —
Long-term debt	(1,981,000,000)	(2,405,000,000)
Preferred and preference stock	(150,000,000)	0
Short-term borrowings	(409,000,000)	(475,000,000)
Distributions to noncontrolling interests	(89,000,000)	(22,000,000)
Capital contributions from noncontrolling interests	79,000,000	367,000,000
Purchase of membership interests from noncontrolling interests	0	(129,000,000)
Payment of common stock dividends	(1,716,000,000)	(1,553,000,000)
Other financing activities	(113,000,000)	(185,000,000)
Net cash provided from financing activities	1,300,000,000	13,609,000,000
Net Change in Cash and Cash Equivalents	(134,000,000)	1,265,000,000
Cash and Cash Equivalents at Beginning of Period	1,975,000,000	1,404,000,000
Cash and Cash Equivalents at End of Period	1,841,000,000	2,669,000,000
Cash paid (received) during the period for —
Interest	1,286,000,000	766,000,000
Income taxes, net	(187,000,000)	(151,000,000)
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	805,000,000	578,000,000
ALABAMA POWER CO
Operating Activities:
Net Income	743,000,000	734,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	666,000,000	634,000,000
Deferred income taxes	260,000,000	267,000,000
Allowance for equity funds used during construction	(27,000,000)	(23,000,000)
Pension, postretirement, and other employee benefits	(4,000,000)	(14,000,000)
Other, net	43,000,000	(12,000,000)
Changes in certain current assets and liabilities —
-Receivables	(163,000,000)	(4,000,000)
-Fossil fuel stock	34,000,000	18,000,000
-Other current assets	(58,000,000)	(46,000,000)
-Accounts payable	(125,000,000)	(113,000,000)
-Accrued taxes	159,000,000	207,000,000
-Accrued compensation	(48,000,000)	(22,000,000)
-Retail fuel cost over recovery	(76,000,000)	(104,000,000)
-Other current liabilities	7,000,000	19,000,000
Net cash provided from operating activities	1,411,000,000	1,541,000,000
Investing Activities:
Property additions	(1,211,000,000)	(947,000,000)
Nuclear decommissioning trust fund purchases	(174,000,000)	(275,000,000)
Nuclear decommissioning trust fund sales	174,000,000	275,000,000
Cost of removal, net of salvage	(82,000,000)	(70,000,000)
Change in construction payables, net	105,000,000	(37,000,000)
Other investing activities	(29,000,000)	(28,000,000)
Net cash used for investing activities	(1,217,000,000)	(1,082,000,000)
Proceeds —
Senior notes	550,000,000	400,000,000
Capital contributions from parent company	337,000,000	253,000,000
Preferred stock	250,000,000	0
Other long-term debt	0	45,000,000
Redemptions and repurchases —
Pollution control revenue bonds	(36,000,000)	0
Senior notes	(200,000,000)	(200,000,000)
Payment of common stock dividends	(536,000,000)	(574,000,000)
Other financing activities	(26,000,000)	(21,000,000)
Net cash provided from financing activities	339,000,000	(97,000,000)
Net Change in Cash and Cash Equivalents	533,000,000	362,000,000
Cash and Cash Equivalents at Beginning of Period	420,000,000	194,000,000
Cash and Cash Equivalents at End of Period	953,000,000	556,000,000
Cash paid (received) during the period for —
Interest	217,000,000	215,000,000
Income taxes, net	146,000,000	(70,000,000)
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	189,000,000	84,000,000
GEORGIA POWER CO
Operating Activities:
Net Income	1,201,000,000	1,230,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	821,000,000	794,000,000
Deferred income taxes	328,000,000	346,000,000
Allowance for equity funds used during construction	(29,000,000)	(36,000,000)
Deferred expenses	(30,000,000)	(40,000,000)
Pension, postretirement, and other employee benefits	(42,000,000)	(14,000,000)
Settlement of asset retirement obligations	(95,000,000)	(93,000,000)
Other, net	(21,000,000)	7,000,000
Changes in certain current assets and liabilities —
-Receivables	(254,000,000)	(162,000,000)
-Fossil fuel stock	(2,000,000)	128,000,000
-Other current assets	(29,000,000)	62,000,000
-Accounts payable	(161,000,000)	39,000,000
-Accrued taxes	(52,000,000)	(22,000,000)
-Accrued compensation	(60,000,000)	(26,000,000)
-Retail fuel cost over recovery	(84,000,000)	9,000,000
-Other current liabilities	(11,000,000)	44,000,000
Net cash provided from operating activities	1,480,000,000	2,266,000,000
Investing Activities:
Property additions	(1,907,000,000)	(1,566,000,000)
Nuclear decommissioning trust fund purchases	(411,000,000)	(563,000,000)
Nuclear decommissioning trust fund sales	406,000,000	558,000,000
Cost of removal, net of salvage	(54,000,000)	(45,000,000)
Change in construction payables, net of joint owner portion	180,000,000	(139,000,000)
Payments pursuant to LTSAs	(59,000,000)	(27,000,000)
Sale of property	63,000,000	10,000,000
Other investing activities	(52,000,000)	14,000,000
Net cash used for investing activities	(1,834,000,000)	(1,758,000,000)
Financing Activities:
Increase (decrease) in notes payable, net	(391,000,000)	(63,000,000)
Proceeds —
Senior notes	1,350,000,000	650,000,000
FFB loan	0	300,000,000
Capital contributions from parent company	412,000,000	294,000,000
Short-term borrowings	700,000,000	0
Other long-term debt	370,000,000	0
Redemptions and repurchases —
Pollution control revenue bonds	(65,000,000)	(4,000,000)
Senior notes	(450,000,000)	(700,000,000)
Short-term borrowings	(300,000,000)	0
Payment of common stock dividends	(961,000,000)	(979,000,000)
Other financing activities	(48,000,000)	(26,000,000)
Net cash provided from financing activities	617,000,000	(528,000,000)
Net Change in Cash and Cash Equivalents	263,000,000	(20,000,000)
Cash and Cash Equivalents at Beginning of Period	3,000,000	67,000,000
Cash and Cash Equivalents at End of Period	266,000,000	47,000,000
Cash paid (received) during the period for —
Interest	284,000,000	277,000,000
Income taxes, net	369,000,000	188,000,000
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	470,000,000	226,000,000
GULF POWER CO
Operating Activities:
Net Income	121,000,000	115,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	100,000,000	134,000,000
Deferred income taxes	57,000,000	15,000,000
Loss on Plant Scherer Unit 3	33,000,000	0
Other, net	(5,000,000)	(2,000,000)
Changes in certain current assets and liabilities —
-Receivables	(65,000,000)	(9,000,000)
-Fossil fuel stock	7,000,000	49,000,000
-Other current assets	11,000,000	3,000,000
-Accrued taxes	21,000,000	40,000,000
-Accrued compensation	(10,000,000)	(5,000,000)
-Over recovered regulatory clause revenues	(8,000,000)	26,000,000
-Other current liabilities	10,000,000	8,000,000
Net cash provided from operating activities	272,000,000	374,000,000
Investing Activities:
Property additions	(142,000,000)	(106,000,000)
Cost of removal, net of salvage	(16,000,000)	(8,000,000)
Change in construction payables, net	(9,000,000)	(7,000,000)
Other investing activities	(6,000,000)	(6,000,000)
Net cash used for investing activities	(173,000,000)	(127,000,000)
Financing Activities:
Increase (decrease) in notes payable, net	(268,000,000)	(42,000,000)
Proceeds —
Senior notes	300,000,000	0
Capital contributions from parent company	7,000,000	10,000,000
Common stock	175,000,000	0
Redemptions and repurchases —
Preferred and preference stock	(150,000,000)	0
Senior notes	(85,000,000)	(125,000,000)
Payment of common stock dividends	(94,000,000)	(90,000,000)
Other financing activities	(3,000,000)	(5,000,000)
Net cash provided from financing activities	(118,000,000)	(252,000,000)
Net Change in Cash and Cash Equivalents	(19,000,000)	(5,000,000)
Cash and Cash Equivalents at Beginning of Period	56,000,000	74,000,000
Cash and Cash Equivalents at End of Period	37,000,000	69,000,000
Cash paid (received) during the period for —
Interest	24,000,000	29,000,000
Income taxes, net	19,000,000	14,000,000
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	25,000,000	13,000,000
MISSISSIPPI POWER CO
Operating Activities:
Net Income	(2,033,000,000)	40,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	144,000,000	115,000,000
Deferred income taxes	(1,159,000,000)	34,000,000
Allowance for equity funds used during construction	(72,000,000)	(90,000,000)
Estimated loss on Kemper IGCC	3,148,000,000	222,000,000
Other, net	(26,000,000)	(1,000,000)
Changes in certain current assets and liabilities —
-Receivables	438,000,000	3,000,000
-Fossil fuel stock	21,000,000	8,000,000
-Other current assets	(9,000,000)	34,000,000
-Accounts payable	(21,000,000)	5,000,000
-Accrued taxes	20,000,000	96,000,000
-Accrued compensation	(12,000,000)	(5,000,000)
-Over recovered regulatory clause revenues	(47,000,000)	(20,000,000)
-Customer liability associated with Kemper refunds	0	(73,000,000)
-Other current liabilities	(31,000,000)	5,000,000
Net cash provided from operating activities	361,000,000	373,000,000
Investing Activities:
Property additions	(411,000,000)	(592,000,000)
Change in construction payables, net	(47,000,000)	(25,000,000)
Government grant proceeds	0	137,000,000
Other investing activities	(25,000,000)	(29,000,000)
Net cash used for investing activities	(483,000,000)	(509,000,000)
Financing Activities:
Increase (decrease) in notes payable, net	(23,000,000)	0
Proceeds —
Capital contributions from parent company	1,002,000,000	227,000,000
Short-term borrowings	113,000,000	0
Long-term debt to parent company	40,000,000	200,000,000
Other long-term debt	0	900,000,000
Redemptions and repurchases —
Short-term borrowings	(109,000,000)	(475,000,000)
Long-term debt to parent company	(591,000,000)	(225,000,000)
Other long-term debt	(300,000,000)	(425,000,000)
Other financing activities	(3,000,000)	(5,000,000)
Net cash provided from financing activities	129,000,000	197,000,000
Net Change in Cash and Cash Equivalents	7,000,000	61,000,000
Cash and Cash Equivalents at Beginning of Period	224,000,000	98,000,000
Cash and Cash Equivalents at End of Period	231,000,000	159,000,000
Cash paid (received) during the period for —
Interest	45,000,000	36,000,000
Income taxes, net	(209,000,000)	(231,000,000)
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	32,000,000	80,000,000
SOUTHERN POWER CO
Operating Activities:
Net Income	324,000,000	354,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	404,000,000	262,000,000
Deferred income taxes	240,000,000	(668,000,000)
Amortization of investment tax credits	(42,000,000)	(25,000,000)
Collateral deposits	(1,000,000)	(80,000,000)
Income taxes receivable, non-current	(42,000,000)	0
Other, net	(2,000,000)	19,000,000
Changes in certain current assets and liabilities —
-Receivables	(77,000,000)	(82,000,000)
-Other current assets	38,000,000	(15,000,000)
-Accounts payable	(31,000,000)	7,000,000
-Accrued taxes	79,000,000	483,000,000
-Other current liabilities	5,000,000	14,000,000
Net cash provided from operating activities	895,000,000	269,000,000
Investing Activities:
Business acquisitions, net of cash acquired	(1,032,000,000)	(1,134,000,000)
Property additions	(218,000,000)	(1,702,000,000)
Investment in restricted cash	(16,000,000)	(750,000,000)
Distribution of restricted cash	33,000,000	746,000,000
Change in construction payables, net	(166,000,000)	(69,000,000)
Payments pursuant to LTSAs	(99,000,000)	(58,000,000)
Other investing activities	7,000,000	(41,000,000)
Net cash used for investing activities	(1,491,000,000)	(3,008,000,000)
Financing Activities:
Increase (decrease) in notes payable, net	(89,000,000)	692,000,000
Proceeds —
Senior notes	0	1,531,000,000
Capital contributions from parent company	0	800,000,000
Other long-term debt	43,000,000	63,000,000
Redemptions and repurchases —
Other long-term debt	(4,000,000)	(84,000,000)
Distributions to noncontrolling interests	(89,000,000)	(22,000,000)
Capital contributions from noncontrolling interests	79,000,000	367,000,000
Purchase of membership interests from noncontrolling interests	0	(129,000,000)
Payment of common stock dividends	(238,000,000)	(204,000,000)
Other financing activities	(27,000,000)	(14,000,000)
Net cash provided from financing activities	(325,000,000)	3,000,000,000
Net Change in Cash and Cash Equivalents	(921,000,000)	261,000,000
Cash and Cash Equivalents at Beginning of Period	1,099,000,000	830,000,000
Cash and Cash Equivalents at End of Period	178,000,000	1,091,000,000
Cash paid (received) during the period for —
Interest	144,000,000	49,000,000
Income taxes, net	(343,000,000)	71,000,000
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	16,000,000	210,000,000
SOUTHERN Co GAS
Operating Activities:
Net Income	303,000,000
Adjustments to reconcile consolidated net income to net cash provided from operating activities —
Depreciation and amortization, total	370,000,000
Deferred income taxes	265,000,000
Pension, postretirement, and other employee benefits	(4,000,000)
Stock based compensation expense	25,000,000
Hedge settlements	0
Mark-to-market adjustments	(32,000,000)
Other, net	(67,000,000)
Changes in certain current assets and liabilities —
-Receivables	534,000,000
-Natural gas for sale, net of temporary LIFO liquidation	0
-Prepaid income taxes	(7,000,000)
-Other current assets	(42,000,000)
-Accounts payable	(169,000,000)
-Accrued taxes	(24,000,000)
-Accrued compensation	(11,000,000)
-Other current liabilities	8,000,000
Net cash provided from operating activities	1,149,000,000
Investing Activities:
Property additions	(1,093,000,000)
Cost of removal, net of salvage	(45,000,000)
Change in construction payables, net	49,000,000
Investment in unconsolidated subsidiaries	(128,000,000)
Returned investment in unconsolidated subsidiaries	22,000,000
Other investing activities	3,000,000
Net cash used for investing activities	(1,192,000,000)
Financing Activities:
Increase (decrease) in notes payable, net	(323,000,000)
First mortgage bonds	0
Proceeds —
First mortgage bonds	200,000,000
Senior notes	450,000,000
Capital contributions from parent company	79,000,000
Redemptions and repurchases —
Long-term debt	(22,000,000)
Senior notes	0
Distributions to noncontrolling interests	0
Payment of common stock dividends	(332,000,000)
Other financing activities	(7,000,000)
Net cash provided from financing activities	45,000,000
Net Change in Cash and Cash Equivalents	2,000,000
Cash and Cash Equivalents at Beginning of Period	19,000,000
Cash and Cash Equivalents at End of Period	21,000,000	48,000,000
Cash paid (received) during the period for —
Interest	146,000,000
Income taxes, net	17,000,000
Noncash Investing and Financing Items [Abstract]
Noncash transactions — Accrued property additions at end of period	$ 112,000,000
Predecessor | SOUTHERN Co GAS
Redemptions and repurchases —
Cash and Cash Equivalents at Beginning of Period		$ 19,000,000

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net cash paid for capitalized interest			$ 72	$ 94
ALABAMA POWER CO
Net cash paid for capitalized interest			10	8
GEORGIA POWER CO
Net cash paid for capitalized interest			17	15
GULF POWER CO
Net cash paid for capitalized interest			0	0
MISSISSIPPI POWER CO
Interest paid			73	72
Net cash paid for capitalized interest			28	36
SOUTHERN POWER CO
Net cash paid for capitalized interest			7	$ 32
SOUTHERN Co GAS
Net cash paid for capitalized interest	$ 2		$ 9
Predecessor | SOUTHERN Co GAS
Net cash paid for capitalized interest		$ 3

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 1,841	$ 1,975
Receivables —
Customer accounts receivable	1,744	1,583
Energy marketing receivables	427	623
Unbilled revenues	595	706
Under recovered fuel clause revenues	62	0
Income taxes receivable, current	138	544
Other accounts and notes receivable	578	377
Accumulated provision for uncollectible accounts	(43)	(43)
Materials and supplies	1,499	1,462
Fossil fuel for generation	571	689
Natural gas for sale	631	631
Prepaid expenses	365	364
Other regulatory assets, current	585	581
Other current assets	209	230
Total current assets	9,202	9,722
Property, Plant, and Equipment:
In service	102,014	98,416
Less: Accumulated depreciation	31,164	29,852
Plant in service, net of depreciation	70,850	68,564
Nuclear fuel, at amortized cost	865	905
Construction work in progress	8,026	8,977
Total property, plant, and equipment	79,741	78,446
Other Property and Investments:
Goodwill	6,267	6,251
Equity investments in unconsolidated subsidiaries	1,637	1,549
Other intangible assets, net of amortization	902	970
Nuclear decommissioning trusts, at fair value	1,783	1,606
Leveraged leases	788	774
Miscellaneous property and investments	236	270
Total other property and investments	11,613	11,420
Deferred Charges and Other Assets:
Deferred charges related to income taxes	1,318	1,629
Unamortized loss on reacquired debt	210	223
Other regulatory assets, deferred	6,718	6,851
Other deferred charges and assets	1,513	1,406
Total deferred charges and other assets	9,759	10,109
Total Assets	110,315	109,697
Current Liabilities:
Securities due within one year	3,505	2,587
Notes payable	2,579	2,241
Energy marketing trade payables	451	597
Accounts payable —
Accounts payable	2,353	2,228
Customer deposits	550	558
Accrued taxes —
Accrued income taxes	176	193
Unrecognized tax benefits	17	385
Other accrued taxes	690	667
Accrued interest	443	518
Accrued compensation	703	915
Asset retirement obligations, current	245	378
Acquisitions payable	0	489
Other regulatory liabilities, current	139	236
Other current liabilities	752	925
Total current liabilities	12,603	12,917
Long-term Debt	44,042	42,629
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	14,321	14,092
Accumulated deferred ITCs	2,290	2,228
Employee benefit obligations	2,139	2,299
Asset retirement obligations, deferred	4,356	4,136
Other cost of removal obligations	2,708	2,748
Other regulatory liabilities, deferred	449	476
Other deferred credits and liabilities	1,048	1,278
Total deferred credits and other liabilities	27,311	27,257
Total Liabilities	83,956	82,803
Redeemable Preferred Stock of Subsidiaries	361	118
Redeemable Noncontrolling Interests	59	164
Common Stockholders' Equity:
Par value	5,018	4,952
Paid-in capital	10,300	9,661
Treasury, at cost	(35)	(31)
Retained earnings (accumulated deficit)	8,981	10,356
Accumulated other comprehensive income (loss)	(182)	(180)
Total Common Stockholders' Equity	24,082	24,758
Preferred and Preference Stock of Subsidiaries	462	609
Noncontrolling Interests	1,395	1,245
Total Stockholders' Equity	25,939	26,612
Total Liabilities and Stockholders' Equity	110,315	109,697
ALABAMA POWER CO
Current Assets:
Cash and cash equivalents	953	420
Receivables —
Customer accounts receivable	428	348
Unbilled revenues	149	146
Other accounts and notes receivable	47	27
Affiliated	45	40
Accumulated provision for uncollectible accounts	(8)	(10)
Materials and supplies	455	435
Fossil fuel for generation	171	205
Prepaid expenses	58	34
Other regulatory assets, current	122	149
Other current assets	5	11
Total current assets	2,425	1,805
Property, Plant, and Equipment:
In service	26,619	26,031
Less: Accumulated depreciation	9,463	9,112
Plant in service, net of depreciation	17,156	16,919
Nuclear fuel, at amortized cost	314	336
Construction work in progress	928	491
Total property, plant, and equipment	18,398	17,746
Other Property and Investments:
Equity investments in unconsolidated subsidiaries	65	66
Nuclear decommissioning trusts, at fair value	869	792
Miscellaneous property and investments	121	112
Total other property and investments	1,055	970
Deferred Charges and Other Assets:
Deferred charges related to income taxes	525	525
Deferred under recovered regulatory clause revenues	17	150
Other regulatory assets, deferred	1,191	1,157
Other deferred charges and assets	178	163
Total deferred charges and other assets	1,911	1,995
Total Assets	23,789	22,516
Current Liabilities:
Securities due within one year	325	561
Accounts payable —
Affiliated	275	297
Other	376	433
Customer deposits	92	88
Accrued taxes —
Accrued income taxes	115	45
Other accrued taxes	128	42
Accrued interest	75	78
Accrued compensation	151	193
Other regulatory liabilities, current	4	85
Other current liabilities	50	76
Total current liabilities	1,591	1,898
Long-term Debt	7,083	6,535
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	4,919	4,654
Deferred credits related to income taxes	60	65
Accumulated deferred ITCs	118	110
Employee benefit obligations	289	300
Asset retirement obligations, deferred	1,564	1,503
Other cost of removal obligations	630	684
Other regulatory liabilities, deferred	93	100
Other deferred credits and liabilities	51	63
Total deferred credits and other liabilities	7,724	7,479
Total Liabilities	16,398	15,912
Redeemable Preferred Stock	329	85
Preference Stock	196	196
Common Stockholders' Equity:
Par value	1,222	1,222
Paid-in capital	2,961	2,613
Retained earnings (accumulated deficit)	2,711	2,518
Accumulated other comprehensive income (loss)	(28)	(30)
Total Common Stockholders' Equity	6,866	6,323
Total Liabilities and Stockholders' Equity	23,789	22,516
GEORGIA POWER CO
Current Assets:
Cash and cash equivalents	266	3
Receivables —
Customer accounts receivable	670	523
Unbilled revenues	276	224
Under recovered fuel clause revenues	62	0
Joint owner accounts receivable	222	57
Other accounts and notes receivable	82	81
Affiliated	21	18
Accumulated provision for uncollectible accounts	(3)	(3)
Materials and supplies	480	479
Fossil fuel for generation	300	298
Prepaid expenses	82	105
Other regulatory assets, current	200	193
Other current assets	27	38
Total current assets	2,685	2,016
Property, Plant, and Equipment:
In service	34,589	33,841
Less: Accumulated depreciation	11,655	11,317
Plant in service, net of depreciation	22,934	22,524
Nuclear fuel, at amortized cost	551	569
Construction work in progress	5,751	4,939
Total property, plant, and equipment	29,236	28,032
Other Property and Investments:
Equity investments in unconsolidated subsidiaries	53	60
Nuclear decommissioning trusts, at fair value	914	814
Miscellaneous property and investments	51	46
Total other property and investments	1,018	920
Deferred Charges and Other Assets:
Deferred charges related to income taxes	669	676
Other regulatory assets, deferred	2,890	2,774
Other deferred charges and assets	608	417
Total deferred charges and other assets	4,167	3,867
Total Assets	37,106	34,835
Current Liabilities:
Securities due within one year	261	460
Notes payable	400	391
Accounts payable —
Affiliated	396	438
Other	1,012	589
Customer deposits	270	265
Accrued taxes —
Accrued income taxes	353	407
Accrued interest	121	106
Accrued compensation	164	224
Asset retirement obligations, current	214	299
Other current liabilities	192	297
Total current liabilities	3,383	3,476
Long-term Debt	11,610	10,225
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	6,328	6,000
Accumulated deferred ITCs	248	256
Employee benefit obligations	665	703
Asset retirement obligations, deferred	2,367	2,233
Other deferred credits and liabilities	232	320
Total deferred credits and other liabilities	9,840	9,512
Total Liabilities	24,833	23,213
Redeemable Preferred Stock	45	45
Preference Stock	221	221
Common Stockholders' Equity:
Par value	398	398
Paid-in capital	7,308	6,885
Retained earnings (accumulated deficit)	4,311	4,086
Accumulated other comprehensive income (loss)	(10)	(13)
Total Common Stockholders' Equity	12,007	11,356
Total Liabilities and Stockholders' Equity	37,106	34,835
GULF POWER CO
Current Assets:
Cash and cash equivalents	37	56
Receivables —
Customer accounts receivable	96	72
Unbilled revenues	68	55
Under recovered fuel clause revenues	15	17
Income taxes receivable, current	15	0
Other accounts and notes receivable	12	6
Affiliated	13	17
Accumulated provision for uncollectible accounts	(1)	(1)
Materials and supplies	58	55
Fossil fuel for generation	64	71
Other regulatory assets, current	55	44
Other current assets	15	30
Total current assets	447	422
Property, Plant, and Equipment:
In service	5,181	5,140
Less: Accumulated depreciation	1,457	1,382
Plant in service, net of depreciation	3,724	3,758
Construction work in progress	75	51
Total property, plant, and equipment	3,799	3,809
Deferred Charges and Other Assets:
Deferred charges related to income taxes	56	58
Other regulatory assets, deferred	499	512
Other deferred charges and assets	22	21
Total deferred charges and other assets	577	591
Total Assets	4,823	4,822
Current Liabilities:
Securities due within one year	7	87
Notes payable	0	268
Accounts payable —
Affiliated	46	59
Other	55	54
Customer deposits	35	35
Accrued taxes —
Other accrued taxes	41	20
Accrued interest	20	8
Accrued compensation	30	40
Deferred capacity expense, current	22	22
Other regulatory liabilities, current	1	16
Other current liabilities	37	40
Total current liabilities	294	649
Long-term Debt	1,285	987
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	1,003	948
Employee benefit obligations	90	96
Deferred capacity expense	103	119
Asset retirement obligations, deferred	125	120
Other cost of removal obligations	218	249
Other regulatory liabilities, deferred	45	47
Other deferred credits and liabilities	71	71
Total deferred credits and other liabilities	1,655	1,650
Total Liabilities	3,234	3,286
Preference Stock	0	147
Common Stockholders' Equity:
Par value	678	503
Paid-in capital	600	589
Retained earnings (accumulated deficit)	312	296
Accumulated other comprehensive income (loss)	(1)	1
Total Common Stockholders' Equity	1,589	1,389
Total Liabilities and Stockholders' Equity	4,823	4,822
MISSISSIPPI POWER CO
Current Assets:
Cash and cash equivalents	231	224
Receivables —
Customer accounts receivable	38	29
Unbilled revenues	41	42
Income taxes receivable, current	102	544
Other accounts and notes receivable	15	14
Affiliated	15	15
Materials and supplies	45	76
Fossil fuel for generation	20	100
Other regulatory assets, current	113	115
Other current assets	8	8
Total current assets	628	1,167
Property, Plant, and Equipment:
In service	4,836	4,865
Less: Accumulated depreciation	1,312	1,289
Plant in service, net of depreciation	3,524	3,576
Construction work in progress	75	2,545
Total property, plant, and equipment	3,599	6,121
Other Property and Investments:
Total other property and investments	28	12
Deferred Charges and Other Assets:
Deferred charges related to income taxes	62	361
Other regulatory assets, deferred	436	518
Accumulated deferred income taxes	279	0
Other deferred charges and assets	23	56
Total deferred charges and other assets	800	935
Total Assets	5,055	8,235
Current Liabilities:
Securities due within one year - parent	0	551
Securities due within one year	1,028	78
Notes payable	4	23
Accounts payable —
Affiliated	56	62
Other	82	135
Customer deposits	16	16
Accrued taxes —
Unrecognized tax benefits	2	383
Other accrued taxes	78	99
Accrued interest	16	46
Accrued compensation	29	42
Asset retirement obligations, current	15	32
Over recovered fuel clause liabilities	4	51
Other current liabilities	67	20
Total current liabilities	1,397	1,538
Long-term Debt	1,167	2,424
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	0	756
Employee benefit obligations	109	115
Asset retirement obligations, deferred	150	146
Other cost of removal obligations	175	170
Other regulatory liabilities, deferred	87	84
Other deferred credits and liabilities	23	26
Total deferred credits and other liabilities	544	1,297
Total Liabilities	3,108	5,259
Redeemable Preferred Stock	33	33
Common Stockholders' Equity:
Par value	38	38
Paid-in capital	4,529	3,525
Retained earnings (accumulated deficit)	(2,650)	(616)
Accumulated other comprehensive income (loss)	(3)	(4)
Total Common Stockholders' Equity	1,914	2,943
Total Liabilities and Stockholders' Equity	5,055	8,235
SOUTHERN POWER CO
Current Assets:
Cash and cash equivalents	178	1,099
Receivables —
Customer accounts receivable	148	102
Other accounts and notes receivable	61	34
Affiliated	74	57
Materials and supplies	351	337
Fossil fuel for generation	15	15
Prepaid income taxes	51	74
Other current assets	26	39
Total current assets	904	1,757
Property, Plant, and Equipment:
In service	13,734	12,728
Less: Accumulated depreciation	1,823	1,484
Plant in service, net of depreciation	11,911	11,244
Construction work in progress	425	398
Total property, plant, and equipment	12,336	11,642
Other Property and Investments:
Goodwill	2	2
Intangible assets, net (including goodwill)	417	436
Total other property and investments	417	436
Deferred Charges and Other Assets:
Prepaid LTSAs	77	101
Accumulated deferred income taxes	400	594
Income taxes receivable, non-current	53	11
Other deferred charges and assets — affiliated	6	13
Other deferred charges and assets	455	615
Total deferred charges and other assets	991	1,334
Total Assets	14,648	15,169
Current Liabilities:
Securities due within one year	864	560
Notes payable	120	209
Accounts payable —
Affiliated	93	88
Other	84	278
Accrued taxes —
Accrued income taxes	101	148
Other accrued taxes	30	7
Accrued interest	36	36
Acquisitions payable	0	461
Contingent consideration	15	46
Other current liabilities	58	70
Total current liabilities	1,401	1,903
Long-term Debt	4,946	5,068
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	191	152
Accumulated deferred ITCs	1,900	1,839
Asset retirement obligations, deferred	76	64
Other deferred credits and liabilities	232	304
Total deferred credits and other liabilities	2,399	2,359
Total Liabilities	8,746	9,330
Redeemable Noncontrolling Interests	59	164
Common Stockholders' Equity:
Par value	0	0
Paid-in capital	3,661	3,671
Retained earnings (accumulated deficit)	762	724
Accumulated other comprehensive income (loss)	25	35
Total Common Stockholders' Equity	4,448	4,430
Noncontrolling interests	1,395	1,245
Total Stockholders' Equity	5,843	5,675
Total Liabilities and Stockholders' Equity	14,648	15,169
SOUTHERN Co GAS
Current Assets:
Cash and cash equivalents	21	19
Receivables —
Customer accounts receivable	221	364
Energy marketing receivables	427	623
Unbilled revenues	61	239
Other accounts and notes receivable	61	76
Accumulated provision for uncollectible accounts	(26)	(27)
Materials and supplies	24	26
Natural gas for sale	631	631
Prepaid expenses	103	80
Assets from risk management activities, net of collateral	103	128
Other regulatory assets, current	96	81
Other current assets	25	10
Total current assets	1,747	2,250
Property, Plant, and Equipment:
In service	15,383	14,508
Less: Accumulated depreciation	4,567	4,439
Plant in service, net of depreciation	10,816	10,069
Construction work in progress	596	496
Total property, plant, and equipment	11,412	10,565
Other Property and Investments:
Goodwill	5,967	5,967
Equity investments in unconsolidated subsidiaries	1,609	1,541
Other intangible assets, net of amortization	300	366
Miscellaneous property and investments	21	21
Total other property and investments	7,897	7,895
Deferred Charges and Other Assets:
Other regulatory assets, deferred	944	973
Other deferred charges and assets	190	170
Total deferred charges and other assets	1,134	1,143
Total Assets	22,190	21,853
Current Liabilities:
Securities due within one year	0	22
Notes payable	934	1,257
Energy marketing trade payables	451	597
Accounts payable —
Accounts payable	368	348
Customer deposits	137	153
Accrued taxes —
Accrued income taxes	0	26
Other accrued taxes	70	68
Accrued interest	66	48
Accrued compensation	46	58
Liabilities from risk management activities, net of collateral	28	62
Other regulatory liabilities, current	126	102
Accrued environmental remediation, current	54	69
Other current liabilities	112	108
Total current liabilities	2,392	2,918
Long-term Debt	5,862	5,259
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	2,214	1,975
Employee benefit obligations	431	441
Accrued environmental remediation	345	357
Other cost of removal obligations	1,656	1,616
Other regulatory liabilities, deferred	35	51
Other deferred credits and liabilities	88	127
Total deferred credits and other liabilities	4,769	4,567
Total Liabilities	13,023	12,744
Common Stockholders' Equity:
Par value	0	0
Paid-in capital	9,185	9,095
Retained earnings (accumulated deficit)	(41)	(12)
Accumulated other comprehensive income (loss)	23	26
Total Stockholders' Equity	9,167	9,109
Total Liabilities and Stockholders' Equity	$ 22,190	$ 21,853

Condensed Consolidated Balance9

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Other intangible assets, accumulated amortization	$ 156	$ 62
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	1,500,000,000	1,500,000,000
Common stock, shares issued (in shares)	1,000,000,000	991,000,000
Treasury shares at cost (in shares)	900,000	800,000
ALABAMA POWER CO
Common stock, par value (in dollars per share)	$ 40	$ 40
Common stock, shares authorized (in shares)	40,000,000	40,000,000
Common stock, shares outstanding (in shares)	30,537,500	30,537,500
GEORGIA POWER CO
Common stock, shares authorized (in shares)	20,000,000	20,000,000
Common stock, shares outstanding (in shares)	9,261,500	9,261,500
GULF POWER CO
Common stock, shares authorized (in shares)	20,000,000	20,000,000
Common stock, shares outstanding (in shares)	7,392,717	5,642,717
MISSISSIPPI POWER CO
Common stock, shares authorized (in shares)	1,130,000	1,130,000
Common stock, shares outstanding (in shares)	1,121,000	1,121,000
SOUTHERN POWER CO
Other intangible assets, accumulated amortization	$ 41	$ 22
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	1,000,000	1,000,000
Common stock, shares outstanding (in shares)	1,000	1,000
SOUTHERN Co GAS
Other intangible assets, accumulated amortization	$ 100	$ 34
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares outstanding (in shares)	100	100

Introduction

Introduction	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
INTRODUCTION	INTRODUCTION The condensed quarterly financial statements of each registrant included herein have been prepared by such registrant, without audit, pursuant to the rules and regulations of the SEC. The Condensed Balance Sheets as of December 31, 2016 have been derived from the audited financial statements of each registrant. In the opinion of each registrant's management, the information regarding such registrant furnished herein reflects all adjustments, which, except as otherwise disclosed, are of a normal recurring nature, necessary to present fairly the results of operations for the periods ended September 30, 2017 and 2016 . Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations, although each registrant believes that the disclosures regarding such registrant are adequate to make the information presented not misleading. Disclosures which would substantially duplicate the disclosures in the Form 10-K and details which have not changed significantly in amount or composition since the filing of the Form 10-K are generally omitted from this Quarterly Report on Form 10-Q unless specifically required by GAAP. Therefore, these Condensed Financial Statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K. Due to the seasonal variations in the demand for energy, operating results for the periods presented are not necessarily indicative of the operating results to be expected for the full year. Southern Company's financial statements reflect its investments in its subsidiaries, including Southern Company Gas as a result of the Merger, on a consolidated basis. Southern Company Gas' results of operations and cash flows for the three and nine months ended September 30, 2017 and the three months ended September 30, 2016 , as well as its financial condition as of September 30, 2017 and December 31, 2016 , are reflected within Southern Company's consolidated amounts in these accompanying notes herein. The equity method is used for entities in which Southern Company has significant influence but does not control, including Southern Company Gas' investment in SNG, and for variable interest entities where Southern Company has an equity investment but is not the primary beneficiary. See Note (I) under " Southern Company – Merger with Southern Company Gas " for additional information regarding the Merger. Pursuant to the Merger, Southern Company pushed down the application of the acquisition method of accounting to the consolidated financial statements of Southern Company Gas such that the assets and liabilities are recorded at their respective fair values, and goodwill has been established for the excess of the purchase price over the fair value of net identifiable assets. Accordingly, the consolidated financial statements of Southern Company Gas for periods before and after July 1, 2016 (acquisition date) reflect different bases of accounting, and the financial positions and results of operations of those periods are not comparable. Throughout Southern Company Gas' condensed consolidated financial statements and the accompanying notes herein, periods prior to July 1, 2016 are identified as "predecessor," while periods after the acquisition date are identified as "successor." Certain prior year data presented in the financial statements have been reclassified to conform to the current year presentation. These reclassifications had no impact on the results of operations, financial position, or cash flows of any registrant. Recently Issued Accounting Standards See Note 1 to the financial statements of the registrants under "Recently Issued Accounting Standards" in Item 8 of the Form 10-K for additional information. In 2014, the FASB issued ASC 606, Revenue from Contracts with Customers (ASC 606), replacing the existing accounting standard and industry specific guidance for revenue recognition with a five-step model for recognizing and measuring revenue from contracts with customers. The underlying principle of the standard is to recognize revenue to depict the transfer of goods or services to customers at the amount expected to be collected. The new standard also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenue and the related cash flows arising from contracts with customers. While the registrants expect most of their revenue to be included in the scope of ASC 606, they have not fully completed the evaluation of all revenue arrangements. The majority of Southern Company's, the traditional electric operating companies', and Southern Company Gas' revenue, including energy provided to customers, is from tariff offerings that provide electricity or natural gas without a defined contractual term, as well as longer-term contractual commitments , including PPAs and non-derivative natural gas asset management and optimization arrangements . The majority of Southern Power's revenues includes longer-term PPAs for generation capacity and energy. The registrants expect the adoption of ASC 606 will not result in a significant shift from the current timing of revenue recognition for such transactions . The registrants' ongoing evaluation of other revenue streams and related contracts includes unregulated sales to customers. Some revenue arrangements, such as certain PPAs , energy-related derivatives , and alternative revenue programs, are excluded from the scope of ASC 606 and, therefore, will be accounted for and disclosed or presented separately from revenues under ASC 606 on the registrants' financial statements. In addition, the power and utilities industry continues to evaluate other specific industry issues, including the applicability of ASC 606 to contributions in aid of construction (CIAC). Although final implementation guidance has not been issued, Southern Company, the traditional electric operating companies, and Southern Company Gas expect CIAC to be out of the scope of ASC 606. The new standard is effective for interim and annual reporting periods beginning after December 15, 2017. The registrants intend to use the modified retrospective method of adoption effective January 1, 2018. The registrants have also elected to utilize practical expedients which allow them to apply the standard to open contracts at the date of adoption and to reflect the aggregate effect of all modifications when identifying performance obligations and allocating the transaction price for contracts modified before the effective date. Under the modified retrospective method of adoption, prior year reported results are not restated; however, a cumulative-effect adjustment to retained earnings at January 1, 2018 is recorded. In addition, disclosures will include comparative information on 2018 financial statement line items under current guidance. While the adoption of ASC 606, including the cumulative-effect adjustment, is not expected to have a material impact on either the timing or amount of revenues recognized in the registrants' financial statements, the registrants will continue to evaluate the requirements, as well as any additional clarifying guidance that may be issued. On January 26, 2017, the FASB issued ASU No. 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment (ASU 2017-04). ASU 2017-04 removes the requirement to compare the implied fair value of goodwill with the carrying amount as part of Step 2 of the goodwill impairment test. Under the new standard, the goodwill impairment loss will be measured as the excess of a reporting unit's carrying amount over its fair value, not exceeding the total amount of goodwill allocated to that reporting unit, which may increase the frequency of goodwill impairment charges if a future goodwill impairment test does not pass the Step 1 evaluation. ASU 2017-04 is effective prospectively for annual and interim periods beginning on or after December 15, 2019, and early adoption is permitted on testing dates after January 1, 2017. Southern Company and Southern Company Gas are evaluating the standard and expect to early adopt ASU 2017-04 effective January 1, 2018. On March 10, 2017, the FASB issued ASU No. 2017-07, Compensation – Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost (ASU 2017-07). ASU 2017-07 requires that an employer report the service cost component in the same line item or items as other compensation costs and requires the other components of net periodic pension and postretirement benefit costs to be separately presented in the income statement outside income from operations. Additionally, only the service cost component is eligible for capitalization, when applicable. However, all cost components remain eligible for capitalization under FERC regulations. ASU 2017-07 will be applied retrospectively for the presentation of the service cost component and the other components of net periodic pension and postretirement benefit costs in the income statement. The capitalization of the service cost component of net periodic pension and postretirement benefit costs in assets will be applied on a prospective basis. ASU 2017-07 is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. The presentation changes required for net periodic pension and postretirement benefit costs will result in a decrease in Southern Company's , the traditional electric operating companies', and Southern Company Gas' operating income and an increase in other income for 2016 and 2017 and are expected to result in a decrease in operating income and an increase in other income for 2018. The adoption of ASU 2017-07 is not expected to have a material impact on Southern Company's , the traditional electric operating companies', or Southern Company Gas' financial statements. On August 28, 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities (ASU 2017-12), amending the hedge accounting recognition and presentation requirements. ASU 2017-12 makes more financial and non-financial hedging strategies eligible for hedge accounting, amends the related presentation and disclosure requirements, and simplifies hedge effectiveness assessment requirements. ASU 2017-12 is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years, with early adoption permitted. The registrants are evaluating the standard and expect to early adopt ASU 2017-12 effective January 1, 2018. The adoption of ASU 2017-12 is not expected to have a material impact on the registrants' financial statements. Affiliate Transactions Prior to the completion of Southern Company Gas' acquisition of its 50% equity interest in SNG, SCS (as agent for Alabama Power, Georgia Power, and Southern Power) and Southern Company Gas had entered into long-term interstate natural gas transportation agreements with SNG. The interstate transportation service provided to Alabama Power, Georgia Power, Southern Power, and Southern Company Gas by SNG pursuant to these agreements is governed by the terms and conditions of SNG's natural gas tariff and is subject to FERC regulation. For the nine months ended September 30, 2017 , transportation costs under these agreements for Alabama Power, Georgia Power, Southern Power, and Southern Company Gas were approximately $8 million , $77 million , $19 million , and $24 million , respectively. For the period subsequent to Southern Company Gas' investment in SNG through September 30, 2016 , transportation costs under these agreements for Alabama Power, Georgia Power, Southern Power, and Southern Company Gas were approximately $1 million , $8 million , $2 million , and $4 million , respectively. SCS, as agent for Georgia Power and Southern Power, has agreements with certain subsidiaries of Southern Company Gas to purchase natural gas. For the nine months ended September 30, 2017 , natural gas purchases made by Georgia Power and Southern Power from Southern Company Gas' subsidiaries were approximately $18 million and $94 million , respectively. For the period subsequent to Southern Company's acquisition of Southern Company Gas through September 30, 2016 , natural gas purchases made by Georgia Power and Southern Power from Southern Company Gas' subsidiaries were approximately $7 million and $2 million , respectively. Goodwill and Other Intangible Assets At September 30, 2017 and December 31, 2016 , goodwill was as follows: Goodwill At September 30, 2017 At December 31, 2016 (in millions) Southern Company $ 6,267 $ 6,251 Southern Power $ 2 $ 2 Southern Company Gas Gas distribution operations $ 4,702 $ 4,702 Gas marketing services 1,265 1,265 Southern Company Gas total $ 5,967 $ 5,967 Goodwill is not amortized, but is subject to an annual impairment test during the fourth quarter of each year, or more frequently if impairment indicators arise. Other intangible assets were as follows: At September 30, 2017 At December 31, 2016 Gross Carrying Amount Accumulated Amortization Other Intangible Assets, Net Gross Carrying Amount Accumulated Amortization Other (in millions) (in millions) Southern Company Other intangible assets subject to amortization: Customer relationships $ 288 $ (70 ) $ 218 $ 268 $ (32 ) $ 236 Trade names 159 (15 ) 144 158 (5 ) 153 Storage and transportation contracts 64 (27 ) 37 64 (2 ) 62 PPA fair value adjustments 456 (41 ) 415 456 (22 ) 434 Other 16 (3 ) 13 11 (1 ) 10 Total other intangible assets subject to amortization $ 983 $ (156 ) $ 827 $ 957 $ (62 ) $ 895 Other intangible assets not subject to amortization: Federal Communications Commission licenses $ 75 $ — $ 75 $ 75 $ — $ 75 Total other intangible assets $ 1,058 $ (156 ) $ 902 $ 1,032 $ (62 ) $ 970 Southern Power Other intangible assets subject to amortization: PPA fair value adjustments $ 456 $ (41 ) $ 415 $ 456 $ (22 ) $ 434 Southern Company Gas Other intangible assets subject to amortization: Gas marketing services Customer relationships $ 221 $ (65 ) $ 156 $ 221 $ (30 ) $ 191 Trade names 115 (8 ) 107 115 (2 ) 113 Wholesale gas services Storage and transportation contracts 64 (27 ) 37 64 (2 ) 62 Total other intangible assets subject to amortization $ 400 $ (100 ) $ 300 $ 400 $ (34 ) $ 366 Amortization associated with other intangible assets was as follows: Three Months Ended Nine Months Ended September 30, 2017 (in millions) Southern Company $ 29 $ 94 Southern Power $ 6 $ 19 Southern Company Gas $ 20 $ 66 See Note 12 to the financial statements of Southern Company under "Southern Power" and Note 2 to the financial statements of Southern Power in Item 8 of the Form 10-K for additional information regarding Southern Power's PPA fair value adjustments related to its business acquisitions. Also see Note (I) under " Southern Company – Acquisition of PowerSecure " and " – Merger with Southern Company Gas " for additional information. Property Damage Reserve See Note 1 to the financial statements of Gulf Power under "Property Damage Reserve" in Item 8 of the Form 10-K for additional information. Gulf Power's cost of repairing damages from major storms and other uninsured property damages, including uninsured damages to transmission and distribution facilities, generation facilities, and other property is charged to Gulf Power's property damage reserve. In accordance with a settlement agreement approved by the Florida PSC on April 4, 2017 (2017 Rate Case Settlement Agreement), Gulf Power suspended further property damage reserve accruals effective April 2017. Gulf Power may make discretionary accruals and is required to resume accruals of $3.5 million annually if the reserve balance falls below zero . In addition, Gulf Power may initiate a storm surcharge to recover costs associated with any tropical systems named by the National Hurricane Center or other catastrophic storm events that reduce the property damage reserve in the aggregate by approximately $31 million ( 75% of the April 1, 2017 balance) or more. The storm surcharge would begin, on an interim basis, 60 days following the filing of a cost recovery petition, would be limited to $4.00 /month for a 1,000 KWH residential customer unless Gulf Power incurs in excess of $100 million in qualified storm recovery costs in a calendar year, and would replenish the property damage reserve to approximately $40 million . As of September 30, 2017 , Gulf Power's property damage reserve totaled approximately $39 million . See Note (B) under " Regulatory Matters – Gulf Power – Retail Base Rate Cases " for additional details regarding the 2017 Rate Case Settlement Agreement. Natural Gas for Sale Southern Company Gas' natural gas distribution utilities, with the exception of Nicor Gas, carry natural gas inventory on a WACOG basis. Nicor Gas' natural gas inventory is carried at cost on a LIFO basis. Inventory decrements occurring during the year that are restored prior to year end are charged to cost of natural gas at the estimated annual replacement cost. Inventory decrements that are not restored prior to year end are charged to cost of natural gas at the actual LIFO cost of the inventory layers liquidated. Southern Company Gas had no inventory decrement at September 30, 2017 . The cost of natural gas, including inventory costs, is recovered from customers under a purchased gas recovery mechanism adjusted for differences between actual costs and amounts billed; therefore, LIFO liquidations have no impact on Southern Company's or Southern Company Gas' net income. Natural gas inventories for Southern Company Gas' non-utility businesses are carried at the lower of weighted average cost or current market price, with cost determined on a WACOG basis. For any declines in market prices below the WACOG considered to be other than temporary, an adjustment is recorded to reduce the value of natural gas inventories to market value. Southern Company Gas had no material LOCOM adjustment in any period presented.

Contingencies and Regulatory Ma

Contingencies and Regulatory Matters	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
CONTINGENCIES AND REGULATORY MATTERS	CONTINGENCIES AND REGULATORY MATTERS See Note 3 to the financial statements of the registrants in Item 8 of the Form 10-K for information relating to various lawsuits, other contingencies, and regulatory matters. General Litigation Matters Each registrant is subject to certain claims and legal actions arising in the ordinary course of business. In addition, the business activities of Southern Company's subsidiaries are subject to extensive governmental regulation related to public health and the environment, such as regulation of air emissions and water discharges. Litigation over environmental issues and claims of various types, including property damage, personal injury, common law nuisance, and citizen enforcement of environmental requirements such as air quality and water standards, has occurred throughout the U.S. This litigation has included claims for damages alleged to have been caused by CO 2 and other emissions, CCR, and alleged exposure to hazardous materials, and/or requests for injunctive relief in connection with such matters. The ultimate outcome of such pending or potential litigation against each registrant and any subsidiaries cannot be predicted at this time; however, for current proceedings not specifically reported herein, management does not anticipate that the ultimate liabilities, if any, arising from such current proceedings would have a material effect on such registrant's financial statements. Southern Company On January 20, 2017, a purported securities class action complaint was filed against Southern Company, certain of its officers, and certain former Mississippi Power officers in the U.S. District Court for the Northern District of Georgia, Atlanta Division, by Monroe County Employees' Retirement System on behalf of all persons who purchased shares of Southern Company's common stock between April 25, 2012 and October 29, 2013. The complaint alleges that Southern Company, certain of its officers, and certain former Mississippi Power officers made materially false and misleading statements regarding the Kemper IGCC in violation of certain provisions under the Securities Exchange Act of 1934, as amended. The complaint seeks, among other things, compensatory damages and litigation costs and attorneys' fees. On June 12, 2017, the plaintiffs filed an amended complaint that provided additional detail about their claims, increased the purported class period by one day, and added certain other former Mississippi Power officers as defendants. On July 27, 2017, the defendants filed a motion to dismiss the plaintiffs' amended complaint with prejudice, to which the plaintiffs filed an opposition on September 11, 2017. On February 27, 2017, Jean Vineyard filed a shareholder derivative lawsuit in the U.S. District Court for the Northern District of Georgia that names as defendants Southern Company, certain of its directors, certain of its officers, and certain former Mississippi Power officers. The complaint alleges that the defendants caused Southern Company to make false or misleading statements regarding the Kemper IGCC cost and schedule. Further, the complaint alleges that the defendants were unjustly enriched and caused the waste of corporate assets. The plaintiff seeks to recover, on behalf of Southern Company, unspecified actual damages and, on her own behalf, attorneys' fees and costs in bringing the lawsuit. The plaintiff also seeks certain changes to Southern Company's corporate governance and internal processes. On March 27, 2017, the court deferred this lawsuit until 30 days after certain further action in the purported securities class action complaint discussed above. On May 15, 2017, Helen E. Piper Survivor's Trust filed a shareholder derivative lawsuit in the Superior Court of Gwinnett County, State of Georgia and, on May 31, 2017, Judy Mesirov filed a shareholder derivative lawsuit in the U.S. District Court for the Northern District of Georgia. Each of these lawsuits names as defendants Southern Company, certain of its directors, certain of its officers, and certain former Mississippi Power officers. Each complaint alleges that the individual defendants, among other things, breached their fiduciary duties in connection with schedule delays and cost overruns associated with the construction of the Kemper IGCC. Each complaint further alleges that the individual defendants authorized or failed to correct false and misleading statements regarding the Kemper IGCC schedule and cost and failed to implement necessary internal controls to prevent harm to Southern Company. Each plaintiff seeks to recover, on behalf of Southern Company, unspecified actual damages and disgorgement of profits and, on its behalf, attorneys' fees and costs in bringing the lawsuit. Each plaintiff also seeks certain unspecified changes to Southern Company's corporate governance and internal processes. On August 15, 2017, these two shareholder derivative lawsuits were consolidated in the U.S. District Court for the Northern District of Georgia and the court deferred the consolidated case until 30 days after certain further action in the purported securities class action complaint discussed above. Southern Company believes these legal challenges have no merit; however, an adverse outcome in any of these proceedings could have an impact on Southern Company's results of operations, financial condition, and liquidity. Southern Company will vigorously defend itself in these matters, the ultimate outcome of which cannot be determined at this time. Georgia Power In 2011, plaintiffs filed a putative class action against Georgia Power in the Superior Court of Fulton County, Georgia alleging that Georgia Power's collection in rates of municipal franchise fees (all of which are remitted to municipalities) exceeded the amounts allowed in orders of the Georgia PSC and alleging certain state tort law claims. In November 2016, the Georgia Court of Appeals reversed the trial court's previous dismissal of the case and remanded the case to the trial court for further proceedings. Georgia Power filed a petition for writ of certiorari with the Georgia Supreme Court, which was granted on August 28, 2017. A decision from the Georgia Supreme Court is not expected until 2018. Georgia Power believes the plaintiffs' claims have no merit and intends to vigorously defend itself in this matter. The ultimate outcome of this matter cannot be determined at this time. Southern Power During 2015, Southern Power indirectly acquired a 51% membership interest in RE Roserock LLC (Roserock), the owner of the Roserock facility in Pecos County, Texas, which was under construction by Recurrent Energy, LLC and was subsequently placed in service in November 2016. Prior to placing the facility in service, certain solar panels were damaged during installation. While the facility currently is generating energy consistent with operational expectations and PPA obligations, Southern Power is pursuing remedies under its insurance policies and other contracts to repair or replace these solar panels. In connection therewith, Southern Power is withholding payments of approximately $26 million from the construction contractor, who has placed a lien on the Roserock facility for the same amount. The amounts withheld are included in other accounts and notes payable and other current liabilities on Southern Company's consolidated balance sheets and other accounts payable and other current liabilities on Southern Power's consolidated balance sheets. On May 18, 2017, Roserock filed a lawsuit in the state district court in Pecos County, Texas, against X.L. America, Inc. (XL) and North American Elite Insurance Company (North American Elite) seeking recovery from an insurance policy for damages resulting from a hail storm and certain installation practices by the construction contractor, McCarthy Building Companies, Inc. (McCarthy). On May 19, 2017, Roserock filed a separate lawsuit against McCarthy in the state district court in Travis County, Texas alleging breach of contract and breach of warranty for the damages sustained at the Roserock facility, which has since been moved to the U.S. District Court for the Western District of Texas. On May 22, 2017, McCarthy filed a counter lawsuit against Roserock, Array Technologies, Inc., Canadian Solar (USA), Inc., XL, and North American Elite in the U.S. District Court for the Western District of Texas alleging, among other things, breach of contract, and requesting foreclosure of mechanic's liens against Roserock. On July 18, 2017, the U.S. District Court for the Western District of Texas consolidated the two pending lawsuits. Southern Power intends to vigorously pursue and defend these matters, the ultimate outcome of which cannot be determined at this time. Southern Company Gas Nicor Gas and Nicor Energy Services Company, wholly-owned subsidiaries of Southern Company Gas, and Nicor Inc. were defendants in a putative class action initially filed in 2011 in the state court in Cook County, Illinois. The plaintiffs purported to represent a class of the customers who purchased the Gas Line Comfort Guard product from Nicor Energy Services Company and variously alleged that the marketing, sale, and billing of the Gas Line Comfort Guard product violated the Illinois Consumer Fraud and Deceptive Business Practices Act, constituting common law fraud and resulting in unjust enrichment of these entities. The plaintiffs sought, on behalf of the classes they purported to represent, actual and punitive damages, interest, costs, attorney fees, and injunctive relief. On February 8, 2017, the judge denied the plaintiffs' motion for class certification and Southern Company Gas' motion for summary judgment. On March 7, 2017, the parties reached a settlement, which was finalized and effective on April 3, 2017. The settlement did not have a material impact on Southern Company's or Southern Company Gas' financial statements. Environmental Matters Environmental Remediation The Southern Company system must comply with environmental laws and regulations that cover the handling and disposal of waste and releases of hazardous substances. Under these various laws and regulations, the Southern Company system could incur substantial costs to clean up affected sites. The traditional electric operating companies and the natural gas distribution utilities in Illinois, New Jersey, Georgia, and Florida have each received authority from their respective state PSCs or other applicable state regulatory agencies to recover approved environmental compliance costs through regulatory mechanisms. These regulatory mechanisms are adjusted annually or as necessary within limits approved by the state PSCs or other applicable state regulatory agencies. Georgia Power's environmental remediation liability was $26 million and $17 million as of September 30, 2017 and December 31, 2016, respectively. Georgia Power has been designated or identified as a potentially responsible party at sites governed by the Georgia Hazardous Site Response Act and/or by the federal Comprehensive Environmental Response, Compensation, and Liability Act, and assessment and potential cleanup of such sites is expected. Gulf Power's environmental remediation liability includes estimated costs of environmental remediation projects of approximately $53 million and $44 million as of September 30, 2017 and December 31, 2016, respectively. These estimated costs primarily relate to site closure criteria by the Florida Department of Environmental Protection (FDEP) for potential impacts to soil and groundwater from herbicide applications at Gulf Power's substations. The schedule for completion of the remediation projects is subject to FDEP approval. The projects have been approved by the Florida PSC for recovery through Gulf Power's environmental cost recovery clause; therefore, these liabilities have no impact on net income. Southern Company Gas' environmental remediation liability was $399 million and $426 million as of September 30, 2017 and December 31, 2016, respectively, based on the estimated cost of environmental investigation and remediation associated with known current and former manufactured gas plant operating sites. These environmental remediation expenditures are recoverable from customers through rate mechanisms approved by the applicable state regulatory agencies of the natural gas distribution utilities, with the exception of one site representing $5 million of the total accrued remediation costs. The final outcome of these matters cannot be determined at this time. However, the final disposition of these matters is not expected to have a material impact on the financial statements of Southern Company, Georgia Power, Gulf Power, or Southern Company Gas. Natural Gas Storage A wholly-owned subsidiary of Southern Company Gas owns and operates a natural gas storage facility consisting of two salt dome caverns in Louisiana. Periodic integrity tests are required in accordance with rules of the Louisiana Department of Natural Resources (LDNR). In August 2017, in connection with an ongoing integrity project, updated seismic mapping indicated the proximity of one of the caverns to the edge of the salt dome may be less than the required minimum and could result in Southern Company Gas retiring the cavern early. At September 30, 2017, the facility's property, plant, and equipment had a net book value of $111 million , of which the cavern itself represents approximately 20% . A potential early retirement of this cavern is dependent upon several factors including the results of ongoing third-party technical engineering reviews, testing, and compliance with an order from the LDNR detailing the requirements to place the cavern back in service, which includes, among other things, obtaining a core sample to determine the composition of the sheath surrounding the edge of the salt dome. Early retirement of the cavern could trigger impairment of other long-lived assets associated with the natural gas storage facility. The ultimate outcome of this matter cannot be determined at this time, but could have a significant impact on Southern Company's financial statements and a material impact on Southern Company Gas' financial statements. Nuclear Fuel Disposal Costs See Note 3 to the financial statements of Southern Company, Alabama Power, and Georgia Power under "Nuclear Fuel Disposal Costs" in Item 8 of the Form 10-K for additional information regarding legal remedies pursued by Alabama Power and Georgia Power against the U.S. government for its partial breach of contract relating to the disposal of spent nuclear fuel and high level radioactive waste generated at each company's nuclear plants. On October 10, 2017, Alabama Power and Georgia Power filed additional lawsuits against the U.S. government in the Court of Federal Claims for the costs of continuing to store spent nuclear fuel at Plant Farley, Plant Hatch, and Plant Vogtle Units 1 and 2 for the period from January 1, 2015 through December 31, 2017. Damages will continue to accumulate until the issue is resolved or storage is provided. No amounts have been recognized in the financial statements as of September 30, 2017 for any potential recoveries from the additional lawsuits. The final outcome of these matters cannot be determined at this time; however, no material impact on Southern Company's, Alabama Power's, or Georgia Power's net income is expected. FERC Matters Municipal and Rural Associations Tariff See Note 3 to the financial statements of Mississippi Power under "FERC Matters" in Item 8 of the Form 10-K for additional information regarding a settlement agreement entered into by Mississippi Power regarding the establishment of a regulatory asset for Kemper IGCC-related costs. See " Integrated Coal Gasification Combined Cycle " herein for information regarding the Kemper IGCC. In March 2016, Mississippi Power reached a settlement agreement with its wholesale customers, which was subsequently approved by the FERC, for an increase in wholesale base revenues under the MRA cost-based electric tariff, primarily as a result of placing scrubbers for Plant Daniel Units 1 and 2 in service in 2015. The settlement agreement became effective for services rendered beginning May 1, 2016, resulting in an estimated annual revenue increase of $7 million under the MRA cost-based electric tariff. Additionally, under the settlement agreement, the tariff customers agreed to similar regulatory treatment for MRA tariff ratemaking as the treatment approved for retail ratemaking through an order issued by the Mississippi PSC in December 2015 (In-Service Asset Rate Order). This regulatory treatment primarily includes (i) recovery of the Kemper IGCC assets currently operational and providing service to customers and other related costs, (ii) amortization of the Kemper IGCC-related regulatory assets included in rates under the settlement agreement over the 36 months ending April 30, 2019, (iii) Kemper IGCC-related expenses included in rates under the settlement agreement no longer being deferred and charged to expense, and (iv) removing all of the Kemper IGCC CWIP from rate base with a corresponding increase in accrual of AFUDC. The additional resulting AFUDC totaled approximately $22 million through the suspension of Kemper IGCC start-up activities. See " Integrated Coal Gasification Combined Cycle " herein for additional information. Fuel Cost Recovery Mississippi Power has a wholesale MRA and a Market Based (MB) fuel cost recovery factor. At September 30, 2017 , the amount of over-recovered wholesale MRA fuel costs included in the balance sheets was $3 million compared to $13 million at December 31, 2016 . Over-recovered wholesale MB fuel costs included in the balance sheets were immaterial at September 30, 2017 and December 31, 2016 . See Note 3 to the financial statements of Mississippi Power under "FERC Matters – Fuel Cost Recovery" in Item 8 of the Form 10-K for additional information. Market-Based Rate Authority See Note 3 to the financial statements of Southern Company and Mississippi Power under "FERC Matters – Market-Based Rate Authority" and Note 3 to the financial statements of Alabama Power, Georgia Power, Gulf Power, and Southern Power under "FERC Matters" in Item 8 of the Form 10-K for additional information regarding the traditional electric operating companies' and Southern Power's market power proceeding and amendment to their market-rate tariff. On May 17, 2017, the FERC accepted the traditional electric operating companies' and Southern Power's compliance filing accepting the terms of the FERC's February 2, 2017 order regarding an amendment by the traditional electric operating companies and Southern Power to their market-based rate tariff. While the FERC's order references the traditional electric operating companies' and Southern Power's market power proceeding related to their 2014 triennial updated market power analysis, that proceeding remains a separate, ongoing matter. On October 25, 2017, the FERC issued an order in response to the traditional electric operating companies' and Southern Power's June 30, 2017 triennial updated market power analysis. The FERC directed the traditional electric operating companies and Southern Power to show cause within 60 days why market-based rate authority should not be revoked in certain areas adjacent to the area presently under mitigation in accordance with the February 2, 2017 order, or to provide a mitigation plan to further address market power concerns. The traditional electric operating companies and Southern Power expect to make a filing within the specified 60 days responding to the FERC's order. The ultimate outcome of these matters cannot be determined at this time. Regulatory Matters Alabama Power See Note 3 to the financial statements of Southern Company and Alabama Power under "Regulatory Matters – Alabama Power" and "Retail Regulatory Matters," respectively, in Item 8 of the Form 10-K for additional information regarding Alabama Power's recovery of retail costs through various regulatory clauses and accounting orders. The balance of each regulatory clause recovery on the balance sheet follows: Regulatory Clause Balance Sheet Line Item September 30, December 31, (in millions) Rate CNP Compliance Deferred over recovered regulatory clause revenues $ 9 $ — Rate CNP Compliance (*) Deferred under recovered regulatory clause revenues — 9 Rate CNP PPA Deferred under recovered regulatory clause revenues 17 142 Retail Energy Cost Recovery (*) Other regulatory liabilities, current — 76 Natural Disaster Reserve Other regulatory liabilities, deferred 51 69 (*) In accordance with an accounting order issued on February 17, 2017 by the Alabama PSC, Alabama Power reclassified $36 million of its under recovered balance for Rate CNP Compliance and $11 million of its under recovered balance for Retail Energy Cost Recovery to a deferred regulatory asset account. Georgia Power Rate Plans See Note 3 to the financial statements of Southern Company and Georgia Power under "Regulatory Matters – Georgia Power – Rate Plans" and "Retail Regulatory Matters – Rate Plans," respectively, in Item 8 of the Form 10-K for additional information. Georgia Power's revenues from regulated retail operations are collected through various rate mechanisms subject to the oversight of the Georgia PSC. Georgia Power currently recovers its costs from the regulated retail business through the 2013 ARP, which includes traditional base tariff rates, Demand-Side Management tariffs, Environmental Compliance Cost Recovery tariffs, and Municipal Franchise Fee tariffs. In addition, financing costs related to the construction of Plant Vogtle Units 3 and 4 are being collected through the NCCR tariff and fuel costs are collected through a separate fuel cost recovery tariff. See " Nuclear Construction " herein and Note 3 to the financial statements of Southern Company under "Regulatory Matters – Georgia Power – Nuclear Construction" and Georgia Power under "Retail Regulatory Matters – Nuclear Construction" in Item 8 of the Form 10-K for additional information regarding the NCCR tariff. Also see " Fuel Cost Recovery " herein and Note 3 to the financial statements of Southern Company under "Regulatory Matters – Georgia Power – Fuel Cost Recovery" and Georgia Power under "Retail Regulatory Matters – Fuel Cost Recovery" in Item 8 of the Form 10-K for additional information regarding fuel cost recovery. Integrated Resource Plan See Note 3 to the financial statements of Southern Company and Georgia Power under "Regulatory Matters – Georgia Power – Integrated Resource Plan" and "Retail Regulatory Matters – Integrated Resource Plan," respectively, in Item 8 of the Form 10-K for additional information regarding Georgia Power's triennial Integrated Resource Plan. On March 7, 2017, the Georgia PSC approved Georgia Power's decision to suspend work at a future generation site in Stewart County, Georgia, due to changing economics, including load forecasts and lower fuel costs. The timing of recovery for costs incurred of approximately $50 million will be determined by the Georgia PSC in a future base rate case. The ultimate outcome of this matter cannot be determined at this time. Fuel Cost Recovery See Note 3 to the financial statements of Southern Company and Georgia Power under "Regulatory Matters – Georgia Power – Fuel Cost Recovery" and "Retail Regulatory Matters – Fuel Cost Recovery," respectively, in Item 8 of the Form 10-K for additional information. As of September 30, 2017 , Georgia Power's under recovered fuel balance totaled $100 million and is included in current assets and other deferred charges and assets on Southern Company's and Georgia Power's condensed balance sheets. As of December 31, 2016 , Georgia Power's over recovered fuel balance totaled $84 million and is included in other current liabilities on Southern Company's and Georgia Power's condensed balance sheets. Fuel cost recovery revenues are adjusted for differences in actual recoverable fuel costs and amounts billed in current regulated rates. Accordingly, changes in the billing factor will not have a significant effect on Southern Company's or Georgia Power's revenues or net income, but will affect cash flow. Storm Damage Recovery Georgia Power is accruing $30 million annually through December 31, 2019, as provided in the 2013 ARP, for incremental operating and maintenance costs of damage from major storms to its transmission and distribution facilities. During September 2017, Hurricane Irma caused significant damage to Georg ia Power's transmission and distribution facilities. The total amount of incremental restoration costs related to this hurricane is estimated to be approximately $150 million . As of September 30, 2017 , Georgia Power had deferred approximately $145 million in a regulatory asset related to storm damage. As of September 30, 2017, the total balance in Georgia Power's regulatory asset related to storm damage was $360 million . The rate of storm damage cost recovery is expected to be adjusted as part of Georgia Power's next base rate case required to be filed by July 1, 2019. As a result of this regulatory treatment, costs related to storms are not expected to have a material impact on Southern Company's or Georgia Power's financial statements. See Note 3 to the financial statements of Southern Company under "Regulatory Matters – Georgia Power – Storm Damage Recovery" and Note 1 to the financial statements of Georgia Power under "Storm Damage Recovery" in Item 8 of the Form 10-K for additional information regarding Georgia Power's storm damage reserve. Nuclear Construction See Note 3 to the financial statements of Southern Company and Georgia Power under "Regulatory Matters – Georgia Power – Nuclear Construction" and "Retail Regulatory Matters – Nuclear Construction," respectively, in Item 8 of the Form 10-K for additional information regarding Georgia Power's construction of Plant Vogtle Units 3 and 4, Vogtle Construction Monitoring (VCM) reports, the NCCR tariff, and the Contractor Settlement Agreement. Vogtle 3 and 4 Agreement and EPC Contractor Bankruptcy In 2008, Georgia Power, acting for itself and as agent for the Vogtle Owners, entered into the Vogtle 3 and 4 Agreement, pursuant to which the EPC Contractor agreed to design, engineer, procure, construct, and test Plant Vogtle Units 3 and 4. Under the terms of the Vogtle 3 and 4 Agreement, the Vogtle Owners agreed to pay a purchase price subject to certain price escalations and adjustments, including fixed escalation amounts and index-based adjustments, as well as adjustments for change orders, and performance bonuses for early completion and unit performance. Georgia Power's proportionate share of Plant Vogtle Units 3 and 4 is 45.7% . The Vogtle 3 and 4 Agreement also provided for liquidated damages upon the EPC Contractor's failure to fulfill the schedule and certain performance guarantees, each subject to an aggregate cap of 10% of the contract price, or approximately $920 million (approximately $420 million based on Georgia Power's ownership interest). Under the Toshiba Guarantee, Toshiba guaranteed certain payment obligations of the EPC Contractor, including any liability of the EPC Contractor for abandonment of work. In January 2016, Westinghouse delivered to the Vogtle Owners $920 million of letters of credit from financial institutions (Westinghouse Letters of Credit) to secure a portion of the EPC Contractor's potential obligations under the Vogtle 3 and 4 Agreement. The Westinghouse Letters of Credit are subject to annual renewals through June 30, 2020 and require 60 days' written notice to Georgia Power in the event the Westinghouse Letters of Credit will not be renewed. Under the terms of the Vogtle 3 and 4 Agreement, the EPC Contractor did not have the right to terminate the Vogtle 3 and 4 Agreement for convenience. In the event of an abandonment of work by the EPC Contractor, the maximum liability of the EPC Contractor under the Vogtle 3 and 4 Agreement was 40% of the contract price (approximately $1.7 billion based on Georgia Power's ownership interest). On March 29, 2017, the EPC Contractor filed for bankruptcy protection under Chapter 11 of the U.S. Bankruptcy Code. To provide for a continuation of work at Plant Vogtle Units 3 and 4, Georgia Power, acting for itself and as agent for the Vogtle Owners, entered into an interim assessment agreement with the EPC Contractor (Interim Assessment Agreement), which the bankruptcy court approved on March 30, 2017. The Interim Assessment Agreement provided, among other items, that during the term of the Interim Assessment Agreement Georgia Power was obligated to pay, on behalf of the Vogtle Owners, all costs accrued by the EPC Contractor for subcontractors and vendors for services performed or goods provided. The Interim Assessment Agreement, as amended, expired on July 27, 2017. Subsequent to the EPC Contractor bankruptcy filing, a number of subcontractors to the EPC Contractor, including Fluor Enterprises, Inc., a subsidiary of Fluor Corporation, alleged non-payment by the EPC Contractor for amounts owed for work performed on Plant Vogtle Units 3 and 4. Georgia Power, acting for itself and as agent for the Vogtle Owners, has taken, and continues to take, actions to remove liens filed by these subcontractors through the posting of surety bonds. Georgia Power estimates the aggregate liability, through September 30, 2017 , of the Vogtle Owners for the removal of subcontractor liens and payment of other EPC Contractor pre-petition accounts payable to total approximately $386 million , of which $340 million had been paid or accrued as of September 30, 2017 . Georgia Power's proportionate share of this aggregate liability totaled approximately $176 million . On June 9, 2017, Georgia Power and the other Vogtle Owners and Toshiba entered into a settlement agreement regarding the Toshiba Guarantee (Guarantee Settlement Agreement). Pursuant to the Guarantee Settlement Agreement, Toshiba acknowledged the amount of its obligation under the Toshiba Guarantee is $3.68 billion (Guarantee Obligations), of which Georgia Power's proportionate share is approximately $1.7 billion , and that the Guarantee Obligations exist regardless of whether Plant Vogtle Units 3 and 4 are completed. The Guarantee Settlement Agreement also provides for a schedule of payments for the Guarantee Obligations, which will reduce CWIP, beginning in October 2017 and continuing through January 2021. In the event Toshiba receives certain payments, including sale proceeds, from or related to Westinghouse (or its subsidiaries) or Toshiba Nuclear Energy Holdings (UK) Limited (or its subsidiaries), it will hold a portion of such payments in trust for the Vogtle Owners and promptly pay them as offsets against any remaining Guarantee Obligations. Under the Guarantee Settlement Agreement, the Vogtle Owners will forbear from exercising certain remedies, including drawing on the Westinghouse Letters of Credit, until June 30, 2020, unless certain events of nonpayment, insolvency, or other material breach of the Guarantee Settlement Agreement by Toshiba occur. If such an event occurs, the balance of the Guarantee Obligations will become immediately due and payable, and the Vogtle Owners may exercise any and all rights and remedies, including drawing on the Westinghouse Letters of Credit without restriction. In addition, the Guarantee Settlement Agreement does not restrict the Vogtle Owners from fully drawing on the Westinghouse Letters of Credit in the event they are not renewed or replaced prior to the expiration date. On October 2, 2017, Georgia Power received the first installment of the Guarantee Obligations of $300 million from Toshiba, of which Georgia Power's proportionate share was $137 million . Georgia Power is considering potential options with respect to its right to future payments under the Guarantee Settlement Agreement and its claims against the EPC Contractor in the EPC Contractor's bankruptcy proceeding, including a potential sale of those payment rights and bankruptcy claims. Any such transaction cannot be assured and would be subject to DOE consents and related approvals under the Loan Guarantee Agreement and related agreements. On August 10, 2017, Toshiba released its financial results for the quarter ended June 30, 2017, which reflected a negative shareholders' equity balance of approximately $4.5 billion as of June 30, 2017. Toshiba previously announced the existence of material events and conditions that raise substantial d

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS As of September 30, 2017 , assets and liabilities measured at fair value on a recurring basis during the period, together with their associated level of the fair value hierarchy, were as follows: Fair Value Measurements Using: As of September 30, 2017: Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Net Asset Value as a Practical Expedient (NAV) Total (in millions) Southern Company Assets: Energy-related derivatives (a)(b) $ 231 $ 184 $ — $ — $ 415 Interest rate derivatives — 5 — — 5 Foreign currency derivatives — 103 — — 103 Nuclear decommissioning trusts (c) 752 1,004 — 26 1,782 Cash equivalents 1,271 — — — 1,271 Other investments 9 — 1 — 10 Total $ 2,263 $ 1,296 $ 1 $ 26 $ 3,586 Liabilities: Energy-related derivatives (a)(b) $ 265 $ 146 $ — $ — $ 411 Interest rate derivatives — 24 — — 24 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 20 — 20 Total $ 265 $ 193 $ 20 $ — $ 478 Alabama Power Assets: Energy-related derivatives $ — $ 9 $ — $ — $ 9 Nuclear decommissioning trusts: (d) Domestic equity 422 81 — — 503 Foreign equity 60 57 — — 117 U.S. Treasury and government agency securities — 27 — — 27 Corporate bonds 19 150 — — 169 Mortgage and asset backed securities — 18 — — 18 Private Equity — — — 26 26 Other — 8 — — 8 Cash equivalents 808 — — — 808 Total $ 1,309 $ 350 $ — $ 26 $ 1,685 Liabilities: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Fair Value Measurements Using: As of September 30, 2017: Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Net Asset Value as a Practical Expedient (NAV) Total (in millions) Georgia Power Assets: Energy-related derivatives $ — $ 18 $ — $ — $ 18 Interest rate derivatives — 1 — — 1 Nuclear decommissioning trusts: (d) (e) Domestic equity 235 1 — — 236 Foreign equity — 156 — — 156 U.S. Treasury and government agency securities — 225 — — 225 Municipal bonds — 64 — — 64 Corporate bonds — 160 — — 160 Mortgage and asset backed securities — 38 — — 38 Other 16 19 — — 35 Cash equivalents 112 — — — 112 Total $ 363 $ 682 $ — $ — $ 1,045 Liabilities: Energy-related derivatives $ — $ 11 $ — $ — $ 11 Interest rate derivatives — 3 — — 3 Total $ — $ 14 $ — $ — $ 14 Gulf Power Assets: Cash equivalents $ 21 $ — $ — $ — $ 21 Liabilities: Energy-related derivatives $ — $ 22 $ — $ — $ 22 Mississippi Power Assets: Energy-related derivatives $ — $ 3 $ — $ — $ 3 Interest rate derivatives — 2 — — 2 Cash equivalents 209 — — — 209 Total $ 209 $ 5 $ — $ — $ 214 Liabilities: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Fair Value Measurements Using: As of September 30, 2017: Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Net Asset Value as a Practical Expedient (NAV) Total (in millions) Southern Power Assets: Energy-related derivatives $ — $ 9 $ — $ — $ 9 Foreign currency derivatives — 103 — — 103 Cash equivalents 90 — — — 90 Total $ 90 $ 112 $ — $ — $ 202 Liabilities: Energy-related derivatives $ — $ 4 $ — $ — $ 4 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 20 — 20 Total $ — $ 27 $ 20 $ — $ 47 Southern Company Gas Assets: Energy-related derivatives (a)(b) $ 231 $ 145 $ — $ — $ 376 Liabilities: Energy-related derivatives (a)(b) $ 265 $ 95 $ — $ — $ 360 (a) Excludes $13 million associated with certain weather derivatives accounted for based on intrinsic value rather than fair value. (b) Excludes cash collateral of $76 million . (c) For additional detail, see the nuclear decommissioning trusts sections for Alabama Power and Georgia Power in this table. (d) Excludes receivables related to investment income, pending investment sales, payables related to pending investment purchases, and currencies. (e) Includes the investment securities pledged to creditors and collateral received and excludes payables related to the securities lending program. As of September 30, 2017 , approximately $66 million of the fair market value of Georgia Power's nuclear decommissioning trust funds' securities were on loan to creditors under the funds' managers' securities lending program. Southern Company, Alabama Power, and Georgia Power continue to elect the option to fair value investment securities held in the nuclear decommissioning trust funds. The fair value of the funds at Southern Company, including reinvested interest and dividends and excluding the funds' expenses, increased by $50 million and $168 million , respectively, for the three and nine months ended September 30, 2017 , and by $49 million and $116 million , respectively, for the three and nine months ended September 30, 2016 . Alabama Power recorded increases in fair value of $25 million and $87 million , respectively, for the three and nine months ended September 30, 2017 and $26 million and $66 million , respectively, for the three and nine months ended September 30, 2016 as a change in regulatory liabilities related to its AROs. Georgia Power recorded increases in fair value of $25 million and $81 million , respectively, for the three and nine months ended September 30, 2017 and $23 million and $50 million , respectively, for the three and nine months ended September 30, 2016 as a change in its regulatory asset related to its AROs. Valuation Methodologies The energy-related derivatives primarily consist of exchange-traded and over-the-counter financial products for natural gas and physical power products, including, from time to time, basis swaps. These are standard products used within the energy industry and are valued using the market approach. The inputs used are mainly from observable market sources, such as forward natural gas prices, power prices, implied volatility, and overnight index swap interest rates. Interest rate derivatives are also standard over-the-counter products that are valued using observable market data and assumptions commonly used by market participants. The fair value of interest rate derivatives reflects the net present value of expected payments and receipts under the swap agreement based on the market's expectation of future interest rates. Additional inputs to the net present value calculation may include the contract terms, counterparty credit risk, and occasionally, implied volatility of interest rate options. The fair value of cross-currency swaps reflects the net present value of expected payments and receipts under the swap agreement based on the market's expectation of future foreign currency exchange rates. Additional inputs to the net present value calculation may include the contract terms, counterparty credit risk, and discount rates. The interest rate derivatives and cross-currency swaps are categorized as Level 2 under Fair Value Measurements as these inputs are based on observable data and valuations of similar instruments. See Note (H) for additional information on how these derivatives are used. The NRC requires licensees of commissioned nuclear power reactors to establish a plan for providing reasonable assurance of funds for future decommissioning. For fair value measurements of the investments within the nuclear decommissioning trusts, external pricing vendors are designated for each asset class with each security specifically assigned a primary pricing source. For investments held within commingled funds, fair value is determined at the end of each business day through the net asset value, which is established by obtaining the underlying securities' individual prices from the primary pricing source. A market price secured from the primary source vendor is then evaluated by management in its valuation of the assets within the trusts. As a general approach, fixed income market pricing vendors gather market data (including indices and market research reports) and integrate relative credit information, observed market movements, and sector news into proprietary pricing models, pricing systems, and mathematical tools. Dealer quotes and other market information, including live trading levels and pricing analysts' judgments, are also obtained when available. See Note 1 to the financial statements of Southern Company, Alabama Power, and Georgia Power under "Nuclear Decommissioning" in Item 8 of the Form 10-K for additional information. Southern Power has contingent payment obligations related to certain acquisitions whereby Southern Power is obligated to make generation-based payments to the seller over a period ranging from 10 to 30 years, beginning at the commercial operation date. The obligation is categorized as Level 3 under Fair Value Measurements as the fair value is determined using significant unobservable inputs for the forecasted facility generation in MW-hours, as well as other inputs such as a fixed dollar amount per MW-hour, and a discount rate, and is evaluated periodically. The fair value of contingent consideration reflects the net present value of expected payments and any periodic change arising from forecasted generation is expected to be immaterial. "Other investments" include investments that are not traded in the open market. The fair value of these investments has been determined based on market factors including comparable multiples and the expectations regarding cash flows and business plan executions. As of September 30, 2017 , the fair value measurements of private equity investments held in the nuclear decommissioning trust that are calculated at net asset value per share (or its equivalent) as a practical expedient, as well as the nature and risks of those investments, were as follows: As of September 30, 2017: Fair Value Unfunded Commitments Redemption Frequency Redemption Notice Period (in millions) Southern Company $ 26 $ 24 Not Applicable Not Applicable Alabama Power $ 26 $ 24 Not Applicable Not Applicable Private equity funds include a fund-of-funds that invests in high-quality private equity funds across several market sectors, funds that invest in real estate assets, and a fund that acquires companies to create resale value. Private equity funds do not have redemption rights. Distributions from these funds will be received as the underlying investments in the funds are liquidated. Liquidations are expected to occur at various times over the next 10 years . As of September 30, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: Carrying Amount Fair Value (in millions) Long-term debt, including securities due within one year: Southern Company $ 47,269 $ 49,348 Alabama Power $ 7,404 $ 8,031 Georgia Power $ 11,713 $ 12,237 Gulf Power $ 1,292 $ 1,352 Mississippi Power $ 2,123 $ 2,117 Southern Power $ 5,810 $ 5,916 Southern Company Gas $ 5,862 $ 6,230 The fair values are determined using Level 2 measurements and are based on quoted market prices for the same or similar issues or on the current rates available to Southern Company, Alabama Power, Georgia Power, Gulf Power, Mississippi Power, Southern Power, and Southern Company Gas.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS' EQUITY Earnings per Share For Southern Company, the only difference in computing basic and diluted earnings per share is attributable to awards outstanding under the stock option and performance share plans. See Note 8 to the financial statements of Southern Company in Item 8 of the Form 10-K for information on the stock option and performance share plans. The effect of both stock options and performance share award units was determined using the treasury stock method. Shares used to compute diluted earnings per share were as follows: Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 (in millions) As reported shares 1,003 968 998 940 Effect of options and performance share award units 7 7 7 5 Diluted shares 1,010 975 1,005 945 Stock options and performance share award units that were not included in the diluted earnings per share calculation because they were anti-dilutive were immaterial for the three and nine months ended September 30, 2017 and 2016 . Changes in Stockholders' Equity The following table presents year-to-date changes in stockholders' equity of Southern Company: Number of Common Shares Common Preferred and Preference Stock of Subsidiaries Total Issued Treasury Noncontrolling Interests (*) (in thousands) (in millions) Balance at December 31, 2016 991,213 (819 ) $ 24,758 $ 609 $ 1,245 $ 26,612 Consolidated net income attributable to Southern Company — — 347 — — 347 Other comprehensive income (loss) — — (2 ) — — (2 ) Stock issued 13,308 — 613 — — 613 Stock-based compensation — — 97 — — 97 Cash dividends on common stock — — (1,716 ) — — (1,716 ) Preference stock redemption — — — (150 ) — (150 ) Contributions from noncontrolling interests — — — — 77 77 Distributions to noncontrolling interests — — — — (87 ) (87 ) Net income attributable to noncontrolling interests — — — — 45 45 Reclassification from redeemable noncontrolling interests — — — — 114 114 Other — (75 ) (15 ) 3 1 (11 ) Balance at September 30, 2017 1,004,521 (894 ) $ 24,082 $ 462 $ 1,395 $ 25,939 Balance at December 31, 2015 915,073 (3,352 ) $ 20,592 $ 609 $ 781 $ 21,982 Consolidated net income attributable to Southern Company — — 2,251 — — 2,251 Other comprehensive income (loss) — — (95 ) — — (95 ) Stock issued 65,725 2,599 3,265 — — 3,265 Stock-based compensation — — 94 — — 94 Cash dividends on common stock — — (1,553 ) — — (1,553 ) Contributions from noncontrolling interests — — — — 357 357 Distributions to noncontrolling interests — — — — (21 ) (21 ) Purchase of membership interests from noncontrolling interests — — — — (129 ) (129 ) Net income attributable to noncontrolling interests — — — — 36 36 Other — (46 ) (7 ) — — (7 ) Balance at September 30, 2016 980,798 (799 ) $ 24,547 $ 609 $ 1,024 $ 26,180 (*) Related to Southern Power Company and excludes redeemable noncontrolling interests. In April 2017, approximately $114 million was reclassified from redeemable noncontrolling interests to noncontrolling interests, included in stockholder's equity, due to the expiration of SunPower Corp's option to require Southern Power to purchase its membership interests in one of the solar partnerships. See Note 10 to the financial statements of Southern Power in Item 8 of the Form 10-K for additional information.

Financing

Financing	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
FINANCING	FINANCING Going Concern As of September 30, 2017 , Mississippi Power's current liabilities exceeded current assets by approximately $769 million primarily due to approximately $935 million that will be required through September 30, 2018 to fund maturities of long-term debt and $4 million that will be required to fund maturities of short-term debt. In addition, Mississippi Power has $40 million of tax-exempt variable rate demand obligations that are supported by short-term credit facilities and $50 million of fixed rate pollution control revenue bonds that are required to be remarketed over the next 12 months. Mississippi Power intends to utilize operating cash flows, lines of credit, and bank term loans, as market conditions permit, as well as, under certain circumstances, commercial paper and/or equity contributions and/or loans from Southern Company to fund Mississippi Power's short-term capital needs. Specifically, Mississippi Power has been informed by Southern Company that in the event sufficient funds are not available from external sources, Southern Company intends to provide Mississippi Power with loans and/or equity contributions sufficient to fund the remaining indebtedness scheduled to mature and other cash needs over the next 12 months. Therefore, Mississippi Power's financial statement presentation contemplates continuation of Mississippi Power as a going concern as a result of Southern Company's anticipated ongoing financial support of Mississippi Power. For additional information, see Notes 1 and 6 to the financial statements of Mississippi Power under "Recently Issued Accounting Standards" and "Going Concern," respectively, in Item 8 of the Form 10-K and Note (B) under " Integrated Coal Gasification Combined Cycle ." DOE Loan Guarantee Borrowings See Note 6 to the financial statements of Southern Company and Georgia Power in Item 8 of the Form 10-K for additional information regarding Georgia Power's Loan Guarantee Agreement with the DOE and related multi-advance term loan facility (FFB Credit Facility) with the FFB. On July 27, 2017, Georgia Power entered into an amendment to the Loan Guarantee Agreement (LGA Amendment) in connection with the DOE's consent to Georgia Power's entry into the Services Agreement and the related intellectual property licenses (IP Licenses). Under the terms of the Loan Guarantee Agreement, upon termination of the Vogtle 3 and 4 Agreement, further advances are conditioned upon the DOE's approval of any agreements entered into in replacement of the Vogtle 3 and 4 Agreement. Under the terms of the LGA Amendment, Georgia Power will not request any advances unless and until such time as Georgia Power has (i) completed the cost-to-complete and cancellation cost assessments prepared as a result of the bankruptcy of the EPC Contractor (Cost Assessments) and made a determination to continue construction of Plant Vogtle Units 3 and 4, (ii) delivered to the DOE an updated project schedule, construction budget, and other information, (iii) entered into one or more agreements with a construction contractor or contractors that will be primarily responsible for construction of Plant Vogtle Units 3 and 4 and such agreements have been approved by the DOE (together with the Services Agreement and the IP Licenses, the Replacement EPC Arrangements), and (iv) entered into a further amendment to the Loan Guarantee Agreement with the DOE to reflect the Replacement EPC Arrangements. Upon satisfaction of the conditions described above, advances may be requested under the FFB Credit Facility on a quarterly basis through 2020. The final maturity date for each advance under the FFB Credit Facility is February 20, 2044. Interest is payable quarterly and principal payments will begin on February 20, 2020. Borrowings under the FFB Credit Facility will bear interest at the applicable U.S. Treasury rate plus a spread equal to 0.375% . In addition to the conditions described above, future advances are subject to satisfaction of customary conditions, as well as certification of compliance with the requirements of the Title XVII Loan Guarantee Program, accuracy of project-related representations and warranties, delivery of updated project-related information, absence of liens on Georgia Power's ownership interest in Plant Vogtle Units 3 and 4 other than permitted liens, evidence of compliance with the prevailing wage requirements of the Davis-Bacon Act of 1931, as amended, and certification from the DOE's consulting engineer that proceeds of the advances are used to reimburse Eligible Project Costs. Under the Loan Guarantee Agreement, Georgia Power is subject to customary borrower affirmative and negative covenants and events of default. In addition, Georgia Power is subject to project-related reporting requirements and other project-specific covenants and events of default. In the event certain mandatory prepayment events occur, the FFB's commitment to make further advances under the FFB Credit Facility will terminate and Georgia Power will be required to prepay the outstanding principal amount of all borrowings under the FFB Credit Facility over a period of five years (with level principal amortization). Among other things, these mandatory prepayment events include (i) the termination of the Services Agreement or rejection of the Services Agreement in bankruptcy if Georgia Power does not maintain access to intellectual property rights under the IP Licenses; (ii) a decision by Georgia Power not to continue construction of Plant Vogtle Units 3 and 4; (iii) a failure by Georgia Power to complete the Cost Assessments or enter into Replacement EPC Arrangements by December 31, 2017; (iv) cancellation of Plant Vogtle Units 3 and 4 by the Georgia PSC, or by Georgia Power if authorized by the Georgia PSC; and (v) cost disallowances by the Georgia PSC that could have a material adverse effect on completion of Plant Vogtle Units 3 and 4 or Georgia Power's ability to repay the outstanding borrowings under the FFB Credit Facility. Under certain circumstances, insurance proceeds and any proceeds from an event of taking must be applied to immediately prepay outstanding borrowings under the FFB Credit Facility. In addition, under certain circumstances Georgia Power may be required to make additional prepayments in connection with its receipt of payments under the Guarantee Settlement Agreement or from the EPC Contractor under the Vogtle 3 and 4 Agreement. Georgia Power also may voluntarily prepay outstanding borrowings under the FFB Credit Facility. Under the FFB Credit Facility, any prepayment (whether mandatory or optional) will be made with a make-whole premium or discount, as applicable. On September 28, 2017, the DOE issued a conditional commitment to Georgia Power for up to approximately $1.67 billion of additional guaranteed loans under the Loan Guarantee Agreement. Final approval and issuance of these additional loan guarantees by the DOE cannot be assured and are subject to the negotiation of definitive agreements, completion of due diligence by the DOE, receipt of any necessary regulatory approvals, and satisfaction of other conditions. See Note (B) under " Regulatory Matters – Georgia Power – Nuclear Construction " for additional information regarding Plant Vogtle Units 3 and 4. Bank Credit Arrangements Bank credit arrangements provide liquidity support to the registrants' commercial paper borrowings and the traditional electric operating companies' pollution control revenue bonds. The amount of variable rate pollution control revenue bonds of the traditional electric operating companies outstanding requiring liquidity support as of September 30, 2017 was approximately $1.5 billion (comprised of approximately $854 million at Alabama Power, $550 million at Georgia Power, $82 million at Gulf Power, and $40 million at Mississippi Power). In June 2017, Georgia Power remarketed $318 million of variable rate pollution control bonds in index rate modes, reducing the liquidity support utilized under Georgia Power's bank credit arrangement. In addition, at September 30, 2017 , the traditional electric operating companies had approximately $699 million (comprised of approximately $509 million at Georgia Power, $140 million at Gulf Power, and $50 million at Mississippi Power) of pollution control revenue bonds outstanding that were required to be remarketed within the next 12 months. Subsequent to September 30, 2017, $40 million of these pollution control revenue bonds of Georgia Power which were in an index rate mode were remarketed to the public in a long-term fixed rate mode. See Note 6 to the financial statements of each registrant under "Bank Credit Arrangements" in Item 8 of the Form 10-K and " Financing Activities " herein for additional information. The following table outlines the committed credit arrangements by company as of September 30, 2017 : Expires Executable Term Loans Expires Within One Year Company 2017 2018 2019 2020 2022 Total Unused One Year Two Years Term Out No Term Out (in millions) Southern Company (a) $ — $ — $ — $ — $ 2,000 $ 2,000 $ 2,000 $ — $ — $ — $ — Alabama Power — 35 — 500 800 1,335 1,335 — — — 35 Georgia Power — — — — 1,750 1,750 1,732 — — — — Gulf Power 30 195 25 30 — 280 280 45 — — 40 Mississippi Power 100 — — — — 100 100 — — — 100 Southern Power Company (b) — — — — 750 750 728 — — — — Southern Company Gas (c) — — — — 1,900 1,900 1,861 — — — — Other — 30 — — — 30 30 20 — 20 10 Southern Company Consolidated $ 130 $ 260 $ 25 $ 530 $ 7,200 $ 8,145 $ 8,066 $ 65 $ — $ 20 $ 185 (a) Represents the Southern Company parent entity. (b) Does not include Southern Power's $120 million continuing letter of credit facility for standby letters of credit expiring in 2019, of which $111 million has been used for letters of credit and $9 million remains unused at September 30, 2017 . (c) Southern Company Gas, as the parent entity, guarantees the obligations of Southern Company Gas Capital, which is the borrower of $1.2 billion of these arrangements. Southern Company Gas' committed credit arrangements also include $700 million for which Nicor Gas is the borrower and which is restricted for working capital needs of Nicor Gas. As reflected in the table above, in May 2017, Southern Company, Alabama Power, Georgia Power, and Southern Power Company each amended certain of their multi-year credit arrangements, which, among other things, extended the maturity dates from 2020 to 2022. Southern Company and Southern Power Company increased their borrowing ability under these arrangements to $2.0 billion from $1.25 billion and to $750 million from $600 million , respectively. Southern Company also terminated its $1.0 billion facility maturing in 2018. Also in May 2017, Southern Company Gas Capital and Nicor Gas terminated their existing credit arrangements for $1.3 billion and $700 million , respectively, which were to mature in 2017 and 2018, and entered into a new multi-year credit arrangement currently allocated for $1.2 billion and $700 million , respectively, with a maturity date of 2022. Pursuant to the new multi-year credit arrangement, the allocations may be adjusted. In September 2017, Alabama Power amended its $500 million multi-year credit arrangement, which, among other things, extended the maturity date from 2018 to 2020. Subject to applicable market conditions, Southern Company and its subsidiaries expect to renew or replace their bank credit arrangements as needed, prior to expiration. In connection therewith, Southern Company and its subsidiaries may extend the maturity dates and/or increase or decrease the lending commitments thereunder. Financing Activities The following table outlines the long-term debt financing activities for Southern Company and its subsidiaries for the first nine months of 2017 : Company Senior Note Issuances Senior Note Maturities and Redemptions Revenue Bond Other Long-Term Debt Issuances Other Long-Term Debt Redemptions and Maturities (a) (in millions) Southern Company (b) $ 300 $ 400 $ — $ 500 $ 400 Alabama Power 550 200 36 — — Georgia Power 1,350 450 65 370 13 Gulf Power 300 85 — 6 — Mississippi Power — — — 40 893 Southern Power — — — 43 4 Southern Company Gas (c) 450 — — 200 22 Other — — — — 12 Elimination (d) — — — (40 ) (599 ) Southern Company Consolidated $ 2,950 $ 1,135 $ 101 $ 1,119 $ 745 (a) Includes reductions in capital lease obligations resulting from cash payments under capital leases. (b) Represents the Southern Company parent entity. (c) The senior notes were issued by Southern Company Gas Capital and guaranteed by the Southern Company Gas parent entity. Other long-term debt issued represents first mortgage bonds issued by Nicor Gas. (d) Includes intercompany loans from Southern Company to Mississippi Power and reductions in affiliate capital lease obligations at Georgia Power. These transactions are eliminated in Southern Company's Consolidated Financial Statements. Southern Company In June 2017, Southern Company issued $500 million aggregate principal amount of Series 2017A 5.325% Junior Subordinated Notes due June 21, 2057 and $300 million aggregate principal amount of Series 2017A Floating Rate Senior Notes due September 30, 2020, which bear interest at a floating rate based on three -month LIBOR. The proceeds were used to repay short-term indebtedness and for other general corporate purposes. Also in June 2017, Southern Company entered into two $100 million aggregate principal amount floating rate bank term loan agreements, which mature on June 21, 2018 and June 29, 2018 and bear interest based on one -month LIBOR. The proceeds were used for working capital and other general corporate purposes. In August 2017, Southern Company borrowed $250 million pursuant to an uncommitted bank credit arrangement, which bears interest at a rate agreed upon by Southern Company and the bank from time to time and is payable on no less than 30 days' demand by the bank. The proceeds were used for working capital and other general corporate purposes. Alabama Power In March 2017, Alabama Power issued $550 million aggregate principal amount of Series 2017A 2.45% Senior Notes due March 30, 2022. The proceeds were used to repay Alabama Power's short-term indebtedness and for general corporate purposes, including Alabama Power's continuous construction program. In September 2017, Alabama Power issued 10 million shares ( $250 million aggregate stated capital) of 5.00% Class A Preferred Stock, Cumulative, Par Value $1 Per Share (Stated Capital $25 Per Share). The proceeds were used in October 2017 to redeem all 2 million shares ( $50 million aggregate stated capital) of Alabama Power's 6.50% Series Preference Stock, 6 million shares ( $150 million aggregate stated capital) of Alabama Power's 6.45% Series Preference Stock, and 1.52 million shares ( $38 million aggregate stated capital) of Alabama Power's 5.83% Class A Preferred Stock and for other general corporate purposes, including Alabama Power's continuous construction program. Georgia Power In March 2017, Georgia Power issued $450 million aggregate principal amount of Series 2017A 2.00% Senior Notes due March 30, 2020 and $400 million aggregate principal amount of Series 2017B 3.25% Senior Notes due March 30, 2027. The proceeds were used to repay a portion of Georgia Power's short-term indebtedness and for general corporate purposes, including Georgia Power's continuous construction program. In April 2017, Georgia Power purchased and held $27 million aggregate principal amount of Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fifth Series 1995. Subsequent to September 30, 2017, Georgia Power remarketed these bonds to the public. In June 2017, Georgia Power entered into three floating rate bank loans in aggregate principal amounts of $50 million , $150 million , and $100 million , with maturity dates of December 1, 2017, May 31, 2018, and June 28, 2018, respectively, which bear interest based on one -month LIBOR. Also in June 2017, Georgia Power borrowed $500 million pursuant to an uncommitted bank credit arrangement, which bears interest at a rate agreed upon by Georgia Power and the bank from time to time and is payable on no less than 30 days' demand by the bank. The proceeds from these bank loans were used to repay a portion of Georgia Power's existing indebtedness and for working capital and other general corporate purposes, including Georgia Power's continuous construction program. In August 2017, Georgia Power repaid $250 million of the $500 million aggregate principal amount outstanding pursuant to its uncommitted bank credit arrangement. Also in August 2017, Georgia Power amended its $100 million floating rate bank loan to extend the maturity date from June 28, 2018 to October 26, 2018. Also in August 2017, Georgia Power issued $500 million aggregate principal amount of Series 2017C 2.00% Senior Notes due September 8, 2020. The proceeds were used to repay Georgia Power's $50 million floating rate bank loan due December 1, 2017 and outstanding commercial paper borrowings and for general corporate purposes. Also in August 2017, Georgia Power purchased and held $38 million aggregate principal amount of Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 1997. Subsequent to September 30, 2017, Georgia Power remarketed these bonds to the public. In September 2017, Georgia Power issued $270 million aggregate principal amount of Series 2017A 5.00% Junior Subordinated Notes due October 1, 2077. The proceeds were used in October 2017 to redeem all 1.8 million shares ( $45 million aggregate liquidation amount) of Georgia Power's 6.125% Series Class A Preferred Stock and 2.25 million shares ( $225 million aggregate liquidation amount) of Georgia Power's 6.50% Series 2007A Preference Stock. Gulf Power In March 2017, Gulf Power extended the maturity of a $100 million short-term floating rate bank loan bearing interest based on one -month LIBOR from April 2017 to October 2017 and subsequently repaid the loan in May 2017. In May 2017, Gulf Power issued $300 million aggregate principal amount of Series 2017A 3.30% Senior Notes due May 30, 2027. The proceeds, together with other funds, were used to repay at maturity $85 million aggregate principal amount of Series 2007A 5.90% Senior Notes due June 15, 2017; to repay outstanding commercial paper borrowings; to repay a $100 million short-term floating rate bank loan, as discussed above; and to redeem, in June 2017, 550,000 shares ( $55 million aggregate liquidation amount) of Gulf Power's 6.00% Series Preference Stock, 450,000 shares ( $45 million aggregate liquidation amount) of Gulf Power's Series 2007A 6.45% Preference Stock, and 500,000 shares ( $50 million aggregate liquidation amount) of Gulf Power's Series 2013A 5.60% Preference Stock. Mississippi Power In March 2017, Mississippi Power issued a $9 million short-term bank note bearing interest at 5% per annum, which was repaid in April 2017. In February 2017, Mississippi Power amended $551 million in promissory notes to Southern Company extending the maturity dates of the notes from December 1, 2017 to July 31, 2018. In the second quarter 2017, Mississippi Power borrowed an additional $40 million under a promissory note issued to Southern Company. In June 2017, Southern Company made equity contributions totaling $1.0 billion to Mississippi Power. Mississippi Power used a portion of the proceeds to (i) prepay $300 million of the outstanding principal amount under its $1.2 billion unsecured term loan, which matures on March 30, 2018; (ii) repay all of the $591 million outstanding principal amount of promissory notes to Southern Company; and (iii) repay a $10 million short-term bank loan. In August 2017, Mississippi Power repaid a $12.5 million short-term bank note. In September 2017, Mississippi Power issued a floating rate promissory note to Southern Company in an aggregate principal amount of up to $150 million bearing interest based on one -month LIBOR. Mississippi Power borrowed $109 million under this promissory note primarily to satisfy its federal income tax obligations for the quarter ending September 30, 2017 and subsequently repaid the promissory note upon receipt of its income tax refund from the U.S. federal government related to the settlement concerning deductible R&E expenditures. See Note (G) under "Section 174 Research and Experimental Deduction" for additional information. Southern Power In September 2017, Southern Power amended its $60 million aggregate principal amount floating rate bank loan to, among other things, increase the aggregate principal amount to $100 million and extend the maturity date from September 2017 to October 2018. The additional $40 million of proceeds were used to repay existing indebtedness and for other general corporate purposes. Southern Company Gas In May 2017, Southern Company Gas Capital issued $450 million aggregate principal amount of Series 2017A 4.40% Senior Notes due May 30, 2047. The proceeds were used to repay Southern Company Gas' short-term indebtedness and for general corporate purposes. In July 2017, Nicor Gas agreed to issue $400 million aggregate principal amount of first mortgage bonds in a private placement. On August 10, 2017, Nicor Gas issued $100 million aggregate principal amount of First Mortgage Bonds 3.03% Series due August 10, 2027 and $100 million aggregate principal amount of First Mortgage Bonds 3.62% Series due August 10, 2037. The proceeds were used to repay short-term indebtedness incurred under the Nicor Gas commercial paper program and for other working capital needs. The remaining $200 million is expected to be issued in November 2017.

Retirement Benefits

Retirement Benefits	9 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS Southern Company has a defined benefit, trusteed, pension plan covering substantially all employees, with the exception of employees at Southern Company Gas, as discussed below, and PowerSecure. The Southern Company qualified pension plan is funded in accordance with requirements of the Employee Retirement Income Security Act of 1974, as amended (ERISA). No mandatory contributions to the Southern Company qualified pension plan are anticipated for the year ending December 31, 2017 . Southern Company also provides certain defined benefit pension plans for a selected group of management and highly compensated employees. Benefits under these non-qualified pension plans are funded on a cash basis. In addition, Southern Company provides certain medical care and life insurance benefits for retired employees through other postretirement benefit plans. The traditional electric operating companies fund related other postretirement trusts to the extent required by their respective regulatory commissions. In addition, Southern Company Gas has a qualified defined benefit, trusteed, pension plan covering certain eligible employees, which was closed in 2012 to new employees. This qualified pension plan is funded in accordance with requirements of ERISA. No mandatory contributions to the Southern Company Gas qualified pension plan are anticipated for the year ending December 31, 2017 . Southern Company Gas also provides certain non-qualified defined benefit and defined contribution pension plans for a selected group of management and highly compensated employees. Benefits under these non-qualified pension plans are funded on a cash basis. In addition, Southern Company Gas provides certain medical care and life insurance benefits for eligible retired employees through a postretirement benefit plan. Southern Company Gas also has a separate unfunded supplemental retirement health care plan that provides medical care and life insurance benefits to employees of discontinued businesses. See Note 2 to the financial statements of Southern Company, Alabama Power, Georgia Power, Gulf Power, Mississippi Power, and Southern Company Gas in Item 8 of the Form 10-K for additional information. Components of the net periodic benefit costs for the three and nine months ended September 30, 2017 and 2016 are presented in the following tables. Pension Plans Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power (in millions) Three Months Ended September 30, 2017 Service cost $ 73 $ 15 $ 19 $ 3 $ 4 Interest cost 114 25 34 5 5 Expected return on plan assets (224 ) (49 ) (71 ) (10 ) (9 ) Amortization: Prior service costs 3 1 — — — Net (gain)/loss 41 10 15 2 1 Net periodic pension cost (income) $ 7 $ 2 $ (3 ) $ — $ 1 Nine Months Ended September 30, 2017 Service cost $ 220 $ 47 $ 56 $ 10 $ 11 Interest cost 341 73 103 15 15 Expected return on plan assets (673 ) (147 ) (212 ) (29 ) (29 ) Amortization: Prior service costs 9 2 2 — 1 Net (gain)/loss 122 31 43 5 5 Net periodic pension cost (income) $ 19 $ 6 $ (8 ) $ 1 $ 3 Three Months Ended September 30, 2016 Service cost $ 68 $ 14 $ 17 $ 3 $ 3 Interest cost 110 23 34 5 4 Expected return on plan assets (203 ) (46 ) (64 ) (9 ) (9 ) Amortization: Prior service costs 3 1 1 — 1 Net (gain)/loss 45 10 14 2 2 Net periodic pension cost $ 23 $ 2 $ 2 $ 1 $ 1 Nine Months Ended September 30, 2016 Service cost $ 192 $ 43 $ 52 $ 9 $ 9 Interest cost 311 71 102 14 14 Expected return on plan assets (577 ) (138 ) (193 ) (26 ) (26 ) Amortization: Prior service costs 10 2 4 1 1 Net (gain)/loss 120 30 41 5 5 Net periodic pension cost $ 56 $ 8 $ 6 $ 3 $ 3 Pension Plans Southern Company Gas (in millions) Successor – Three Months Ended September 30, 2017 Service cost $ 6 Interest cost 10 Expected return on plan assets (18 ) Amortization of net (gain)/loss 5 Net periodic pension cost $ 3 Successor – Nine Months Ended September 30, 2017 Service cost $ 17 Interest cost 30 Expected return on plan assets (53 ) Amortization: Prior service costs (1 ) Net (gain)/loss 15 Net periodic pension cost $ 8 Successor – July 1, 2016 through September 30, 2016 Service cost $ 7 Interest cost 10 Expected return on plan assets (17 ) Amortization of regulatory asset 6 Net periodic pension cost $ 6 Predecessor – January 1, 2016 through June 30, 2016 Service cost $ 13 Interest cost 21 Expected return on plan assets (33 ) Amortization: Prior service costs (1 ) Net (gain)/loss 13 Net periodic pension cost $ 13 Postretirement Benefits Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power (in millions) Three Months Ended September 30, 2017 Service cost $ 6 $ 1 $ 2 $ — $ — Interest cost 19 4 6 1 1 Expected return on plan assets (16 ) (5 ) (6 ) — — Amortization: Prior service costs 2 1 — — — Net (gain)/loss 3 — 3 — — Net periodic postretirement benefit cost $ 14 $ 1 $ 5 $ 1 $ 1 Nine Months Ended September 30, 2017 Service cost $ 18 $ 4 $ 5 $ 1 $ 1 Interest cost 59 13 21 2 3 Expected return on plan assets (49 ) (19 ) (18 ) (1 ) (1 ) Amortization: Prior service costs 5 3 1 — — Net (gain)/loss 10 1 6 — — Net periodic postretirement benefit cost $ 43 $ 2 $ 15 $ 2 $ 3 Three Months Ended September 30, 2016 Service cost $ 6 $ 1 $ 2 $ — $ — Interest cost 20 5 7 1 — Expected return on plan assets (16 ) (6 ) (6 ) — — Amortization: Prior service costs 1 1 — — — Net (gain)/loss 5 — 3 — 1 Net periodic postretirement benefit cost $ 16 $ 1 $ 6 $ 1 $ 1 Nine Months Ended September 30, 2016 Service cost $ 17 $ 4 $ 5 $ 1 $ 1 Interest cost 55 14 22 2 2 Expected return on plan assets (44 ) (19 ) (17 ) (1 ) (1 ) Amortization: Prior service costs 4 3 1 — — Net (gain)/loss 12 1 7 — 1 Net periodic postretirement benefit cost $ 44 $ 3 $ 18 $ 2 $ 3 Postretirement Benefits Southern Company Gas (in millions) Successor – Three Months Ended September 30, 2017 Service cost $ 1 Interest cost 3 Expected return on plan assets (2 ) Amortization: Prior service costs (1 ) Net (gain)/loss 1 Net periodic postretirement benefit cost $ 2 Successor – Nine Months Ended September 30, 2017 Service cost $ 2 Interest cost 8 Expected return on plan assets (5 ) Amortization: Prior service costs (2 ) Net (gain)/loss 3 Net periodic postretirement benefit cost $ 6 Successor – July 1, 2016 through September 30, 2016 Service cost $ 1 Interest cost 2 Expected return on plan assets (2 ) Amortization of regulatory asset 1 Net periodic postretirement benefit cost $ 2 Predecessor – January 1, 2016 through June 30, 2016 Service cost $ 1 Interest cost 5 Expected return on plan assets (3 ) Amortization: Prior service costs (1 ) Net (gain)/loss 2 Net periodic postretirement benefit cost $ 4

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES See Note 5 to the financial statements of each registrant in Item 8 of the Form 10-K for additional tax information. Current and Deferred Income Taxes Tax Credit Carryforwards Southern Company had federal ITC and PTC carryforwards (primarily related to Southern Power) totaling $1.9 billion as of September 30, 2017 compared to $1.8 billion as of December 31, 2016 . The federal ITC carryforwards begin expiring in 2032 but are expected to be fully utilized by 2022. The PTC carryforwards begin expiring in 2036 but are expected to be utilized by 2022. The expected utilization of tax credit carryforwards could be further delayed by numerous factors. These factors include the acquisition of additional renewable projects, increased generation at existing wind facilities, carrying back the federal net operating loss, and potential tax reform legislation, as well as additional deductions in the event of an asset abandonment. The ultimate outcome of these matters cannot be determined at this time. Valuation Allowances At September 30, 2017 , valuation allowances were as follows: Mississippi Power Southern Company Gas Southern Company (in millions) Federal $ — $ 18 $ 18 State (net of federal benefit) 46 1 64 Balance at September 30, 2017 $ 46 $ 19 $ 82 Southern Company had valuation allowances, net of the federal benefit, of $82 million at September 30, 2017 compared to $21 million at December 31, 2016. The increase was primarily due to Mississippi Power's projected inability to utilize the State of Mississippi net operating loss. Effective Tax Rate Southern Company Southern Company's effective tax rate is typically lower than the statutory rate due to employee stock plans' dividend deduction, non-taxable AFUDC equity, and federal income tax benefits from ITCs and PTCs. Southern Company's effective tax rate was 42.6% for the nine months ended September 30, 2017 compared to 28.3% for the corresponding period in 2016 . The effective tax rate increase was primarily due to the estimated probable losses on the Kemper IGCC, net of the non-deductible AFUDC equity portion. Other factors include a decrease in tax benefits from solar ITCs and an increase in state valuation allowances, partially offset by an increase in tax benefits from wind PTCs. Southern Company recognizes PTCs when wind energy is generated and sold (using the prescribed KWH rate in applicable federal and state statutes), which may differ significantly from amounts computed on a quarterly basis using an overall estimated annual effective income tax rate. Southern Company uses this method of recognition since the amount of PTCs can be significantly impacted by wind generation. This method can significantly affect the effective income tax rate for the period depending on the amount of pretax income. Mississippi Power Mississippi Power's effective tax (benefit) rate was (30.3)% for the nine months ended September 30, 2017 compared to (282.8)% for the corresponding period in 2016 . The effective tax rate increase was primarily due to the estimated probable losses on the Kemper IGCC, net of the non-deductible AFUDC equity portion and the related state valuation allowances. Southern Power Southern Power's effective tax (benefit) rate was (66.5)% for the nine months ended September 30, 2017 compared to (88.9)% for the corresponding period in 2016 . The effective tax rate increase was primarily due to a decrease in tax benefits from solar ITCs, partially offset by additional wind PTCs and state apportionment rate changes. Southern Power recognizes PTCs when wind energy is generated and sold (using the prescribed KWH rate in applicable federal and state statutes), which may differ significantly from amounts computed on a quarterly basis using an overall estimated annual effective income tax rate. Southern Power uses this method of recognition since the amount of PTCs can be significantly impacted by wind generation. This method can significantly affect the effective income tax rate for the period depending on the amount of pretax income. During the third quarter 2017, Southern Power began a legal entity reorganization of various direct and indirect subsidiaries that own and operate solar facilities, including certain subsidiaries owned in partnership with various third parties. Southern Power's ownership interests in the various solar entities and facilities will not be affected by the reorganization. The reorganization is expected to result in estimated tax benefits totaling approximately $40 million that will be recorded in the fourth quarter 2017 related to certain changes in state apportionment rates and net operating loss carryforward utilization. The ultimate outcome of this matter cannot be determined at this time. Southern Company Gas Southern Company Gas' effective tax rate was 43.4% for the successor nine months ended September 30, 2017 compared to 60.3% for the successor period of July 1, 2016 through September 30, 2016 and 37.6% for the predecessor period of January 1, 2016 through June 30, 2016. The effective tax rate for the successor year-to-date 2017 was impacted by State of Illinois tax legislation enacted during July 2017, the allocation of new tax apportionment factors in several states for the inclusion of Southern Company Gas into the consolidated Southern Company state tax filings, and higher pre-tax earnings. The effective tax rates for the periods in 2016 were impacted by the non-deductibility of certain Merger-related expenses. The effective tax rate for the successor period of July 1, 2016 through September 30, 2016 was also impacted by nondeductible expenses associated with certain compensation costs. Unrecognized Tax Benefits See Note 5 to the financial statements of each registrant under "Unrecognized Tax Benefits" in Item 8 of the Form 10-K for additional information. Changes during the nine months ended September 30, 2017 for unrecognized tax benefits were as follows: Mississippi Power Southern Power Southern Company (in millions) Unrecognized tax benefits as of December 31, 2016 $ 465 $ 17 $ 484 Tax positions from current periods 2 — 9 Tax positions from prior periods (175 ) (17 ) (186 ) Reductions due to settlements (290 ) — (290 ) Balance as of September 30, 2017 $ 2 $ — $ 17 The tax positions from current and prior periods primarily relate to state tax benefits, deductions for R&E expenditures, and charitable contribution carryforwards that were impacted as a result of the settlement of R&E expenditures associated with the Kemper IGCC, as well as federal income tax benefits from deferred ITCs. See " Section 174 Research and Experimental Deduction " herein for additional information. These amounts are presented on a gross basis without considering the related federal or state income tax impact. The impact on the effective tax rate, if recognized, is as follows: As of September 30, 2017 As of December 31, 2016 Mississippi Power Southern Company Southern Company (in millions) Tax positions impacting the effective tax rate $ 2 $ 17 $ 20 Tax positions not impacting the effective tax rate — — 464 Balance of unrecognized tax benefits $ 2 $ 17 $ 484 The tax positions impacting the effective tax rate primarily relate to state tax benefits and charitable contribution carryforwards that were impacted as a result of the settlement of R&E expenditures associated with the Kemper IGCC. See " Section 174 Research and Experimental Deduction " herein for additional information. Accrued interest for all tax positions was immaterial for all periods presented. All of the registrants classify interest on tax uncertainties as interest expense. None of the registrants accrued any penalties on uncertain tax positions. It is reasonably possible that the amount of the unrecognized tax benefits could change within 12 months . The settlement of federal and state audits could impact the balances significantly. At this time, an estimate of the range of reasonably possible outcomes cannot be determined. The IRS has finalized its audits of Southern Company's consolidated federal income tax returns through 2016. Southern Company is a participant in the Compliance Assurance Process of the IRS. However, the pre-Merger Southern Company Gas 2014, 2015, and June 30, 2016 federal tax returns are currently under audit. The audits for Southern Company's state income tax returns have either been concluded, or the statute of limitations has expired, for years prior to 2011. Section 174 Research and Experimental Deduction Southern Company has reflected deductions for R&E expenditures related to the Kemper IGCC in its federal income tax calculations since 2013 and filed amended federal income tax returns for 2008 through 2013 to also include such deductions. In December 2016, Southern Company and the IRS reached a proposed settlement, which was approved on September 8, 2017 by the U.S. Congress Joint Committee on Taxation (JCT), resolving a methodology for these deductions. As a result of the JCT approval, Southern Company recognized $176 million of previously unrecognized tax benefits and reversed $36 million of associated accrued interest. If the suspension of the Kemper IGCC start-up activities results in an abandonment, any amount not allowed under IRC Section 174 would be claimed as a deduction under IRC Section 165 in the year an abandonment is determined. The ultimate outcome of this matter cannot be determined at this time.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVES	DERIVATIVES Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas are exposed to market risks, including commodity price risk, interest rate risk, weather risk, and occasionally foreign currency exchange rate risk. To manage the volatility attributable to these exposures, each company nets its exposures, where possible, to take advantage of natural offsets and enters into various derivative transactions for the remaining exposures pursuant to each company's policies in areas such as counterparty exposure and risk management practices. Southern Company Gas' wholesale gas operations use various contracts in its commercial activities that generally meet the definition of derivatives. For the traditional electric operating companies, Southern Power, and Southern Company Gas' other businesses, each company's policy is that derivatives are to be used primarily for hedging purposes and mandates strict adherence to all applicable risk management policies. Derivative positions are monitored using techniques including, but not limited to, market valuation, value at risk, stress testing, and sensitivity analysis. Derivative instruments are recognized at fair value in the balance sheets as either assets or liabilities and are presented on a net basis. See Note (C) for additional information. In the statements of cash flows, the cash impacts of settled energy-related and interest rate derivatives are recorded as operating activities. The cash impacts of settled foreign currency derivatives are classified as operating or financing activities to correspond with classification of the hedged interest or principal, respectively. Energy-Related Derivatives Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas enter into energy-related derivatives to hedge exposures to electricity, natural gas, and other fuel price changes. However, due to cost-based rate regulations and other various cost recovery mechanisms, the traditional electric operating companies and the natural gas distribution utilities have limited exposure to market volatility in energy-related commodity prices. Each of the traditional electric operating companies and certain of the natural gas distribution utilities of Southern Company Gas manage fuel-hedging programs, implemented per the guidelines of their respective state PSCs or other applicable state regulatory agencies, through the use of financial derivative contracts, which is expected to continue to mitigate price volatility. The Florida PSC extended the moratorium on Gulf Power's fuel-hedging program until January 1, 2021 in connection with the 2017 Rate Case Settlement Agreement. The moratorium does not have an impact on the recovery of existing hedges entered into under the previously-approved hedging program. The traditional electric operating companies (with respect to wholesale generating capacity) and Southern Power have limited exposure to market volatility in energy-related commodity prices because their long-term sales contracts shift substantially all fuel cost responsibility to the purchaser. However, the traditional electric operating companies and Southern Power may be exposed to market volatility in energy-related commodity prices to the extent any uncontracted capacity is used to sell electricity. Southern Company Gas retains exposure to price changes that can, in a volatile energy market, be material and can adversely affect its results of operations. Southern Company Gas also enters into weather derivative contracts as economic hedges of operating margins in the event of warmer-than-normal weather. Exchange-traded options are carried at fair value, with changes reflected in operating revenues. Non exchange-traded options are accounted for using the intrinsic value method. Changes in the intrinsic value for non-exchange-traded contracts are reflected in the statements of income. Energy-related derivative contracts are accounted for under one of three methods: • Regulatory Hedges — Energy-related derivative contracts which are designated as regulatory hedges relate primarily to the traditional electric operating companies' and the natural gas distribution utilities' fuel-hedging programs, where gains and losses are initially recorded as regulatory liabilities and assets, respectively, and then are included in fuel expense as the underlying fuel is used in operations and ultimately recovered through the respective fuel cost recovery clauses. • Cash Flow Hedges — Gains and losses on energy-related derivatives designated as cash flow hedges (which are mainly used to hedge anticipated purchases and sales) are initially deferred in OCI before being recognized in the statements of income in the same period as the hedged transactions are reflected in earnings. • Not Designated — Gains and losses on energy-related derivative contracts that are not designated or fail to qualify as hedges are recognized in the statements of income as incurred. Some energy-related derivative contracts require physical delivery as opposed to financial settlement, and this type of derivative is both common and prevalent within the electric and natural gas industries. When an energy-related derivative contract is settled physically, any cumulative unrealized gain or loss is reversed and the contract price is recognized in the respective line item representing the actual price of the underlying goods being delivered. At September 30, 2017 , the net volume of energy-related derivative contracts for natural gas positions for the Southern Company system, together with the longest hedge date over which the respective entity is hedging its exposure to the variability in future cash flows for forecasted transactions and the longest non-hedge date for derivatives not designated as hedges, were as follows: Net Purchased mmBtu Longest Hedge Date Longest Non-Hedge Date (in millions) Southern Company (*) 463 2021 2024 Alabama Power 66 2020 — Georgia Power 159 2021 — Gulf Power 28 2020 — Mississippi Power 44 2021 — Southern Power 13 2018 — Southern Company Gas (*) 153 2020 2024 (*) Southern Company's and Southern Company Gas' derivative instruments include both long and short natural gas positions. A long position is a contract to purchase natural gas and a short position is a contract to sell natural gas. Southern Company Gas' volume represents the net of long natural gas positions of 3.3 billion mmBtu and short natural gas positions of 3.1 billion mmBtu as of September 30, 2017 , which is also included in Southern Company's total volume. In addition to the volumes discussed above, the traditional electric operating companies and Southern Power enter into physical natural gas supply contracts that provide the option to sell back excess gas due to operational constraints. The maximum expected volume of natural gas subject to such a feature is 34 million mmBtu for Southern Company, 11 million mmbtu for Georgia Power and Southern Power, 5 million mmbtu for Alabama Power, 3 million mmBtu for Gulf Power, and 4 million mmBtu for Mississippi Power. For cash flow hedges of energy-related derivatives, the amounts expected to be reclassified from accumulated OCI to earnings for the next 12 -month period ending September 30, 2018 are $5 million for Southern Power and immaterial for all other registrants. Interest Rate Derivatives Southern Company and certain subsidiaries may also enter into interest rate derivatives to hedge exposure to changes in interest rates. The derivatives employed as hedging instruments are structured to minimize ineffectiveness. Derivatives related to existing variable rate securities or forecasted transactions are accounted for as cash flow hedges where the effective portion of the derivatives' fair value gains or losses is recorded in OCI and is reclassified into earnings at the same time the hedged transactions affect earnings, with any ineffectiveness recorded directly to earnings. Derivatives related to existing fixed rate securities are accounted for as fair value hedges, where the derivatives' fair value gains or losses and hedged items' fair value gains or losses are both recorded directly to earnings, providing an offset, with any difference representing ineffectiveness. Fair value gains or losses on derivatives that are not designated or fail to qualify as hedges are recognized in the statements of income as incurred. At September 30, 2017 , the following interest rate derivatives were outstanding: Notional Amount Interest Rate Received Weighted Average Interest Rate Paid Hedge Maturity Date Fair Value Gain (Loss) at September 30, 2017 (in millions) (in millions) Cash Flow Hedges of Existing Debt Mississippi Power $ 900 1-month 0.79% March 2018 $ 2 Fair Value Hedges of Existing Debt Southern Company (*) 300 2.75% 3-month June 2020 — Southern Company (*) 1,500 2.35% 1-month July 2021 (19 ) Georgia Power 250 5.40% 3-month June 2018 — Georgia Power 500 1.95% 3-month December 2018 (2 ) Georgia Power 200 4.25% 3-month December 2019 — Southern Company Consolidated $ 3,650 $ (19 ) (*) Represents the Southern Company parent entity. The estimated pre-tax gains (losses) related to interest rate derivatives expected to be reclassified from accumulated OCI to interest expense for the next 12 -month period ending September 30, 2018 are $(19) million for Southern Company and immaterial for all other registrants. Southern Company and certain subsidiaries have deferred gains and losses expected to be amortized into earnings through 2046 . Foreign Currency Derivatives Southern Company and certain subsidiaries may also enter into foreign currency derivatives to hedge exposure to changes in foreign currency exchange rates, such as that arising from the issuance of debt denominated in a currency other than U.S. dollars. Derivatives related to forecasted transactions are accounted for as cash flow hedges where the effective portion of the derivatives' fair value gains or losses is recorded in OCI and is reclassified into earnings at the same time that the hedged transactions affect earnings, including foreign currency gains or losses arising from changes in the U.S. currency exchange rates. Any ineffectiveness is recorded directly to earnings. The derivatives employed as hedging instruments are structured to minimize ineffectiveness. At September 30, 2017 , the following foreign currency derivatives were outstanding: Pay Notional Pay Rate Receive Notional Receive Rate Hedge Fair Value Gain (Loss) at September 30, 2017 (in millions) (in millions) (in millions) Cash Flow Hedges of Existing Debt Southern Power $ 677 2.95% € 600 1.00% June 2022 $ 42 Southern Power 564 3.78% 500 1.85% June 2026 38 Total $ 1,241 € 1,100 $ 80 The estimated pre-tax gains (losses) related to foreign currency derivatives that will be reclassified from accumulated OCI to earnings for the next 12 -month period ending September 30, 2018 are $(23) million for Southern Company and Southern Power. Derivative Financial Statement Presentation and Amounts Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas enter into derivative contracts that may contain certain provisions that permit intra-contract netting of derivative receivables and payables for routine billing and offsets related to events of default and settlements. Southern Company and certain subsidiaries also utilize master netting agreements to mitigate exposure to counterparty credit risk. These agreements may contain provisions that permit netting across product lines and against cash collateral. The fair value amounts of derivative assets and liabilities on the balance sheet are presented net to the extent that there are netting arrangements or similar agreements with the counterparties. The fair value of energy-related derivatives, interest rate derivatives, and foreign currency derivatives was reflected in the balance sheets as follows: As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Southern Company Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 21 $ 25 $ 73 $ 27 Other deferred charges and assets/Other deferred credits and liabilities 13 23 25 33 Total derivatives designated as hedging instruments for regulatory purposes $ 34 $ 48 $ 98 $ 60 Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 8 $ 6 $ 23 $ 7 Interest rate derivatives: Other current assets/Other current liabilities 5 1 12 1 Other deferred charges and assets/Other deferred credits and liabilities — 23 1 28 Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 25 Other deferred charges and assets/Other deferred credits and liabilities 103 — — 33 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 116 $ 53 $ 36 $ 94 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ 271 $ 254 $ 489 $ 483 Other deferred charges and assets/Other deferred credits and liabilities 115 103 66 81 Interest rate derivatives: Other current assets/Other current liabilities — — 1 — Total derivatives not designated as hedging instruments $ 386 $ 357 $ 556 $ 564 Gross amounts recognized $ 536 $ 458 $ 690 $ 718 Gross amounts offset (*) $ (275 ) $ (351 ) $ (462 ) $ (524 ) Net amounts recognized in the Balance Sheets $ 261 $ 107 $ 228 $ 194 As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Alabama Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 6 $ 4 $ 13 $ 5 Other deferred charges and assets/Other deferred credits and liabilities 3 3 7 4 Total derivatives designated as hedging instruments for regulatory purposes $ 9 $ 7 $ 20 $ 9 Gross amounts recognized $ 9 $ 7 $ 20 $ 9 Gross amounts offset $ (5 ) $ (5 ) $ (8 ) $ (8 ) Net amounts recognized in the Balance Sheets $ 4 $ 2 $ 12 $ 1 Georgia Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 10 $ 3 $ 30 $ 1 Other deferred charges and assets/Other deferred credits and liabilities 8 8 14 7 Total derivatives designated as hedging instruments for regulatory purposes $ 18 $ 11 $ 44 $ 8 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ 1 $ 1 $ 2 $ — Other deferred charges and assets/Other deferred credits and liabilities — 2 — 3 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 1 $ 3 $ 2 $ 3 Gross amounts recognized $ 19 $ 14 $ 46 $ 11 Gross amounts offset $ (10 ) $ (10 ) $ (8 ) $ (8 ) Net amounts recognized in the Balance Sheets $ 9 $ 4 $ 38 $ 3 Gulf Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ — $ 13 $ 4 $ 12 Other deferred charges and assets/Other deferred credits and liabilities — 9 1 17 Total derivatives designated as hedging instruments for regulatory purposes $ — $ 22 $ 5 $ 29 Gross amounts recognized $ — $ 22 $ 5 $ 29 Gross amounts offset $ — $ — $ (4 ) $ (4 ) Net amounts recognized in the Balance Sheets $ — $ 22 $ 1 $ 25 As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Mississippi Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 1 $ 4 $ 2 $ 6 Other deferred charges and assets/Other deferred credits and liabilities 2 3 2 5 Total derivatives designated as hedging instruments for regulatory purposes $ 3 $ 7 $ 4 $ 11 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ 2 $ — $ 2 $ — Other deferred charges and assets/Other deferred credits and liabilities — — 1 — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 2 $ — $ 3 $ — Gross amounts recognized $ 5 $ 7 $ 7 $ 11 Gross amounts offset $ (3 ) $ (3 ) $ (3 ) $ (3 ) Net amounts recognized in the Balance Sheets $ 2 $ 4 $ 4 $ 8 Southern Power Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 8 $ 4 $ 18 $ 4 Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 25 Other deferred charges and assets/Other deferred credits and liabilities 103 — — 33 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 111 $ 27 $ 18 $ 62 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ 1 $ — $ 3 $ 1 Interest rate derivatives: Other current assets/Other current liabilities — — 1 — Total derivatives not designated as hedging instruments $ 1 $ — $ 4 $ 1 Gross amounts recognized $ 112 $ 27 $ 22 $ 63 Gross amounts offset $ (1 ) $ (1 ) $ (5 ) $ (5 ) Net amounts recognized in the Balance Sheets $ 111 $ 26 $ 17 $ 58 As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Southern Company Gas Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ 4 $ 1 $ 24 $ 3 Other deferred charges and assets/Other deferred credits and liabilities — — 1 — Total derivatives designated as hedging instruments for regulatory purposes $ 4 $ 1 $ 25 $ 3 Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ — $ 2 $ 4 $ 3 Derivatives not designated as hedging instruments Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ 270 $ 254 $ 486 $ 482 Other deferred charges and assets/Other deferred credits and liabilities 115 103 66 81 Total derivatives not designated as hedging instruments $ 385 $ 357 $ 552 $ 563 Gross amounts of recognized $ 389 $ 360 $ 581 $ 569 Gross amounts offset (*) $ (251 ) $ (327 ) $ (435 ) $ (497 ) Net amounts recognized in the Balance Sheets $ 138 $ 33 $ 146 $ 72 (*) Gross amounts offset include cash collateral held on deposit in broker margin accounts of $76 million and $62 million as of September 30, 2017 and December 31, 2016 , respectively. At September 30, 2017 and December 31, 2016 , the pre-tax effects of unrealized derivative gains (losses) arising from energy-related derivative instruments designated as regulatory hedging instruments and deferred were as follows: Regulatory Hedge Unrealized Gain (Loss) Recognized in the Balance Sheet at September 30, 2017 Derivative Category and Balance Sheet Location Southern Company (b) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (c) (in millions) Energy-related derivatives: Other regulatory assets, current $ (18 ) $ (1 ) $ — $ (13 ) $ (3 ) $ (1 ) Other regulatory assets, deferred (12 ) (1 ) (1 ) (9 ) (1 ) — Other regulatory liabilities, current (a) 14 3 7 — — 4 Other regulatory liabilities, deferred (b) 2 1 1 — — — Total energy-related derivative gains (losses) $ (14 ) $ 2 $ 7 $ (22 ) $ (4 ) $ 3 (a) Georgia Power includes other regulatory liabilities, current in other current liabilities. (b) Georgia Power includes other regulatory liabilities, deferred in other deferred credits and liabilities. (c) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $1 million at September 30, 2017 . Regulatory Hedge Unrealized Gain (Loss) Recognized in the Balance Sheet at December 31, 2016 Derivative Category and Balance Sheet Location Southern Company (c) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (c) (in millions) Energy-related derivatives: Other regulatory assets, current $ (16 ) $ (1 ) $ — $ (9 ) $ (5 ) $ (1 ) Other regulatory assets, deferred (19 ) — — (16 ) (3 ) — Other regulatory liabilities, current (a) 56 8 29 1 1 17 Other regulatory liabilities, deferred (b) 12 4 7 — — 1 Total energy-related derivative gains (losses) $ 33 $ 11 $ 36 $ (24 ) $ (7 ) $ 17 (a) Georgia Power includes other regulatory liabilities, current in other current liabilities. (b) Georgia Power includes other regulatory liabilities, deferred in other deferred credits and liabilities. (c) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $ 8 million at December 31, 2016 . For the three months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives, interest rate derivatives, and foreign currency derivatives designated as cash flow hedging instruments were as follows: Derivatives in Cash Flow Hedging Relationships Gain (Loss) Recognized in OCI on Derivative (Effective Portion) Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) Statements of Income Location Amount 2017 2016 2017 2016 (in millions) (in millions) Southern Company Energy-related derivatives $ (6 ) $ — Depreciation and amortization $ (6 ) $ 1 Interest rate derivatives (1 ) (6 ) Interest expense, net of amounts capitalized (5 ) (6 ) Foreign currency derivatives 46 37 Interest expense, net of amounts capitalized (5 ) (6 ) Other income (expense), net (*) 43 7 Total $ 39 $ 31 $ 27 $ (4 ) Alabama Power Interest rate derivatives $ — $ — Interest expense, net of amounts capitalized $ (2 ) $ (2 ) Georgia Power Interest rate derivatives $ — $ — Interest expense, net of amounts capitalized $ (1 ) $ (1 ) Mississippi Power Interest rate derivatives $ (1 ) $ (1 ) Interest expense, net of amounts capitalized $ — $ — Southern Power Energy-related derivatives $ (6 ) $ — Depreciation and amortization $ (6 ) $ 1 Foreign currency derivatives 46 37 Interest expense, net of amounts capitalized (5 ) (6 ) Other income (expense), net (*) 43 7 Total $ 40 $ 37 $ 32 $ 2 Southern Company Gas Interest rate derivatives $ — $ (5 ) Interest expense, net of amounts capitalized $ — $ — (*) The reclassification from accumulated OCI into other income (expense), net completely offsets currency gains and losses arising from changes in the U.S. currency exchange rates used to record the euro-denominated notes. For the nine months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives, interest rate derivatives, and foreign currency derivatives designated as cash flow hedging instruments were as follows: Derivatives in Cash Flow Gain (Loss) Gain (Loss) Reclassified from Accumulated OCI into Statements of Income Location Amount 2017 2016 2017 2016 (in millions) (in millions) Southern Company Energy-related derivatives $ (26 ) $ (1 ) Depreciation and amortization $ (12 ) $ 1 Interest rate derivatives (2 ) (189 ) Interest expense, net of amounts capitalized (15 ) (13 ) Foreign currency derivatives 114 (1 ) Interest expense, net of amounts capitalized (17 ) (7 ) Other income (expense), net (*) 139 (13 ) Total $ 86 $ (191 ) $ 95 $ (32 ) Alabama Power Interest rate derivatives $ — $ (3 ) Interest expense, net of amounts capitalized $ (5 ) $ (5 ) Georgia Power Interest rate derivatives $ — $ — Interest expense, net of amounts capitalized $ (3 ) $ (3 ) Gulf Power Energy-related derivatives $ (1 ) $ — Depreciation and amortization $ — $ — Interest rate derivatives (1 ) (7 ) Interest expense, net of amounts capitalized — — Total $ (2 ) $ (7 ) $ — $ — Mississippi Power Interest rate derivatives $ — $ (1 ) Interest expense, net of amounts capitalized $ (1 ) $ (1 ) Southern Power Energy-related derivatives $ (21 ) $ (1 ) Depreciation and amortization $ (12 ) $ 1 Interest rate derivatives — — Interest expense, net of amounts capitalized — (1 ) Foreign currency derivatives 114 (1 ) Interest expense, net of amounts capitalized (17 ) (7 ) Other income (expense), net (*) 139 (13 ) Total $ 93 $ (2 ) $ 110 $ (20 ) (*) The reclassification from accumulated OCI into other income (expense), net completely offsets currency gains and losses arising from changes in the U.S. currency exchange rates used to record the euro-denominated notes. For Southern Company Gas, the pre-tax effect of energy related derivatives and interest rate derivatives designated as cash flow hedging instruments recognized in OCI and those reclassified from accumulated OCI into earnings for the successor nine months ended September 30, 2017 , the successor period of July 1, 2016 through September 30, 2016, and the predecessor period of January 1, 2016 through June 30, 2016 were as follows: Gain (Loss) Recognized in OCI on Derivative (Effective Portion) Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) Derivatives in Cash Flow Hedging Relationships Nine Months Ended September 30, 2017 Statements of Income Location Nine Months Ended September 30, 2017 (in millions) (in millions) Energy-related derivatives $ (4 ) Cost of natural gas $ — Gain (Loss) Recognized in OCI on Derivative (Effective Portion) Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) Successor Predecessor Successor Predecessor Derivatives in Cash Flow Hedging Relationships July 1, 2016 through September 30, 2016 January 1, 2016 through June 30, 2016 Statements of Income Location July 1, 2016 January 1, 2016 through June 30, 2016 (in millions) (in millions) (in millions) (in millions) Energy-related derivatives $ — $ — Cost of natural gas $ — $ (1 ) Interest rate derivatives (5 ) (64 ) Interest expense, net of amounts capitalized — — Total $ (5 ) $ (64 ) $ — $ (1 ) For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives and interest rate derivatives designated as cash flow hedging instruments were immaterial for the other registrants. For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives and interest rate derivatives not designated as hedging instruments on the statements of income were as follows: Gain (Loss) Three Months Ended September 30, Nine Months Ended September 30, Derivatives in Non-Designated Hedging Relationships Statements of Income Location 2017 2016 2017 2016 (in millions) (in millions) Southern Company Energy Related derivatives: Natural gas revenues (*) $ (17 ) $ — $ 48 $ — Cost of natural gas 2 6 (2 ) 6 Total derivatives in non-designated hedging relationships $ (15 ) $ 6 $ 46 $ 6 (*) Excludes gains (losses) recorded in cost of natural gas associated with weather derivatives of $15 million for the nine months ended September 30, 2017 and immaterial amounts for all other periods presented. Gain (Loss) Successor Successor Successor Successor Predecessor Derivatives in Non-Designated Hedging Relationships Statements of Income Location Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Nine Months Ended September 30, 2017 July 1, 2016 through September 30, 2016 January 1, 2016 through June 30, 2016 (in millions) (in millions) (in millions) (in millions) Southern Company Gas Energy Related derivatives: Natural gas revenues (*) $ (17 ) $ — $ 48 $ — $ (1 ) Cost of natural gas 2 6 (2 ) 6 (62 ) Total derivatives in non-designated hedging relationships $ (15 ) $ 6 $ 46 $ 6 $ (63 ) (*) Excludes gains recorded in cost of natural gas associated with weather derivatives of $15 million for the successor nine months ended September 30, 2017 and immaterial amounts for all other periods presented. For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives and interest rate derivatives not designated as hedging instruments were immaterial for the traditional electric operating companies and Southern Power. For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of interest rate derivatives designated as fair value hedging instruments were as follows: Derivatives in Fair Value Hedging Relationships Gain (Loss) Three Months Ended September 30, Nine Months Ended September 30, Derivative Category Statements of Income Location 2017 2016 2017 2016 (in millions) (in millions) Southern Company Interest rate derivatives: Interest expense, net of amounts capitalized $ (5 ) $ (9 ) $ (6 ) $ 15 Georgia Power Interest rate derivatives: Interest expense, net of amounts capitalized $ — $ (5 ) $ (1 ) $ 10 For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of interest rate derivatives designated as fair value hedging instruments were offset by changes to the carrying value of long-term debt. There was no material ineffectiveness recorded in earnings for any registrant for any period presented. Contingent Features Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas do not have any credit arrangements that would require material changes in payment schedules or terminations as a result of a credit rating downgrade. There are certain derivatives that could require collateral, but not accelerated payment, in the event of various credit rating changes of certain Southern Company subsidiaries. At September 30, 2017 , the registrants had no collateral posted with derivative counterparties to satisfy these arrangements. At September 30, 2017 , the fair value of derivative liabilities with contingent features was immaterial for all registrants. The maximum potential collateral requirements arising from the credit-risk-related contingent features, at a rating below BBB- and/or Baa3, were immaterial for all registrants. The maximum potential collateral requirements arising from the credit-risk-related contingent features for the traditional electric operating companies and Southern Power include certain agreements that could require collateral in the event that one or more Southern Company power pool participants has a credit rating change to below investment grade. Generally, collateral may be provided by a Southern Company guaranty, letter of credit, or cash. If collateral is required, fair value amounts recognized for the right to reclaim cash collateral or the obligation to return cash collateral are not offset against fair value amounts recognized for derivatives executed with the same counterparty. Alabama Power and Southern Power maintain accounts with certain regional transmission organizations to facilitate financial derivative transactions. Based on the value of the positions in these accounts and the associated margin requirements, Alabama Power and Southern Power may be required to post collateral. At September 30, 2017 , cash collateral posted in these accounts was immaterial. Southern Company Gas maintains accounts with brokers or the clearing houses of certain exchanges to facilitate financial derivative transactions. Based on the value of the positions in these accounts and the associated margin requirements, Southern Company Gas may be required to deposit cash into these accounts. At September 30, 2017 , cash collateral held on deposit in broker margin accounts was $76 million . Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas are exposed to losses related to financial instruments in the event of counterparties' nonperformance. Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas only enter into agreements and material transactions with counterparties that have investment grade credit ratings by Moody's and S&P or with counterparties who have posted collateral to cover potential credit exposure. Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas have also established risk management policies and controls to determine and monitor

Acquisitions and Dispositions

Acquisitions and Dispositions	9 Months Ended
	Sep. 30, 2017
Business Combinations [Abstract]
ACQUISITIONS AND DISPOSITIONS	ACQUISITIONS AND DISPOSITIONS Southern Company Merger with Southern Company Gas Southern Company Gas is an energy services holding company whose primary business is the distribution of natural gas through the natural gas distribution utilities. On July 1, 2016, Southern Company completed the Merger for a total purchase price of approximately $8.0 billion and Southern Company Gas became a wholly-owned, direct subsidiary of Southern Company. The Merger was accounted for using the acquisition method of accounting with the assets acquired and liabilities assumed recognized at fair value as of the acquisition date. The following table presents the final purchase price allocation: Southern Company Gas Purchase Price (in millions) Current assets $ 1,557 Property, plant, and equipment 10,108 Goodwill 5,967 Intangible assets 400 Regulatory assets 1,118 Other assets 229 Current liabilities (2,201 ) Other liabilities (4,742 ) Long-term debt (4,261 ) Noncontrolling interest (174 ) Total purchase price $ 8,001 The excess of the purchase price over the estimated fair values of the assets acquired and liabilities assumed of $6.0 billion is recognized as goodwill, which is primarily attributable to positioning the Southern Company system to provide natural gas infrastructure to meet customers' growing energy needs and to compete for growth across the energy value chain. Southern Company anticipates that much of the value assigned to goodwill will not be deductible for tax purposes. The valuation of identifiable intangible assets included customer relationships, trade names, and storage and transportation contracts with estimated lives of one to 28 years . The estimated fair value measurements of identifiable intangible assets were primarily based on significant unobservable inputs (Level 3). The results of operations for Southern Company Gas have been included in Southern Company's consolidated financial statements from the date of acquisition and consist of operating revenues of $565 million and $2.8 billion and net income of $15 million and $303 million for the three and nine months ended September 30, 2017 , respectively, and operating revenues and net income of $543 million and $4 million , respectively, for the three months ended September 30, 2016. The following summarized unaudited pro forma consolidated statement of earnings information assumes that the acquisition of Southern Company Gas was completed on January 1, 2015. The summarized unaudited pro forma consolidated statement of earnings information includes adjustments for (i) intercompany sales, (ii) amortization of intangible assets, (iii) adjustments to interest expense to reflect current interest rates on Southern Company Gas debt and additional interest expense associated with borrowings by Southern Company to fund the Merger, and (iv) the elimination of nonrecurring expenses associated with the Merger. For the Nine Months Ended September 30, 2016 Operating revenues (in millions) $ 16,609 Net income attributable to Southern Company (in millions) $ 2,394 Basic Earnings Per Share (EPS) $ 2.52 Diluted EPS $ 2.51 These unaudited pro forma results are for comparative purposes only and may not be indicative of the results that would have occurred had this acquisition been completed on January 1, 2015 or the results that would be attained in the future. Acquisition of PowerSecure On May 9, 2016, Southern Company acquired all of the outstanding stock of PowerSecure, a provider of products and services in the areas of distributed generation, energy efficiency, and utility infrastructure, for $18.75 per common share in cash, resulting in an aggregate purchase price of $429 million . As a result, PowerSecure became a wholly-owned subsidiary of Southern Company. The acquisition of PowerSecure was accounted for using the acquisition method of accounting with the assets acquired and liabilities assumed recognized at fair value as of the acquisition date. The following table presents the final purchase price allocation: PowerSecure Purchase Price (in millions) Current assets $ 172 Property, plant, and equipment 46 Intangible assets 106 Goodwill 284 Other assets 4 Current liabilities (121 ) Long-term debt, including current portion (48 ) Deferred credits and other liabilities (14 ) Total purchase price $ 429 The excess of the purchase price over the estimated fair values of the assets acquired and liabilities assumed of $284 million was recognized as goodwill, which is primarily attributable to expected business expansion opportunities for PowerSecure. Southern Company anticipates that the majority of the value assigned to goodwill will not be deductible for tax purposes. The valuation of identifiable intangible assets included customer relationships, trade names, patents, backlog, and software with estimated lives of one to 26 years. The estimated fair value measurements of identifiable intangible assets were primarily based on significant unobservable inputs (Level 3). The results of operations for PowerSecure have been included in Southern Company's consolidated financial statements from the date of acquisition and are immaterial to the consolidated financial results of Southern Company. Pro forma results of operations have not been presented for the acquisition because the effects of the acquisition were immaterial to Southern Company's consolidated financial results for all periods presented. Southern Power See Note 2 to the financial statements of Southern Power and Note 12 to the financial statements of Southern Company under "Southern Power" in Item 8 of the Form 10-K for additional information. Acquisitions During the Nine Months Ended September 30, 2017 During the nine months ended September 30, 2017, in accordance with Southern Power's overall growth strategy, one of Southern Power's wholly-owned subsidiaries acquired the project discussed below . Acquisition-related costs were expensed as incurred and were not material. Project Facility Resource Seller; Acquisition Date Approximate Nameplate Capacity ( MW ) Location Southern Power Percentage Ownership Actual COD PPA Contract Period Bethel Wind Invenergy, 276 Castro County, TX 100 % January 2017 12 years The aggregate amounts of revenue and net income recognized by Southern Power related to the Bethel facility included in Southern Power's condensed consolidated statements of income for year-to-date 2017 were immaterial. The Bethel facility did not have operating revenues or activities prior to completion of construction and the assets being placed in service; therefore, supplemental pro forma information as though the acquisition occurred as of the beginning of 2017 and for the comparable 2016 period is not meaningful and has been omitted. In connection with Southern Power's 2016 acquisitions, allocations of the purchase price to individual assets were finalized during the nine months ended September 30, 2017 with no changes to amounts originally reported for Boulder 1, Grant Plains, Grant Wind, Henrietta, Mankato, Passadumkeag, Salt Fork, Tyler Bluff, and Wake Wind. Subsequent to September 30, 2017, Southern Power purchased all of the redeemable noncontrolling interests, representing 10% of the membership interests, in Southern Turner Renewable Energy, LLC and repaid $14 million of notes payable to Turner Renewable Energy, LLC. Construction Projects Completed and in Progress During the nine months ended September 30, 2017 , in accordance with its overall growth strategy, Southern Power completed construction of and placed in service, or continued construction of, the projects set forth in the following table. Through September 30, 2017 , total costs of construction incurred for these projects were $494 million , of which $122 million remained in CWIP. Total aggregate construction costs, excluding the acquisition costs, are expected to be between $360 million and $415 million for the Mankato and Cactus Flats facilities. The ultimate outcome of these matters cannot be determined at this time. Project Facility Resource Approximate Nameplate Capacity ( MW ) Location Actual/Expected COD PPA Contract Period Projects Completed During the Nine Months Ended September 30, 2017 East Pecos Solar 120 Pecos County, TX March 2017 15 years Lamesa Solar 102 Dawson County, TX April 2017 15 years Projects Under Construction as of September 30, 2017 Cactus Flats (*) Wind 148 Concho County, TX Third quarter 2018 12-15 years Mankato Natural Gas 345 Mankato, MN Second quarter 2019 20 years (*) On July 31, 2017, Southern Power acquired a 100% ownership interest in the Cactus Flats facility, which is in the early stages of construction, from RES America Developments, Inc. Development Projects In December 2016, as part of Southern Power's renewable development strategy, one of Southern Power's wholly-owned subsidiaries entered into a joint development agreement with Renewable Energy Systems Americas, Inc. to develop and construct approximately 3,000 MWs of wind projects. Also in December 2016, Southern Power signed agreements and made payments to purchase wind turbine equipment from Siemens Wind Power, Inc. and Vestas-American Wind Technology, Inc. to be used for construction of the facilities. All of the wind turbine equipment was delivered by April 2017, which allows the projects to qualify for 100% PTCs for 10 years following their expected commercial operation dates between 2018 and 2020. The ultimate outcome of these matters cannot be determined at this time. Southern Company Gas On October 15, 2017, Southern Company Gas subsidiary, Pivotal Utility Holdings, entered into agreements for the sale of the assets of two of its natural gas distribution utilities, Elizabethtown Gas and Elkton Gas, to South Jersey Industries, Inc. for a total cash purchase price of $1.7 billion . The completion of each sale is subject to the satisfaction or waiver of certain closing conditions, including, among others, (i) the expiration or termination of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act; (ii) the receipt of required regulatory approvals, including the FERC, the Federal Communications Commission, the New Jersey BPU, and, with respect to the sale of Elkton Gas, the Maryland PSC; and (iii) other customary closing conditions. The sales are expected to be completed by the end of the third quarter 2018. The ultimate outcome of these matters cannot be determined at this time.

Joint Ownership Agreements

Joint Ownership Agreements	9 Months Ended
	Sep. 30, 2017
Regulated Operations [Abstract]
JOINT OWNERSHIP AGREEMENTS	JOINT OWNERSHIP AGREEMENTS Southern Company Gas See Note 4 to the financial statements of Southern Company Gas in Item 8 of the Form 10-K for additional information. Equity Method Investments The carrying amounts of Southern Company Gas' equity method investments as of September 30, 2017 and December 31, 2016 and related income from those investments for the successor three and nine month periods ended September 30, 2017 , the successor three-month period ended September 30, 2016 , and for the predecessor period January 1, 2016 through June 30, 2016 were as follows: Balance Sheet Information September 30, 2017 December 31, 2016 (in millions) SNG $ 1,385 $ 1,394 Atlantic Coast Pipeline 61 33 PennEast Pipeline 49 22 Triton 43 44 Pivotal JAX LNG, LLC 40 16 Horizon Pipeline 30 30 Other 1 2 Total $ 1,609 $ 1,541 Successor Successor Successor Predecessor Income Statement Information Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Nine Months Ended September 30, 2017 January 1, 2016 through June 30, 2016 (in millions) (in millions) (in millions) (in millions) SNG $ 28 $ 27 $ 86 $ — PennEast Pipeline 1 — 5 — Atlantic Coast Pipeline 1 1 4 — Triton 1 1 3 1 Horizon Pipeline 1 — 2 1 Total $ 32 $ 29 $ 100 $ 2 Southern Natural Gas In September 2016, Southern Company Gas, through a wholly-owned, indirect subsidiary, acquired a 50% equity interest in SNG, which is accounted for as an equity method investment. On March 31, 2017, Southern Company Gas made an additional $50 million contribution to maintain its 50% equity interest in SNG. See Note 11 to the financial statements of Southern Company Gas under "Investment in SNG" in Item 8 of the Form 10-K for additional information on this investment. Selected financial information of SNG for the three and nine months ended September 30, 2017 and for the period September 1, 2016 through September 30, 2016 is as follows: Income Statement Information Three Months Ended September 30, 2017 Nine Months Ended September 30, 2017 September 1, 2016 through September 30, 2016 (in millions) Revenues $ 146 $ 445 $ 82 Operating income $ 71 $ 218 $ 60 Net income $ 57 $ 172 $ 55

Segment and Related Information

Segment and Related Information	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
SEGMENT AND RELATED INFORMATION	SEGMENT AND RELATED INFORMATION Southern Company The primary businesses of the Southern Company system are electricity sales by the traditional electric operating companies and Southern Power and the distribution of natural gas by Southern Company Gas. The four traditional electric operating companies – Alabama Power, Georgia Power, Gulf Power, and Mississippi Power – are vertically integrated utilities providing electric service in four Southeastern states. Southern Power constructs, acquires, owns, and manages power generation assets, including renewable energy projects, and sells electricity at market-based rates in the wholesale market. Southern Company Gas distributes natural gas through the seven natural gas distribution utilities in seven states and is involved in several other complementary businesses including gas marketing services, wholesale gas services, and gas midstream operations. Southern Company's reportable business segments are the sale of electricity by the four traditional electric operating companies, the sale of electricity in the competitive wholesale market by Southern Power, and the sale of natural gas and other complementary products and services by Southern Company Gas. Revenues from sales by Southern Power to the traditional electric operating companies were $105 million and $295 million for the three and nine months ended September 30, 2017 , respectively, and $110 million and $313 million for the three and nine months ended September 30, 2016 , respectively. The "All Other" column includes the Southern Company parent entity, which does not allocate operating expenses to business segments. Also, this category includes segments below the quantitative threshold for separate disclosure. These segments include providing energy technologies and services to electric utilities and large industrial, commercial, institutional, and municipal customers; as well as investments in telecommunications and leveraged lease projects. All other inter-segment revenues are not material. Financial data for business segments and products and services for the three and nine months ended September 30, 2017 and 2016 was as follows: Electric Utilities Traditional Electric Operating Companies Southern Power Eliminations Total Southern Company Gas All Other Eliminations Consolidated (in millions) Three Months Ended Operating revenues $ 5,017 $ 618 $ (112 ) $ 5,523 $ 565 $ 153 $ (40 ) $ 6,201 Segment net income (loss) (a)(b) 1,008 124 — 1,132 15 (80 ) 2 1,069 Nine Months Ended Operating revenues $ 12,960 $ 1,597 $ (318 ) $ 14,239 $ 2,841 $ 442 $ (119 ) $ 17,403 Segment net income (loss) (a)(b)(c) — 276 — 276 303 (232 ) — 347 Total assets at September 30, 2017 $ 73,056 $ 14,648 $ (322 ) $ 87,382 $ 22,190 $ 2,275 $ (1,532 ) $ 110,315 Three Months Ended Operating revenues $ 5,236 $ 500 $ (117 ) $ 5,619 $ 543 $ 139 $ (37 ) $ 6,264 Segment net income (loss) (a)(b) 1,022 176 — 1,198 4 (62 ) (1 ) 1,139 Nine Months Ended Operating revenues $ 13,120 $ 1,189 $ (330 ) $ 13,979 $ 543 $ 311 $ (118 ) $ 14,715 Segment net income (loss) (a)(b) 2,086 315 — 2,401 4 (146 ) (8 ) 2,251 Total assets at December 31, 2016 $ 72,141 $ 15,169 $ (316 ) $ 86,994 $ 21,853 $ 2,474 $ (1,624 ) $ 109,697 (a) Attributable to Southern Company. (b) Segment net income (loss) for the traditional electric operating companies includes pre-tax charges for estimated probable losses on the Kemper IGCC of $34 million ( $21 million after tax) and $88 million ( $54 million after tax) for the three months ended September 30, 2017 and 2016 , respectively, and $3.2 billion ( $2.2 billion after tax) and $222 million ( $137 million after tax) for the nine months ended September 30, 2017 and 2016 , respectively. See Note (B) under " Integrated Coal Gasification Combined Cycle – Kemper IGCC Schedule and Cost Estimate " for additional information. (c) Segment net income (loss) for the traditional electric operating companies also includes a pre-tax charge for the write-down of Gulf Power's ownership of Plant Scherer Unit 3 of $33 million ( $20 million after tax) for the nine months ended September 30, 2017 . See Note (B) under " Regulatory Matters – Gulf Power – Retail Base Rate Cases " for additional information. Products and Services Electric Utilities' Revenues Period Retail Wholesale Other Total (in millions) Three Months Ended September 30, 2017 $ 4,615 $ 718 $ 190 $ 5,523 Three Months Ended September 30, 2016 4,808 613 198 5,619 Nine Months Ended September 30, 2017 $ 11,786 $ 1,867 $ 586 $ 14,239 Nine Months Ended September 30, 2016 11,932 1,455 592 13,979 Southern Company Gas' Revenues Period Gas Gas Other Total (in millions) Three Months Ended September 30, 2017 $ 430 $ 143 $ (8 ) $ 565 Nine Months Ended September 30, 2017 $ 2,119 $ 597 $ 125 $ 2,841 Three and Nine Months Ended September 30, 2016 $ 420 $ 126 $ (3 ) $ 543 Southern Company Gas Southern Company Gas manages its business through four reportable segments – gas distribution operations, gas marketing services, wholesale gas services, and gas midstream operations. The non-reportable segments are combined and presented as all other. Gas distribution operations is the largest component of Southern Company Gas' business and includes natural gas local distribution utilities that construct, manage, and maintain intrastate natural gas pipelines and gas distribution facilities in seven states. Gas marketing services includes natural gas marketing to end-use customers primarily in Georgia and Illinois. Additionally, gas marketing services provides home equipment protection products and services. Wholesale gas services provides natural gas asset management and/or related logistics services for each of Southern Company Gas' utilities except Nicor Gas as well as for non-affiliated companies. Additionally, wholesale gas services engages in natural gas storage and gas pipeline arbitrage and related activities. Gas midstream operations primarily consists of Southern Company Gas' pipeline investments, with storage and fuel operations also aggregated into this segment. The all other column includes segments below the quantitative threshold for separate disclosure, including the subsidiaries that fall below the quantitative threshold for separate disclosure. After the Merger, Southern Company Gas changed its segment performance measure to net income. In order to properly assess net income by segment, Southern Company Gas executed various intercompany note agreements to revise interest charges to its segments. Since such agreements did not exist in the predecessor period, Southern Company Gas is unable to provide the comparable net income. Business segment financial data for the successor three months ended September 30, 2017 and 2016 , the successor nine months ended September 30, 2017 , the successor period of July 1, 2016 through September 30, 2016 , and the predecessor period of January 1, 2016 through June 30, 2016 was as follows: Gas Distribution Operations Gas Marketing Services Wholesale Gas Services (*) Gas Midstream Operations Total All Other Eliminations Consolidated (in millions) Successor – Three Months Ended September 30, 2017: Operating revenues $ 472 $ 143 $ (24 ) $ 16 $ 607 $ 2 $ (44 ) $ 565 Segment net income 52 1 (23 ) 14 44 (29 ) — 15 Successor – Nine Months Ended September 30, 2017: Operating revenues $ 2,255 $ 597 $ 95 $ 53 $ 3,000 $ 7 $ (166 ) $ 2,841 Segment net income 223 36 28 38 325 (22 ) — 303 Successor – Total assets at $ 18,711 $ 2,089 $ 893 $ 2,359 $ 24,052 $ 11,400 $ (13,262 ) $ 22,190 Gas Distribution Operations Gas Marketing Services Wholesale Gas Services (*) Gas Midstream Operations Total All Other Eliminations Consolidated (in millions) Successor – Three Months Ended September 30, 2016: Operating revenues $ 455 $ 126 $ (8 ) $ 13 $ 586 $ 2 $ (45 ) $ 543 Segment net income (loss) 27 (4 ) (11 ) 14 26 (22 ) — 4 Predecessor – January 1, 2016 through June 30, 2016: Operating revenues $ 1,575 $ 435 $ (32 ) $ 25 $ 2,003 $ 4 $ (102 ) $ 1,905 Segment EBIT 353 109 (68 ) (6 ) 388 (60 ) — 328 Successor – Total assets at $ 19,453 $ 2,084 $ 1,127 $ 2,211 $ 24,875 $ 11,145 $ (14,167 ) $ 21,853 (*) The revenues for wholesale gas services are netted with costs associated with its energy and risk management activities. A reconciliation of operating revenues and intercompany revenues is shown in the following table. Third Party Gross Revenues Intercompany Revenues Total Gross Revenues Less Gross Gas Costs Operating Revenues (in millions) Successor – Three Months Ended September 30, 2017 $ 1,411 $ 103 $ 1,514 $ 1,538 $ (24 ) Successor – Nine Months Ended September 30, 2017 4,781 362 5,143 5,048 95 Successor – Three Months Ended September 30 , 2016 1,688 77 1,765 1,773 (8 ) Predecessor – January 1, 2016 through June 30, 2016 $ 2,500 $ 143 $ 2,643 $ 2,675 $ (32 )

Introduction (Policies)

Introduction (Policies)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Accounting	The condensed quarterly financial statements of each registrant included herein have been prepared by such registrant, without audit, pursuant to the rules and regulations of the SEC. The Condensed Balance Sheets as of December 31, 2016 have been derived from the audited financial statements of each registrant. In the opinion of each registrant's management, the information regarding such registrant furnished herein reflects all adjustments, which, except as otherwise disclosed, are of a normal recurring nature, necessary to present fairly the results of operations for the periods ended September 30, 2017 and 2016 . Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations, although each registrant believes that the disclosures regarding such registrant are adequate to make the information presented not misleading. Disclosures which would substantially duplicate the disclosures in the Form 10-K and details which have not changed significantly in amount or composition since the filing of the Form 10-K are generally omitted from this Quarterly Report on Form 10-Q unless specifically required by GAAP. Therefore, these Condensed Financial Statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K. Due to the seasonal variations in the demand for energy, operating results for the periods presented are not necessarily indicative of the operating results to be expected for the full year. Southern Company's financial statements reflect its investments in its subsidiaries, including Southern Company Gas as a result of the Merger, on a consolidated basis. Southern Company Gas' results of operations and cash flows for the three and nine months ended September 30, 2017 and the three months ended September 30, 2016 , as well as its financial condition as of September 30, 2017 and December 31, 2016 , are reflected within Southern Company's consolidated amounts in these accompanying notes herein. The equity method is used for entities in which Southern Company has significant influence but does not control, including Southern Company Gas' investment in SNG, and for variable interest entities where Southern Company has an equity investment but is not the primary beneficiary. See Note (I) under " Southern Company – Merger with Southern Company Gas " for additional information regarding the Merger. Pursuant to the Merger, Southern Company pushed down the application of the acquisition method of accounting to the consolidated financial statements of Southern Company Gas such that the assets and liabilities are recorded at their respective fair values, and goodwill has been established for the excess of the purchase price over the fair value of net identifiable assets. Accordingly, the consolidated financial statements of Southern Company Gas for periods before and after July 1, 2016 (acquisition date) reflect different bases of accounting, and the financial positions and results of operations of those periods are not comparable. Throughout Southern Company Gas' condensed consolidated financial statements and the accompanying notes herein, periods prior to July 1, 2016 are identified as "predecessor," while periods after the acquisition date are identified as "successor."
Reclassification	Certain prior year data presented in the financial statements have been reclassified to conform to the current year presentation. These reclassifications had no impact on the results of operations, financial position, or cash flows of any registrant.
Recently Issued Accounting Standards	Recently Issued Accounting Standards See Note 1 to the financial statements of the registrants under "Recently Issued Accounting Standards" in Item 8 of the Form 10-K for additional information. In 2014, the FASB issued ASC 606, Revenue from Contracts with Customers (ASC 606), replacing the existing accounting standard and industry specific guidance for revenue recognition with a five-step model for recognizing and measuring revenue from contracts with customers. The underlying principle of the standard is to recognize revenue to depict the transfer of goods or services to customers at the amount expected to be collected. The new standard also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenue and the related cash flows arising from contracts with customers. While the registrants expect most of their revenue to be included in the scope of ASC 606, they have not fully completed the evaluation of all revenue arrangements. The majority of Southern Company's, the traditional electric operating companies', and Southern Company Gas' revenue, including energy provided to customers, is from tariff offerings that provide electricity or natural gas without a defined contractual term, as well as longer-term contractual commitments , including PPAs and non-derivative natural gas asset management and optimization arrangements . The majority of Southern Power's revenues includes longer-term PPAs for generation capacity and energy. The registrants expect the adoption of ASC 606 will not result in a significant shift from the current timing of revenue recognition for such transactions . The registrants' ongoing evaluation of other revenue streams and related contracts includes unregulated sales to customers. Some revenue arrangements, such as certain PPAs , energy-related derivatives , and alternative revenue programs, are excluded from the scope of ASC 606 and, therefore, will be accounted for and disclosed or presented separately from revenues under ASC 606 on the registrants' financial statements. In addition, the power and utilities industry continues to evaluate other specific industry issues, including the applicability of ASC 606 to contributions in aid of construction (CIAC). Although final implementation guidance has not been issued, Southern Company, the traditional electric operating companies, and Southern Company Gas expect CIAC to be out of the scope of ASC 606. The new standard is effective for interim and annual reporting periods beginning after December 15, 2017. The registrants intend to use the modified retrospective method of adoption effective January 1, 2018. The registrants have also elected to utilize practical expedients which allow them to apply the standard to open contracts at the date of adoption and to reflect the aggregate effect of all modifications when identifying performance obligations and allocating the transaction price for contracts modified before the effective date. Under the modified retrospective method of adoption, prior year reported results are not restated; however, a cumulative-effect adjustment to retained earnings at January 1, 2018 is recorded. In addition, disclosures will include comparative information on 2018 financial statement line items under current guidance. While the adoption of ASC 606, including the cumulative-effect adjustment, is not expected to have a material impact on either the timing or amount of revenues recognized in the registrants' financial statements, the registrants will continue to evaluate the requirements, as well as any additional clarifying guidance that may be issued. On January 26, 2017, the FASB issued ASU No. 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment (ASU 2017-04). ASU 2017-04 removes the requirement to compare the implied fair value of goodwill with the carrying amount as part of Step 2 of the goodwill impairment test. Under the new standard, the goodwill impairment loss will be measured as the excess of a reporting unit's carrying amount over its fair value, not exceeding the total amount of goodwill allocated to that reporting unit, which may increase the frequency of goodwill impairment charges if a future goodwill impairment test does not pass the Step 1 evaluation. ASU 2017-04 is effective prospectively for annual and interim periods beginning on or after December 15, 2019, and early adoption is permitted on testing dates after January 1, 2017. Southern Company and Southern Company Gas are evaluating the standard and expect to early adopt ASU 2017-04 effective January 1, 2018. On March 10, 2017, the FASB issued ASU No. 2017-07, Compensation – Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost (ASU 2017-07). ASU 2017-07 requires that an employer report the service cost component in the same line item or items as other compensation costs and requires the other components of net periodic pension and postretirement benefit costs to be separately presented in the income statement outside income from operations. Additionally, only the service cost component is eligible for capitalization, when applicable. However, all cost components remain eligible for capitalization under FERC regulations. ASU 2017-07 will be applied retrospectively for the presentation of the service cost component and the other components of net periodic pension and postretirement benefit costs in the income statement. The capitalization of the service cost component of net periodic pension and postretirement benefit costs in assets will be applied on a prospective basis. ASU 2017-07 is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. The presentation changes required for net periodic pension and postretirement benefit costs will result in a decrease in Southern Company's , the traditional electric operating companies', and Southern Company Gas' operating income and an increase in other income for 2016 and 2017 and are expected to result in a decrease in operating income and an increase in other income for 2018. The adoption of ASU 2017-07 is not expected to have a material impact on Southern Company's , the traditional electric operating companies', or Southern Company Gas' financial statements. On August 28, 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities (ASU 2017-12), amending the hedge accounting recognition and presentation requirements. ASU 2017-12 makes more financial and non-financial hedging strategies eligible for hedge accounting, amends the related presentation and disclosure requirements, and simplifies hedge effectiveness assessment requirements. ASU 2017-12 is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years, with early adoption permitted. The registrants are evaluating the standard and expect to early adopt ASU 2017-12 effective January 1, 2018. The adoption of ASU 2017-12 is not expected to have a material impact on the registrants' financial statements.
Affiliate Transactions	Affiliate Transactions Prior to the completion of Southern Company Gas' acquisition of its 50% equity interest in SNG, SCS (as agent for Alabama Power, Georgia Power, and Southern Power) and Southern Company Gas had entered into long-term interstate natural gas transportation agreements with SNG. The interstate transportation service provided to Alabama Power, Georgia Power, Southern Power, and Southern Company Gas by SNG pursuant to these agreements is governed by the terms and conditions of SNG's natural gas tariff and is subject to FERC regulation. For the nine months ended September 30, 2017 , transportation costs under these agreements for Alabama Power, Georgia Power, Southern Power, and Southern Company Gas were approximately $8 million , $77 million , $19 million , and $24 million , respectively. For the period subsequent to Southern Company Gas' investment in SNG through September 30, 2016 , transportation costs under these agreements for Alabama Power, Georgia Power, Southern Power, and Southern Company Gas were approximately $1 million , $8 million , $2 million , and $4 million , respectively. SCS, as agent for Georgia Power and Southern Power, has agreements with certain subsidiaries of Southern Company Gas to purchase natural gas. For the nine months ended September 30, 2017 , natural gas purchases made by Georgia Power and Southern Power from Southern Company Gas' subsidiaries were approximately $18 million and $94 million , respectively. For the period subsequent to Southern Company's acquisition of Southern Company Gas through September 30, 2016 , natural gas purchases made by Georgia Power and Southern Power from Southern Company Gas' subsidiaries were approximately $7 million and $2 million , respectively.
Goodwill and Other Intangible Assets	Goodwill is not amortized, but is subject to an annual impairment test during the fourth quarter of each year, or more frequently if impairment indicators arise.
Property Damage Reserve	Property Damage Reserve See Note 1 to the financial statements of Gulf Power under "Property Damage Reserve" in Item 8 of the Form 10-K for additional information. Gulf Power's cost of repairing damages from major storms and other uninsured property damages, including uninsured damages to transmission and distribution facilities, generation facilities, and other property is charged to Gulf Power's property damage reserve. In accordance with a settlement agreement approved by the Florida PSC on April 4, 2017 (2017 Rate Case Settlement Agreement), Gulf Power suspended further property damage reserve accruals effective April 2017. Gulf Power may make discretionary accruals and is required to resume accruals of $3.5 million annually if the reserve balance falls below zero . In addition, Gulf Power may initiate a storm surcharge to recover costs associated with any tropical systems named by the National Hurricane Center or other catastrophic storm events that reduce the property damage reserve in the aggregate by approximately $31 million ( 75% of the April 1, 2017 balance) or more. The storm surcharge would begin, on an interim basis, 60 days following the filing of a cost recovery petition, would be limited to $4.00 /month for a 1,000 KWH residential customer unless Gulf Power incurs in excess of $100 million in qualified storm recovery costs in a calendar year, and would replenish the property damage reserve to approximately $40 million . As of September 30, 2017 , Gulf Power's property damage reserve totaled approximately $39 million . See Note (B) under " Regulatory Matters – Gulf Power – Retail Base Rate Cases " for additional details regarding the 2017 Rate Case Settlement Agreement.
Natural Gas For Sale	Natural Gas for Sale Southern Company Gas' natural gas distribution utilities, with the exception of Nicor Gas, carry natural gas inventory on a WACOG basis. Nicor Gas' natural gas inventory is carried at cost on a LIFO basis. Inventory decrements occurring during the year that are restored prior to year end are charged to cost of natural gas at the estimated annual replacement cost. Inventory decrements that are not restored prior to year end are charged to cost of natural gas at the actual LIFO cost of the inventory layers liquidated. Southern Company Gas had no inventory decrement at September 30, 2017 . The cost of natural gas, including inventory costs, is recovered from customers under a purchased gas recovery mechanism adjusted for differences between actual costs and amounts billed; therefore, LIFO liquidations have no impact on Southern Company's or Southern Company Gas' net income. Natural gas inventories for Southern Company Gas' non-utility businesses are carried at the lower of weighted average cost or current market price, with cost determined on a WACOG basis. For any declines in market prices below the WACOG considered to be other than temporary, an adjustment is recorded to reduce the value of natural gas inventories to market value. Southern Company Gas had no material LOCOM adjustment in any period presented.
Valuation Methodologies	Valuation Methodologies The energy-related derivatives primarily consist of exchange-traded and over-the-counter financial products for natural gas and physical power products, including, from time to time, basis swaps. These are standard products used within the energy industry and are valued using the market approach. The inputs used are mainly from observable market sources, such as forward natural gas prices, power prices, implied volatility, and overnight index swap interest rates. Interest rate derivatives are also standard over-the-counter products that are valued using observable market data and assumptions commonly used by market participants. The fair value of interest rate derivatives reflects the net present value of expected payments and receipts under the swap agreement based on the market's expectation of future interest rates. Additional inputs to the net present value calculation may include the contract terms, counterparty credit risk, and occasionally, implied volatility of interest rate options. The fair value of cross-currency swaps reflects the net present value of expected payments and receipts under the swap agreement based on the market's expectation of future foreign currency exchange rates. Additional inputs to the net present value calculation may include the contract terms, counterparty credit risk, and discount rates. The interest rate derivatives and cross-currency swaps are categorized as Level 2 under Fair Value Measurements as these inputs are based on observable data and valuations of similar instruments. See Note (H) for additional information on how these derivatives are used. The NRC requires licensees of commissioned nuclear power reactors to establish a plan for providing reasonable assurance of funds for future decommissioning. For fair value measurements of the investments within the nuclear decommissioning trusts, external pricing vendors are designated for each asset class with each security specifically assigned a primary pricing source. For investments held within commingled funds, fair value is determined at the end of each business day through the net asset value, which is established by obtaining the underlying securities' individual prices from the primary pricing source. A market price secured from the primary source vendor is then evaluated by management in its valuation of the assets within the trusts. As a general approach, fixed income market pricing vendors gather market data (including indices and market research reports) and integrate relative credit information, observed market movements, and sector news into proprietary pricing models, pricing systems, and mathematical tools. Dealer quotes and other market information, including live trading levels and pricing analysts' judgments, are also obtained when available. See Note 1 to the financial statements of Southern Company, Alabama Power, and Georgia Power under "Nuclear Decommissioning" in Item 8 of the Form 10-K for additional information. Southern Power has contingent payment obligations related to certain acquisitions whereby Southern Power is obligated to make generation-based payments to the seller over a period ranging from 10 to 30 years, beginning at the commercial operation date. The obligation is categorized as Level 3 under Fair Value Measurements as the fair value is determined using significant unobservable inputs for the forecasted facility generation in MW-hours, as well as other inputs such as a fixed dollar amount per MW-hour, and a discount rate, and is evaluated periodically. The fair value of contingent consideration reflects the net present value of expected payments and any periodic change arising from forecasted generation is expected to be immaterial. "Other investments" include investments that are not traded in the open market. The fair value of these investments has been determined based on market factors including comparable multiples and the expectations regarding cash flows and business plan executions.
Earnings per Share	Earnings per Share For Southern Company, the only difference in computing basic and diluted earnings per share is attributable to awards outstanding under the stock option and performance share plans. See Note 8 to the financial statements of Southern Company in Item 8 of the Form 10-K for information on the stock option and performance share plans. The effect of both stock options and performance share award units was determined using the treasury stock method.
Derivatives	Interest Rate Derivatives Southern Company and certain subsidiaries may also enter into interest rate derivatives to hedge exposure to changes in interest rates. The derivatives employed as hedging instruments are structured to minimize ineffectiveness. Derivatives related to existing variable rate securities or forecasted transactions are accounted for as cash flow hedges where the effective portion of the derivatives' fair value gains or losses is recorded in OCI and is reclassified into earnings at the same time the hedged transactions affect earnings, with any ineffectiveness recorded directly to earnings. Derivatives related to existing fixed rate securities are accounted for as fair value hedges, where the derivatives' fair value gains or losses and hedged items' fair value gains or losses are both recorded directly to earnings, providing an offset, with any difference representing ineffectiveness. Fair value gains or losses on derivatives that are not designated or fail to qualify as hedges are recognized in the statements of income as incurred. Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas are exposed to market risks, including commodity price risk, interest rate risk, weather risk, and occasionally foreign currency exchange rate risk. To manage the volatility attributable to these exposures, each company nets its exposures, where possible, to take advantage of natural offsets and enters into various derivative transactions for the remaining exposures pursuant to each company's policies in areas such as counterparty exposure and risk management practices. Southern Company Gas' wholesale gas operations use various contracts in its commercial activities that generally meet the definition of derivatives. For the traditional electric operating companies, Southern Power, and Southern Company Gas' other businesses, each company's policy is that derivatives are to be used primarily for hedging purposes and mandates strict adherence to all applicable risk management policies. Derivative positions are monitored using techniques including, but not limited to, market valuation, value at risk, stress testing, and sensitivity analysis. Derivative instruments are recognized at fair value in the balance sheets as either assets or liabilities and are presented on a net basis. See Note (C) for additional information. In the statements of cash flows, the cash impacts of settled energy-related and interest rate derivatives are recorded as operating activities. The cash impacts of settled foreign currency derivatives are classified as operating or financing activities to correspond with classification of the hedged interest or principal, respectively. Energy-Related Derivatives Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas enter into energy-related derivatives to hedge exposures to electricity, natural gas, and other fuel price changes. However, due to cost-based rate regulations and other various cost recovery mechanisms, the traditional electric operating companies and the natural gas distribution utilities have limited exposure to market volatility in energy-related commodity prices. Each of the traditional electric operating companies and certain of the natural gas distribution utilities of Southern Company Gas manage fuel-hedging programs, implemented per the guidelines of their respective state PSCs or other applicable state regulatory agencies, through the use of financial derivative contracts, which is expected to continue to mitigate price volatility. The Florida PSC extended the moratorium on Gulf Power's fuel-hedging program until January 1, 2021 in connection with the 2017 Rate Case Settlement Agreement. The moratorium does not have an impact on the recovery of existing hedges entered into under the previously-approved hedging program. The traditional electric operating companies (with respect to wholesale generating capacity) and Southern Power have limited exposure to market volatility in energy-related commodity prices because their long-term sales contracts shift substantially all fuel cost responsibility to the purchaser. However, the traditional electric operating companies and Southern Power may be exposed to market volatility in energy-related commodity prices to the extent any uncontracted capacity is used to sell electricity. Southern Company Gas retains exposure to price changes that can, in a volatile energy market, be material and can adversely affect its results of operations. Southern Company Gas also enters into weather derivative contracts as economic hedges of operating margins in the event of warmer-than-normal weather. Exchange-traded options are carried at fair value, with changes reflected in operating revenues. Non exchange-traded options are accounted for using the intrinsic value method. Changes in the intrinsic value for non-exchange-traded contracts are reflected in the statements of income. Energy-related derivative contracts are accounted for under one of three methods: • Regulatory Hedges — Energy-related derivative contracts which are designated as regulatory hedges relate primarily to the traditional electric operating companies' and the natural gas distribution utilities' fuel-hedging programs, where gains and losses are initially recorded as regulatory liabilities and assets, respectively, and then are included in fuel expense as the underlying fuel is used in operations and ultimately recovered through the respective fuel cost recovery clauses. • Cash Flow Hedges — Gains and losses on energy-related derivatives designated as cash flow hedges (which are mainly used to hedge anticipated purchases and sales) are initially deferred in OCI before being recognized in the statements of income in the same period as the hedged transactions are reflected in earnings. • Not Designated — Gains and losses on energy-related derivative contracts that are not designated or fail to qualify as hedges are recognized in the statements of income as incurred. Some energy-related derivative contracts require physical delivery as opposed to financial settlement, and this type of derivative is both common and prevalent within the electric and natural gas industries. When an energy-related derivative contract is settled physically, any cumulative unrealized gain or loss is reversed and the contract price is recognized in the respective line item representing the actual price of the underlying goods being delivered.

Introduction (Tables)

Introduction (Tables)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Goodwill	At September 30, 2017 and December 31, 2016 , goodwill was as follows: Goodwill At September 30, 2017 At December 31, 2016 (in millions) Southern Company $ 6,267 $ 6,251 Southern Power $ 2 $ 2 Southern Company Gas Gas distribution operations $ 4,702 $ 4,702 Gas marketing services 1,265 1,265 Southern Company Gas total $ 5,967 $ 5,967
Schedule of Goodwill and Other Intangible Assets	Other intangible assets were as follows: At September 30, 2017 At December 31, 2016 Gross Carrying Amount Accumulated Amortization Other Intangible Assets, Net Gross Carrying Amount Accumulated Amortization Other (in millions) (in millions) Southern Company Other intangible assets subject to amortization: Customer relationships $ 288 $ (70 ) $ 218 $ 268 $ (32 ) $ 236 Trade names 159 (15 ) 144 158 (5 ) 153 Storage and transportation contracts 64 (27 ) 37 64 (2 ) 62 PPA fair value adjustments 456 (41 ) 415 456 (22 ) 434 Other 16 (3 ) 13 11 (1 ) 10 Total other intangible assets subject to amortization $ 983 $ (156 ) $ 827 $ 957 $ (62 ) $ 895 Other intangible assets not subject to amortization: Federal Communications Commission licenses $ 75 $ — $ 75 $ 75 $ — $ 75 Total other intangible assets $ 1,058 $ (156 ) $ 902 $ 1,032 $ (62 ) $ 970 Southern Power Other intangible assets subject to amortization: PPA fair value adjustments $ 456 $ (41 ) $ 415 $ 456 $ (22 ) $ 434 Southern Company Gas Other intangible assets subject to amortization: Gas marketing services Customer relationships $ 221 $ (65 ) $ 156 $ 221 $ (30 ) $ 191 Trade names 115 (8 ) 107 115 (2 ) 113 Wholesale gas services Storage and transportation contracts 64 (27 ) 37 64 (2 ) 62 Total other intangible assets subject to amortization $ 400 $ (100 ) $ 300 $ 400 $ (34 ) $ 366
Finite-lived Intangible Assets Amortization Expense	Amortization associated with other intangible assets was as follows: Three Months Ended Nine Months Ended September 30, 2017 (in millions) Southern Company $ 29 $ 94 Southern Power $ 6 $ 19 Southern Company Gas $ 20 $ 66

Contingencies and Regulatory 23

Contingencies and Regulatory Matters (Tables)	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Revised Cost and Schedule	Cancellation Cost Estimate (in billions) CWIP as of September 30, 2017 $ 4.6 Financing costs collected, net of tax 1.5 Cancellation costs (*) 0.4 Guarantee Obligations (1.7 ) Estimated net cancellation cost $ 4.8 (*) The estimate for cancellation costs includes, but is not limited to, costs to terminate contracts for construction and other services, as well as costs to secure the Plant Vogtle Units 3 and 4 construction site. Georgia Power's approximate proportionate share of the remaining estimated cost to complete Plant Vogtle Units 3 and 4 is as follows: (in billions) Estimated cost to complete $ 4.2 CWIP as of September 30, 2017 4.6 Guarantee Obligations (1.7 ) Estimated capital costs $ 7.1 Vogtle Cost Settlement Agreement Revised Forecast (5.7 ) Estimated net additional capital costs $ 1.4
Recovery Balance of Each Regulatory Clause	The balance of each regulatory clause recovery on the balance sheet follows: Regulatory Clause Balance Sheet Line Item September 30, December 31, (in millions) Rate CNP Compliance Deferred over recovered regulatory clause revenues $ 9 $ — Rate CNP Compliance (*) Deferred under recovered regulatory clause revenues — 9 Rate CNP PPA Deferred under recovered regulatory clause revenues 17 142 Retail Energy Cost Recovery (*) Other regulatory liabilities, current — 76 Natural Disaster Reserve Other regulatory liabilities, deferred 51 69 (*) In accordance with an accounting order issued on February 17, 2017 by the Alabama PSC, Alabama Power reclassified $36 million of its under recovered balance for Rate CNP Compliance and $11 million of its under recovered balance for Retail Energy Cost Recovery to a deferred regulatory asset account. The balance of each regulatory clause recovery on the balance sheet follows: Regulatory Clause Balance Sheet Line Item September 30, December 31, (in millions) Fuel Cost Recovery Under recovered regulatory clause revenues $ 13 $ — Fuel Cost Recovery Other regulatory liabilities, current — 15 Purchased Power Capacity Recovery Under recovered regulatory clause revenues 1 — Environmental Cost Recovery Other regulatory liabilities, current 1 — Environmental Cost Recovery Under recovered regulatory clause revenues — 13 Energy Conservation Cost Recovery Under recovered regulatory clause revenues 1 4

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Assets and Liabilities Measured at Fair Value on a Recurring Basis	As of September 30, 2017 , assets and liabilities measured at fair value on a recurring basis during the period, together with their associated level of the fair value hierarchy, were as follows: Fair Value Measurements Using: As of September 30, 2017: Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Net Asset Value as a Practical Expedient (NAV) Total (in millions) Southern Company Assets: Energy-related derivatives (a)(b) $ 231 $ 184 $ — $ — $ 415 Interest rate derivatives — 5 — — 5 Foreign currency derivatives — 103 — — 103 Nuclear decommissioning trusts (c) 752 1,004 — 26 1,782 Cash equivalents 1,271 — — — 1,271 Other investments 9 — 1 — 10 Total $ 2,263 $ 1,296 $ 1 $ 26 $ 3,586 Liabilities: Energy-related derivatives (a)(b) $ 265 $ 146 $ — $ — $ 411 Interest rate derivatives — 24 — — 24 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 20 — 20 Total $ 265 $ 193 $ 20 $ — $ 478 Alabama Power Assets: Energy-related derivatives $ — $ 9 $ — $ — $ 9 Nuclear decommissioning trusts: (d) Domestic equity 422 81 — — 503 Foreign equity 60 57 — — 117 U.S. Treasury and government agency securities — 27 — — 27 Corporate bonds 19 150 — — 169 Mortgage and asset backed securities — 18 — — 18 Private Equity — — — 26 26 Other — 8 — — 8 Cash equivalents 808 — — — 808 Total $ 1,309 $ 350 $ — $ 26 $ 1,685 Liabilities: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Fair Value Measurements Using: As of September 30, 2017: Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Net Asset Value as a Practical Expedient (NAV) Total (in millions) Georgia Power Assets: Energy-related derivatives $ — $ 18 $ — $ — $ 18 Interest rate derivatives — 1 — — 1 Nuclear decommissioning trusts: (d) (e) Domestic equity 235 1 — — 236 Foreign equity — 156 — — 156 U.S. Treasury and government agency securities — 225 — — 225 Municipal bonds — 64 — — 64 Corporate bonds — 160 — — 160 Mortgage and asset backed securities — 38 — — 38 Other 16 19 — — 35 Cash equivalents 112 — — — 112 Total $ 363 $ 682 $ — $ — $ 1,045 Liabilities: Energy-related derivatives $ — $ 11 $ — $ — $ 11 Interest rate derivatives — 3 — — 3 Total $ — $ 14 $ — $ — $ 14 Gulf Power Assets: Cash equivalents $ 21 $ — $ — $ — $ 21 Liabilities: Energy-related derivatives $ — $ 22 $ — $ — $ 22 Mississippi Power Assets: Energy-related derivatives $ — $ 3 $ — $ — $ 3 Interest rate derivatives — 2 — — 2 Cash equivalents 209 — — — 209 Total $ 209 $ 5 $ — $ — $ 214 Liabilities: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Fair Value Measurements Using: As of September 30, 2017: Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Net Asset Value as a Practical Expedient (NAV) Total (in millions) Southern Power Assets: Energy-related derivatives $ — $ 9 $ — $ — $ 9 Foreign currency derivatives — 103 — — 103 Cash equivalents 90 — — — 90 Total $ 90 $ 112 $ — $ — $ 202 Liabilities: Energy-related derivatives $ — $ 4 $ — $ — $ 4 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 20 — 20 Total $ — $ 27 $ 20 $ — $ 47 Southern Company Gas Assets: Energy-related derivatives (a)(b) $ 231 $ 145 $ — $ — $ 376 Liabilities: Energy-related derivatives (a)(b) $ 265 $ 95 $ — $ — $ 360 (a) Excludes $13 million associated with certain weather derivatives accounted for based on intrinsic value rather than fair value. (b) Excludes cash collateral of $76 million . (c) For additional detail, see the nuclear decommissioning trusts sections for Alabama Power and Georgia Power in this table. (d) Excludes receivables related to investment income, pending investment sales, payables related to pending investment purchases, and currencies. (e) Includes the investment securities pledged to creditors and collateral received and excludes payables related to the securities lending program. As of September 30, 2017 , approximately $66 million of the fair market value of Georgia Power's nuclear decommissioning trust funds' securities were on loan to creditors under the funds' managers' securities lending program.
Fair Value Measurements of Investments Calculated at Net Asset Value per Share as Well as the Nature and Risk of Those Investments	As of September 30, 2017 , the fair value measurements of private equity investments held in the nuclear decommissioning trust that are calculated at net asset value per share (or its equivalent) as a practical expedient, as well as the nature and risks of those investments, were as follows: As of September 30, 2017: Fair Value Unfunded Commitments Redemption Frequency Redemption Notice Period (in millions) Southern Company $ 26 $ 24 Not Applicable Not Applicable Alabama Power $ 26 $ 24 Not Applicable Not Applicable
Financial Instruments for Which Carrying Amount Did Not Equal Fair Value	As of September 30, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: Carrying Amount Fair Value (in millions) Long-term debt, including securities due within one year: Southern Company $ 47,269 $ 49,348 Alabama Power $ 7,404 $ 8,031 Georgia Power $ 11,713 $ 12,237 Gulf Power $ 1,292 $ 1,352 Mississippi Power $ 2,123 $ 2,117 Southern Power $ 5,810 $ 5,916 Southern Company Gas $ 5,862 $ 6,230

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Earnings per Share	Shares used to compute diluted earnings per share were as follows: Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 (in millions) As reported shares 1,003 968 998 940 Effect of options and performance share award units 7 7 7 5 Diluted shares 1,010 975 1,005 945
Changes in Stockholders' Equity	The following table presents year-to-date changes in stockholders' equity of Southern Company: Number of Common Shares Common Preferred and Preference Stock of Subsidiaries Total Issued Treasury Noncontrolling Interests (*) (in thousands) (in millions) Balance at December 31, 2016 991,213 (819 ) $ 24,758 $ 609 $ 1,245 $ 26,612 Consolidated net income attributable to Southern Company — — 347 — — 347 Other comprehensive income (loss) — — (2 ) — — (2 ) Stock issued 13,308 — 613 — — 613 Stock-based compensation — — 97 — — 97 Cash dividends on common stock — — (1,716 ) — — (1,716 ) Preference stock redemption — — — (150 ) — (150 ) Contributions from noncontrolling interests — — — — 77 77 Distributions to noncontrolling interests — — — — (87 ) (87 ) Net income attributable to noncontrolling interests — — — — 45 45 Reclassification from redeemable noncontrolling interests — — — — 114 114 Other — (75 ) (15 ) 3 1 (11 ) Balance at September 30, 2017 1,004,521 (894 ) $ 24,082 $ 462 $ 1,395 $ 25,939 Balance at December 31, 2015 915,073 (3,352 ) $ 20,592 $ 609 $ 781 $ 21,982 Consolidated net income attributable to Southern Company — — 2,251 — — 2,251 Other comprehensive income (loss) — — (95 ) — — (95 ) Stock issued 65,725 2,599 3,265 — — 3,265 Stock-based compensation — — 94 — — 94 Cash dividends on common stock — — (1,553 ) — — (1,553 ) Contributions from noncontrolling interests — — — — 357 357 Distributions to noncontrolling interests — — — — (21 ) (21 ) Purchase of membership interests from noncontrolling interests — — — — (129 ) (129 ) Net income attributable to noncontrolling interests — — — — 36 36 Other — (46 ) (7 ) — — (7 ) Balance at September 30, 2016 980,798 (799 ) $ 24,547 $ 609 $ 1,024 $ 26,180 (*) Related to Southern Power Company and excludes redeemable noncontrolling interests. In April 2017, approximately $114 million was reclassified from redeemable noncontrolling interests to noncontrolling interests, included in stockholder's equity, due to the expiration of SunPower Corp's option to require Southern Power to purchase its membership interests in one of the solar partnerships. See Note 10 to the financial statements of Southern Power in Item 8 of the Form 10-K for additional information.

Financing (Tables)

Financing (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Credit Arrangements by Company	The following table outlines the committed credit arrangements by company as of September 30, 2017 : Expires Executable Term Loans Expires Within One Year Company 2017 2018 2019 2020 2022 Total Unused One Year Two Years Term Out No Term Out (in millions) Southern Company (a) $ — $ — $ — $ — $ 2,000 $ 2,000 $ 2,000 $ — $ — $ — $ — Alabama Power — 35 — 500 800 1,335 1,335 — — — 35 Georgia Power — — — — 1,750 1,750 1,732 — — — — Gulf Power 30 195 25 30 — 280 280 45 — — 40 Mississippi Power 100 — — — — 100 100 — — — 100 Southern Power Company (b) — — — — 750 750 728 — — — — Southern Company Gas (c) — — — — 1,900 1,900 1,861 — — — — Other — 30 — — — 30 30 20 — 20 10 Southern Company Consolidated $ 130 $ 260 $ 25 $ 530 $ 7,200 $ 8,145 $ 8,066 $ 65 $ — $ 20 $ 185 (a) Represents the Southern Company parent entity. (b) Does not include Southern Power's $120 million continuing letter of credit facility for standby letters of credit expiring in 2019, of which $111 million has been used for letters of credit and $9 million remains unused at September 30, 2017 . (c) Southern Company Gas, as the parent entity, guarantees the obligations of Southern Company Gas Capital, which is the borrower of $1.2 billion of these arrangements. Southern Company Gas' committed credit arrangements also include $700 million for which Nicor Gas is the borrower and which is restricted for working capital needs of Nicor Gas.
Schedule of Long-term Debt Financing Activities	The following table outlines the long-term debt financing activities for Southern Company and its subsidiaries for the first nine months of 2017 : Company Senior Note Issuances Senior Note Maturities and Redemptions Revenue Bond Other Long-Term Debt Issuances Other Long-Term Debt Redemptions and Maturities (a) (in millions) Southern Company (b) $ 300 $ 400 $ — $ 500 $ 400 Alabama Power 550 200 36 — — Georgia Power 1,350 450 65 370 13 Gulf Power 300 85 — 6 — Mississippi Power — — — 40 893 Southern Power — — — 43 4 Southern Company Gas (c) 450 — — 200 22 Other — — — — 12 Elimination (d) — — — (40 ) (599 ) Southern Company Consolidated $ 2,950 $ 1,135 $ 101 $ 1,119 $ 745 (a) Includes reductions in capital lease obligations resulting from cash payments under capital leases. (b) Represents the Southern Company parent entity. (c) The senior notes were issued by Southern Company Gas Capital and guaranteed by the Southern Company Gas parent entity. Other long-term debt issued represents first mortgage bonds issued by Nicor Gas. (d) Includes intercompany loans from Southern Company to Mississippi Power and reductions in affiliate capital lease obligations at Georgia Power. These transactions are eliminated in Southern Company's Consolidated Financial Statements.

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
Pension Plans and Postretirement Plans	Components of the net periodic benefit costs for the three and nine months ended September 30, 2017 and 2016 are presented in the following tables. Pension Plans Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power (in millions) Three Months Ended September 30, 2017 Service cost $ 73 $ 15 $ 19 $ 3 $ 4 Interest cost 114 25 34 5 5 Expected return on plan assets (224 ) (49 ) (71 ) (10 ) (9 ) Amortization: Prior service costs 3 1 — — — Net (gain)/loss 41 10 15 2 1 Net periodic pension cost (income) $ 7 $ 2 $ (3 ) $ — $ 1 Nine Months Ended September 30, 2017 Service cost $ 220 $ 47 $ 56 $ 10 $ 11 Interest cost 341 73 103 15 15 Expected return on plan assets (673 ) (147 ) (212 ) (29 ) (29 ) Amortization: Prior service costs 9 2 2 — 1 Net (gain)/loss 122 31 43 5 5 Net periodic pension cost (income) $ 19 $ 6 $ (8 ) $ 1 $ 3 Three Months Ended September 30, 2016 Service cost $ 68 $ 14 $ 17 $ 3 $ 3 Interest cost 110 23 34 5 4 Expected return on plan assets (203 ) (46 ) (64 ) (9 ) (9 ) Amortization: Prior service costs 3 1 1 — 1 Net (gain)/loss 45 10 14 2 2 Net periodic pension cost $ 23 $ 2 $ 2 $ 1 $ 1 Nine Months Ended September 30, 2016 Service cost $ 192 $ 43 $ 52 $ 9 $ 9 Interest cost 311 71 102 14 14 Expected return on plan assets (577 ) (138 ) (193 ) (26 ) (26 ) Amortization: Prior service costs 10 2 4 1 1 Net (gain)/loss 120 30 41 5 5 Net periodic pension cost $ 56 $ 8 $ 6 $ 3 $ 3 Pension Plans Southern Company Gas (in millions) Successor – Three Months Ended September 30, 2017 Service cost $ 6 Interest cost 10 Expected return on plan assets (18 ) Amortization of net (gain)/loss 5 Net periodic pension cost $ 3 Successor – Nine Months Ended September 30, 2017 Service cost $ 17 Interest cost 30 Expected return on plan assets (53 ) Amortization: Prior service costs (1 ) Net (gain)/loss 15 Net periodic pension cost $ 8 Successor – July 1, 2016 through September 30, 2016 Service cost $ 7 Interest cost 10 Expected return on plan assets (17 ) Amortization of regulatory asset 6 Net periodic pension cost $ 6 Predecessor – January 1, 2016 through June 30, 2016 Service cost $ 13 Interest cost 21 Expected return on plan assets (33 ) Amortization: Prior service costs (1 ) Net (gain)/loss 13 Net periodic pension cost $ 13 Postretirement Benefits Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power (in millions) Three Months Ended September 30, 2017 Service cost $ 6 $ 1 $ 2 $ — $ — Interest cost 19 4 6 1 1 Expected return on plan assets (16 ) (5 ) (6 ) — — Amortization: Prior service costs 2 1 — — — Net (gain)/loss 3 — 3 — — Net periodic postretirement benefit cost $ 14 $ 1 $ 5 $ 1 $ 1 Nine Months Ended September 30, 2017 Service cost $ 18 $ 4 $ 5 $ 1 $ 1 Interest cost 59 13 21 2 3 Expected return on plan assets (49 ) (19 ) (18 ) (1 ) (1 ) Amortization: Prior service costs 5 3 1 — — Net (gain)/loss 10 1 6 — — Net periodic postretirement benefit cost $ 43 $ 2 $ 15 $ 2 $ 3 Three Months Ended September 30, 2016 Service cost $ 6 $ 1 $ 2 $ — $ — Interest cost 20 5 7 1 — Expected return on plan assets (16 ) (6 ) (6 ) — — Amortization: Prior service costs 1 1 — — — Net (gain)/loss 5 — 3 — 1 Net periodic postretirement benefit cost $ 16 $ 1 $ 6 $ 1 $ 1 Nine Months Ended September 30, 2016 Service cost $ 17 $ 4 $ 5 $ 1 $ 1 Interest cost 55 14 22 2 2 Expected return on plan assets (44 ) (19 ) (17 ) (1 ) (1 ) Amortization: Prior service costs 4 3 1 — — Net (gain)/loss 12 1 7 — 1 Net periodic postretirement benefit cost $ 44 $ 3 $ 18 $ 2 $ 3 Postretirement Benefits Southern Company Gas (in millions) Successor – Three Months Ended September 30, 2017 Service cost $ 1 Interest cost 3 Expected return on plan assets (2 ) Amortization: Prior service costs (1 ) Net (gain)/loss 1 Net periodic postretirement benefit cost $ 2 Successor – Nine Months Ended September 30, 2017 Service cost $ 2 Interest cost 8 Expected return on plan assets (5 ) Amortization: Prior service costs (2 ) Net (gain)/loss 3 Net periodic postretirement benefit cost $ 6 Successor – July 1, 2016 through September 30, 2016 Service cost $ 1 Interest cost 2 Expected return on plan assets (2 ) Amortization of regulatory asset 1 Net periodic postretirement benefit cost $ 2 Predecessor – January 1, 2016 through June 30, 2016 Service cost $ 1 Interest cost 5 Expected return on plan assets (3 ) Amortization: Prior service costs (1 ) Net (gain)/loss 2 Net periodic postretirement benefit cost $ 4

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Summary of Valuation Allowance	At September 30, 2017 , valuation allowances were as follows: Mississippi Power Southern Company Gas Southern Company (in millions) Federal $ — $ 18 $ 18 State (net of federal benefit) 46 1 64 Balance at September 30, 2017 $ 46 $ 19 $ 82
Change in Unrecognized Tax Benefits	Changes during the nine months ended September 30, 2017 for unrecognized tax benefits were as follows: Mississippi Power Southern Power Southern Company (in millions) Unrecognized tax benefits as of December 31, 2016 $ 465 $ 17 $ 484 Tax positions from current periods 2 — 9 Tax positions from prior periods (175 ) (17 ) (186 ) Reductions due to settlements (290 ) — (290 ) Balance as of September 30, 2017 $ 2 $ — $ 17
Impact on Effective Tax Rate, If Recognized	The impact on the effective tax rate, if recognized, is as follows: As of September 30, 2017 As of December 31, 2016 Mississippi Power Southern Company Southern Company (in millions) Tax positions impacting the effective tax rate $ 2 $ 17 $ 20 Tax positions not impacting the effective tax rate — — 464 Balance of unrecognized tax benefits $ 2 $ 17 $ 484

Derivatives (Tables)

Derivatives (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Energy-Related Derivatives	At September 30, 2017 , the net volume of energy-related derivative contracts for natural gas positions for the Southern Company system, together with the longest hedge date over which the respective entity is hedging its exposure to the variability in future cash flows for forecasted transactions and the longest non-hedge date for derivatives not designated as hedges, were as follows: Net Purchased mmBtu Longest Hedge Date Longest Non-Hedge Date (in millions) Southern Company (*) 463 2021 2024 Alabama Power 66 2020 — Georgia Power 159 2021 — Gulf Power 28 2020 — Mississippi Power 44 2021 — Southern Power 13 2018 — Southern Company Gas (*) 153 2020 2024 (*) Southern Company's and Southern Company Gas' derivative instruments include both long and short natural gas positions. A long position is a contract to purchase natural gas and a short position is a contract to sell natural gas. Southern Company Gas' volume represents the net of long natural gas positions of 3.3 billion mmBtu and short natural gas positions of 3.1 billion mmBtu as of September 30, 2017 , which is also included in Southern Company's total volume.
Schedule of Interest Rate Derivatives	At September 30, 2017 , the following interest rate derivatives were outstanding: Notional Amount Interest Rate Received Weighted Average Interest Rate Paid Hedge Maturity Date Fair Value Gain (Loss) at September 30, 2017 (in millions) (in millions) Cash Flow Hedges of Existing Debt Mississippi Power $ 900 1-month 0.79% March 2018 $ 2 Fair Value Hedges of Existing Debt Southern Company (*) 300 2.75% 3-month June 2020 — Southern Company (*) 1,500 2.35% 1-month July 2021 (19 ) Georgia Power 250 5.40% 3-month June 2018 — Georgia Power 500 1.95% 3-month December 2018 (2 ) Georgia Power 200 4.25% 3-month December 2019 — Southern Company Consolidated $ 3,650 $ (19 ) (*) Represents the Southern Company parent entity.
Schedule of Foreign Currency Derivatives	At September 30, 2017 , the following foreign currency derivatives were outstanding: Pay Notional Pay Rate Receive Notional Receive Rate Hedge Fair Value Gain (Loss) at September 30, 2017 (in millions) (in millions) (in millions) Cash Flow Hedges of Existing Debt Southern Power $ 677 2.95% € 600 1.00% June 2022 $ 42 Southern Power 564 3.78% 500 1.85% June 2026 38 Total $ 1,241 € 1,100 $ 80
Fair Value of Energy-Related Derivatives and Interest Rate Derivatives	The fair value of energy-related derivatives, interest rate derivatives, and foreign currency derivatives was reflected in the balance sheets as follows: As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Southern Company Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 21 $ 25 $ 73 $ 27 Other deferred charges and assets/Other deferred credits and liabilities 13 23 25 33 Total derivatives designated as hedging instruments for regulatory purposes $ 34 $ 48 $ 98 $ 60 Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 8 $ 6 $ 23 $ 7 Interest rate derivatives: Other current assets/Other current liabilities 5 1 12 1 Other deferred charges and assets/Other deferred credits and liabilities — 23 1 28 Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 25 Other deferred charges and assets/Other deferred credits and liabilities 103 — — 33 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 116 $ 53 $ 36 $ 94 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ 271 $ 254 $ 489 $ 483 Other deferred charges and assets/Other deferred credits and liabilities 115 103 66 81 Interest rate derivatives: Other current assets/Other current liabilities — — 1 — Total derivatives not designated as hedging instruments $ 386 $ 357 $ 556 $ 564 Gross amounts recognized $ 536 $ 458 $ 690 $ 718 Gross amounts offset (*) $ (275 ) $ (351 ) $ (462 ) $ (524 ) Net amounts recognized in the Balance Sheets $ 261 $ 107 $ 228 $ 194 As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Alabama Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 6 $ 4 $ 13 $ 5 Other deferred charges and assets/Other deferred credits and liabilities 3 3 7 4 Total derivatives designated as hedging instruments for regulatory purposes $ 9 $ 7 $ 20 $ 9 Gross amounts recognized $ 9 $ 7 $ 20 $ 9 Gross amounts offset $ (5 ) $ (5 ) $ (8 ) $ (8 ) Net amounts recognized in the Balance Sheets $ 4 $ 2 $ 12 $ 1 Georgia Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 10 $ 3 $ 30 $ 1 Other deferred charges and assets/Other deferred credits and liabilities 8 8 14 7 Total derivatives designated as hedging instruments for regulatory purposes $ 18 $ 11 $ 44 $ 8 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ 1 $ 1 $ 2 $ — Other deferred charges and assets/Other deferred credits and liabilities — 2 — 3 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 1 $ 3 $ 2 $ 3 Gross amounts recognized $ 19 $ 14 $ 46 $ 11 Gross amounts offset $ (10 ) $ (10 ) $ (8 ) $ (8 ) Net amounts recognized in the Balance Sheets $ 9 $ 4 $ 38 $ 3 Gulf Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ — $ 13 $ 4 $ 12 Other deferred charges and assets/Other deferred credits and liabilities — 9 1 17 Total derivatives designated as hedging instruments for regulatory purposes $ — $ 22 $ 5 $ 29 Gross amounts recognized $ — $ 22 $ 5 $ 29 Gross amounts offset $ — $ — $ (4 ) $ (4 ) Net amounts recognized in the Balance Sheets $ — $ 22 $ 1 $ 25 As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Mississippi Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 1 $ 4 $ 2 $ 6 Other deferred charges and assets/Other deferred credits and liabilities 2 3 2 5 Total derivatives designated as hedging instruments for regulatory purposes $ 3 $ 7 $ 4 $ 11 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ 2 $ — $ 2 $ — Other deferred charges and assets/Other deferred credits and liabilities — — 1 — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 2 $ — $ 3 $ — Gross amounts recognized $ 5 $ 7 $ 7 $ 11 Gross amounts offset $ (3 ) $ (3 ) $ (3 ) $ (3 ) Net amounts recognized in the Balance Sheets $ 2 $ 4 $ 4 $ 8 Southern Power Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 8 $ 4 $ 18 $ 4 Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 25 Other deferred charges and assets/Other deferred credits and liabilities 103 — — 33 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 111 $ 27 $ 18 $ 62 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ 1 $ — $ 3 $ 1 Interest rate derivatives: Other current assets/Other current liabilities — — 1 — Total derivatives not designated as hedging instruments $ 1 $ — $ 4 $ 1 Gross amounts recognized $ 112 $ 27 $ 22 $ 63 Gross amounts offset $ (1 ) $ (1 ) $ (5 ) $ (5 ) Net amounts recognized in the Balance Sheets $ 111 $ 26 $ 17 $ 58 As of September 30, 2017 As of December 31, 2016 Derivative Category and Balance Sheet Location Assets Liabilities Assets Liabilities (in millions) (in millions) Southern Company Gas Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ 4 $ 1 $ 24 $ 3 Other deferred charges and assets/Other deferred credits and liabilities — — 1 — Total derivatives designated as hedging instruments for regulatory purposes $ 4 $ 1 $ 25 $ 3 Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ — $ 2 $ 4 $ 3 Derivatives not designated as hedging instruments Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ 270 $ 254 $ 486 $ 482 Other deferred charges and assets/Other deferred credits and liabilities 115 103 66 81 Total derivatives not designated as hedging instruments $ 385 $ 357 $ 552 $ 563 Gross amounts of recognized $ 389 $ 360 $ 581 $ 569 Gross amounts offset (*) $ (251 ) $ (327 ) $ (435 ) $ (497 ) Net amounts recognized in the Balance Sheets $ 138 $ 33 $ 146 $ 72 (*) Gross amounts offset include cash collateral held on deposit in broker margin accounts of $76 million and $62 million as of September 30, 2017 and December 31, 2016 , respectively.
Pre-tax Effects of Unrealized Derivative Gains (Losses) Arising from Energy-Related Derivative Instruments	At September 30, 2017 and December 31, 2016 , the pre-tax effects of unrealized derivative gains (losses) arising from energy-related derivative instruments designated as regulatory hedging instruments and deferred were as follows: Regulatory Hedge Unrealized Gain (Loss) Recognized in the Balance Sheet at September 30, 2017 Derivative Category and Balance Sheet Location Southern Company (b) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (c) (in millions) Energy-related derivatives: Other regulatory assets, current $ (18 ) $ (1 ) $ — $ (13 ) $ (3 ) $ (1 ) Other regulatory assets, deferred (12 ) (1 ) (1 ) (9 ) (1 ) — Other regulatory liabilities, current (a) 14 3 7 — — 4 Other regulatory liabilities, deferred (b) 2 1 1 — — — Total energy-related derivative gains (losses) $ (14 ) $ 2 $ 7 $ (22 ) $ (4 ) $ 3 (a) Georgia Power includes other regulatory liabilities, current in other current liabilities. (b) Georgia Power includes other regulatory liabilities, deferred in other deferred credits and liabilities. (c) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $1 million at September 30, 2017 . Regulatory Hedge Unrealized Gain (Loss) Recognized in the Balance Sheet at December 31, 2016 Derivative Category and Balance Sheet Location Southern Company (c) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (c) (in millions) Energy-related derivatives: Other regulatory assets, current $ (16 ) $ (1 ) $ — $ (9 ) $ (5 ) $ (1 ) Other regulatory assets, deferred (19 ) — — (16 ) (3 ) — Other regulatory liabilities, current (a) 56 8 29 1 1 17 Other regulatory liabilities, deferred (b) 12 4 7 — — 1 Total energy-related derivative gains (losses) $ 33 $ 11 $ 36 $ (24 ) $ (7 ) $ 17 (a) Georgia Power includes other regulatory liabilities, current in other current liabilities. (b) Georgia Power includes other regulatory liabilities, deferred in other deferred credits and liabilities. (c) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $ 8 million at December 31, 2016 .
Pre-tax Effects of Interest Rate Derivatives, Designated as Cash Flow Hedging Instruments	For the three months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives, interest rate derivatives, and foreign currency derivatives designated as cash flow hedging instruments were as follows: Derivatives in Cash Flow Hedging Relationships Gain (Loss) Recognized in OCI on Derivative (Effective Portion) Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) Statements of Income Location Amount 2017 2016 2017 2016 (in millions) (in millions) Southern Company Energy-related derivatives $ (6 ) $ — Depreciation and amortization $ (6 ) $ 1 Interest rate derivatives (1 ) (6 ) Interest expense, net of amounts capitalized (5 ) (6 ) Foreign currency derivatives 46 37 Interest expense, net of amounts capitalized (5 ) (6 ) Other income (expense), net (*) 43 7 Total $ 39 $ 31 $ 27 $ (4 ) Alabama Power Interest rate derivatives $ — $ — Interest expense, net of amounts capitalized $ (2 ) $ (2 ) Georgia Power Interest rate derivatives $ — $ — Interest expense, net of amounts capitalized $ (1 ) $ (1 ) Mississippi Power Interest rate derivatives $ (1 ) $ (1 ) Interest expense, net of amounts capitalized $ — $ — Southern Power Energy-related derivatives $ (6 ) $ — Depreciation and amortization $ (6 ) $ 1 Foreign currency derivatives 46 37 Interest expense, net of amounts capitalized (5 ) (6 ) Other income (expense), net (*) 43 7 Total $ 40 $ 37 $ 32 $ 2 Southern Company Gas Interest rate derivatives $ — $ (5 ) Interest expense, net of amounts capitalized $ — $ — (*) The reclassification from accumulated OCI into other income (expense), net completely offsets currency gains and losses arising from changes in the U.S. currency exchange rates used to record the euro-denominated notes. For the nine months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives, interest rate derivatives, and foreign currency derivatives designated as cash flow hedging instruments were as follows: Derivatives in Cash Flow Gain (Loss) Gain (Loss) Reclassified from Accumulated OCI into Statements of Income Location Amount 2017 2016 2017 2016 (in millions) (in millions) Southern Company Energy-related derivatives $ (26 ) $ (1 ) Depreciation and amortization $ (12 ) $ 1 Interest rate derivatives (2 ) (189 ) Interest expense, net of amounts capitalized (15 ) (13 ) Foreign currency derivatives 114 (1 ) Interest expense, net of amounts capitalized (17 ) (7 ) Other income (expense), net (*) 139 (13 ) Total $ 86 $ (191 ) $ 95 $ (32 ) Alabama Power Interest rate derivatives $ — $ (3 ) Interest expense, net of amounts capitalized $ (5 ) $ (5 ) Georgia Power Interest rate derivatives $ — $ — Interest expense, net of amounts capitalized $ (3 ) $ (3 ) Gulf Power Energy-related derivatives $ (1 ) $ — Depreciation and amortization $ — $ — Interest rate derivatives (1 ) (7 ) Interest expense, net of amounts capitalized — — Total $ (2 ) $ (7 ) $ — $ — Mississippi Power Interest rate derivatives $ — $ (1 ) Interest expense, net of amounts capitalized $ (1 ) $ (1 ) Southern Power Energy-related derivatives $ (21 ) $ (1 ) Depreciation and amortization $ (12 ) $ 1 Interest rate derivatives — — Interest expense, net of amounts capitalized — (1 ) Foreign currency derivatives 114 (1 ) Interest expense, net of amounts capitalized (17 ) (7 ) Other income (expense), net (*) 139 (13 ) Total $ 93 $ (2 ) $ 110 $ (20 ) (*) The reclassification from accumulated OCI into other income (expense), net completely offsets currency gains and losses arising from changes in the U.S. currency exchange rates used to record the euro-denominated notes. For Southern Company Gas, the pre-tax effect of energy related derivatives and interest rate derivatives designated as cash flow hedging instruments recognized in OCI and those reclassified from accumulated OCI into earnings for the successor nine months ended September 30, 2017 , the successor period of July 1, 2016 through September 30, 2016, and the predecessor period of January 1, 2016 through June 30, 2016 were as follows: Gain (Loss) Recognized in OCI on Derivative (Effective Portion) Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) Derivatives in Cash Flow Hedging Relationships Nine Months Ended September 30, 2017 Statements of Income Location Nine Months Ended September 30, 2017 (in millions) (in millions) Energy-related derivatives $ (4 ) Cost of natural gas $ — Gain (Loss) Recognized in OCI on Derivative (Effective Portion) Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) Successor Predecessor Successor Predecessor Derivatives in Cash Flow Hedging Relationships July 1, 2016 through September 30, 2016 January 1, 2016 through June 30, 2016 Statements of Income Location July 1, 2016 January 1, 2016 through June 30, 2016 (in millions) (in millions) (in millions) (in millions) Energy-related derivatives $ — $ — Cost of natural gas $ — $ (1 ) Interest rate derivatives (5 ) (64 ) Interest expense, net of amounts capitalized — — Total $ (5 ) $ (64 ) $ — $ (1 )
Pre-tax Effect of Interest Rate and Energy Related Derivatives	For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of energy-related derivatives and interest rate derivatives not designated as hedging instruments on the statements of income were as follows: Gain (Loss) Three Months Ended September 30, Nine Months Ended September 30, Derivatives in Non-Designated Hedging Relationships Statements of Income Location 2017 2016 2017 2016 (in millions) (in millions) Southern Company Energy Related derivatives: Natural gas revenues (*) $ (17 ) $ — $ 48 $ — Cost of natural gas 2 6 (2 ) 6 Total derivatives in non-designated hedging relationships $ (15 ) $ 6 $ 46 $ 6 (*) Excludes gains (losses) recorded in cost of natural gas associated with weather derivatives of $15 million for the nine months ended September 30, 2017 and immaterial amounts for all other periods presented. Gain (Loss) Successor Successor Successor Successor Predecessor Derivatives in Non-Designated Hedging Relationships Statements of Income Location Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Nine Months Ended September 30, 2017 July 1, 2016 through September 30, 2016 January 1, 2016 through June 30, 2016 (in millions) (in millions) (in millions) (in millions) Southern Company Gas Energy Related derivatives: Natural gas revenues (*) $ (17 ) $ — $ 48 $ — $ (1 ) Cost of natural gas 2 6 (2 ) 6 (62 ) Total derivatives in non-designated hedging relationships $ (15 ) $ 6 $ 46 $ 6 $ (63 ) (*) Excludes gains recorded in cost of natural gas associated with weather derivatives of $15 million for the successor nine months ended September 30, 2017 and immaterial amounts for all other periods presented.
Pre-tax Effects of Interest Rate Derivatives, Designated as Fair Value Hedging Instruments	For the three and nine months ended September 30, 2017 and 2016 , the pre-tax effects of interest rate derivatives designated as fair value hedging instruments were as follows: Derivatives in Fair Value Hedging Relationships Gain (Loss) Three Months Ended September 30, Nine Months Ended September 30, Derivative Category Statements of Income Location 2017 2016 2017 2016 (in millions) (in millions) Southern Company Interest rate derivatives: Interest expense, net of amounts capitalized $ (5 ) $ (9 ) $ (6 ) $ 15 Georgia Power Interest rate derivatives: Interest expense, net of amounts capitalized $ — $ (5 ) $ (1 ) $ 10

Acquisitions and Dispositions (

Acquisitions and Dispositions (Tables)	9 Months Ended
	Sep. 30, 2017
Business Combinations [Abstract]
Schedule of Business Acquisitions	The following table presents the final purchase price allocation: PowerSecure Purchase Price (in millions) Current assets $ 172 Property, plant, and equipment 46 Intangible assets 106 Goodwill 284 Other assets 4 Current liabilities (121 ) Long-term debt, including current portion (48 ) Deferred credits and other liabilities (14 ) Total purchase price $ 429 The following table presents the final purchase price allocation: Southern Company Gas Purchase Price (in millions) Current assets $ 1,557 Property, plant, and equipment 10,108 Goodwill 5,967 Intangible assets 400 Regulatory assets 1,118 Other assets 229 Current liabilities (2,201 ) Other liabilities (4,742 ) Long-term debt (4,261 ) Noncontrolling interest (174 ) Total purchase price $ 8,001 During the nine months ended September 30, 2017, in accordance with Southern Power's overall growth strategy, one of Southern Power's wholly-owned subsidiaries acquired the project discussed below . Acquisition-related costs were expensed as incurred and were not material. Project Facility Resource Seller; Acquisition Date Approximate Nameplate Capacity ( MW ) Location Southern Power Percentage Ownership Actual COD PPA Contract Period Bethel Wind Invenergy, 276 Castro County, TX 100 % January 2017 12 years
Business Acquisition, Pro Forma Information	The following summarized unaudited pro forma consolidated statement of earnings information assumes that the acquisition of Southern Company Gas was completed on January 1, 2015. The summarized unaudited pro forma consolidated statement of earnings information includes adjustments for (i) intercompany sales, (ii) amortization of intangible assets, (iii) adjustments to interest expense to reflect current interest rates on Southern Company Gas debt and additional interest expense associated with borrowings by Southern Company to fund the Merger, and (iv) the elimination of nonrecurring expenses associated with the Merger. For the Nine Months Ended September 30, 2016 Operating revenues (in millions) $ 16,609 Net income attributable to Southern Company (in millions) $ 2,394 Basic Earnings Per Share (EPS) $ 2.52 Diluted EPS $ 2.51
Schedule of Construction Projects	During the nine months ended September 30, 2017 , in accordance with its overall growth strategy, Southern Power completed construction of and placed in service, or continued construction of, the projects set forth in the following table. Through September 30, 2017 , total costs of construction incurred for these projects were $494 million , of which $122 million remained in CWIP. Total aggregate construction costs, excluding the acquisition costs, are expected to be between $360 million and $415 million for the Mankato and Cactus Flats facilities. The ultimate outcome of these matters cannot be determined at this time. Project Facility Resource Approximate Nameplate Capacity ( MW ) Location Actual/Expected COD PPA Contract Period Projects Completed During the Nine Months Ended September 30, 2017 East Pecos Solar 120 Pecos County, TX March 2017 15 years Lamesa Solar 102 Dawson County, TX April 2017 15 years Projects Under Construction as of September 30, 2017 Cactus Flats (*) Wind 148 Concho County, TX Third quarter 2018 12-15 years Mankato Natural Gas 345 Mankato, MN Second quarter 2019 20 years (*) On July 31, 2017, Southern Power acquired a 100% ownership interest in the Cactus Flats facility, which is in the early stages of construction, from RES America Developments, Inc.

Joint Ownership Agreements (Tab

Joint Ownership Agreements (Tables)	9 Months Ended
	Sep. 30, 2017
Regulated Operations [Abstract]
Equity Method Investments	The carrying amounts of Southern Company Gas' equity method investments as of September 30, 2017 and December 31, 2016 and related income from those investments for the successor three and nine month periods ended September 30, 2017 , the successor three-month period ended September 30, 2016 , and for the predecessor period January 1, 2016 through June 30, 2016 were as follows: Balance Sheet Information September 30, 2017 December 31, 2016 (in millions) SNG $ 1,385 $ 1,394 Atlantic Coast Pipeline 61 33 PennEast Pipeline 49 22 Triton 43 44 Pivotal JAX LNG, LLC 40 16 Horizon Pipeline 30 30 Other 1 2 Total $ 1,609 $ 1,541 Selected financial information of SNG for the three and nine months ended September 30, 2017 and for the period September 1, 2016 through September 30, 2016 is as follows: Income Statement Information Three Months Ended September 30, 2017 Nine Months Ended September 30, 2017 September 1, 2016 through September 30, 2016 (in millions) Revenues $ 146 $ 445 $ 82 Operating income $ 71 $ 218 $ 60 Net income $ 57 $ 172 $ 55
Schedule of Other Nonoperating Income, by Component	Successor Successor Successor Predecessor Income Statement Information Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Nine Months Ended September 30, 2017 January 1, 2016 through June 30, 2016 (in millions) (in millions) (in millions) (in millions) SNG $ 28 $ 27 $ 86 $ — PennEast Pipeline 1 — 5 — Atlantic Coast Pipeline 1 1 4 — Triton 1 1 3 1 Horizon Pipeline 1 — 2 1 Total $ 32 $ 29 $ 100 $ 2

Segment and Related Informati32

Segment and Related Information (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Financial Data for Business Segments	Financial data for business segments and products and services for the three and nine months ended September 30, 2017 and 2016 was as follows: Electric Utilities Traditional Electric Operating Companies Southern Power Eliminations Total Southern Company Gas All Other Eliminations Consolidated (in millions) Three Months Ended Operating revenues $ 5,017 $ 618 $ (112 ) $ 5,523 $ 565 $ 153 $ (40 ) $ 6,201 Segment net income (loss) (a)(b) 1,008 124 — 1,132 15 (80 ) 2 1,069 Nine Months Ended Operating revenues $ 12,960 $ 1,597 $ (318 ) $ 14,239 $ 2,841 $ 442 $ (119 ) $ 17,403 Segment net income (loss) (a)(b)(c) — 276 — 276 303 (232 ) — 347 Total assets at September 30, 2017 $ 73,056 $ 14,648 $ (322 ) $ 87,382 $ 22,190 $ 2,275 $ (1,532 ) $ 110,315 Three Months Ended Operating revenues $ 5,236 $ 500 $ (117 ) $ 5,619 $ 543 $ 139 $ (37 ) $ 6,264 Segment net income (loss) (a)(b) 1,022 176 — 1,198 4 (62 ) (1 ) 1,139 Nine Months Ended Operating revenues $ 13,120 $ 1,189 $ (330 ) $ 13,979 $ 543 $ 311 $ (118 ) $ 14,715 Segment net income (loss) (a)(b) 2,086 315 — 2,401 4 (146 ) (8 ) 2,251 Total assets at December 31, 2016 $ 72,141 $ 15,169 $ (316 ) $ 86,994 $ 21,853 $ 2,474 $ (1,624 ) $ 109,697 (a) Attributable to Southern Company. (b) Segment net income (loss) for the traditional electric operating companies includes pre-tax charges for estimated probable losses on the Kemper IGCC of $34 million ( $21 million after tax) and $88 million ( $54 million after tax) for the three months ended September 30, 2017 and 2016 , respectively, and $3.2 billion ( $2.2 billion after tax) and $222 million ( $137 million after tax) for the nine months ended September 30, 2017 and 2016 , respectively. See Note (B) under " Integrated Coal Gasification Combined Cycle – Kemper IGCC Schedule and Cost Estimate " for additional information. (c) Segment net income (loss) for the traditional electric operating companies also includes a pre-tax charge for the write-down of Gulf Power's ownership of Plant Scherer Unit 3 of $33 million ( $20 million after tax) for the nine months ended September 30, 2017 . See Note (B) under " Regulatory Matters – Gulf Power – Retail Base Rate Cases " for additional information. Business segment financial data for the successor three months ended September 30, 2017 and 2016 , the successor nine months ended September 30, 2017 , the successor period of July 1, 2016 through September 30, 2016 , and the predecessor period of January 1, 2016 through June 30, 2016 was as follows: Gas Distribution Operations Gas Marketing Services Wholesale Gas Services (*) Gas Midstream Operations Total All Other Eliminations Consolidated (in millions) Successor – Three Months Ended September 30, 2017: Operating revenues $ 472 $ 143 $ (24 ) $ 16 $ 607 $ 2 $ (44 ) $ 565 Segment net income 52 1 (23 ) 14 44 (29 ) — 15 Successor – Nine Months Ended September 30, 2017: Operating revenues $ 2,255 $ 597 $ 95 $ 53 $ 3,000 $ 7 $ (166 ) $ 2,841 Segment net income 223 36 28 38 325 (22 ) — 303 Successor – Total assets at $ 18,711 $ 2,089 $ 893 $ 2,359 $ 24,052 $ 11,400 $ (13,262 ) $ 22,190 Gas Distribution Operations Gas Marketing Services Wholesale Gas Services (*) Gas Midstream Operations Total All Other Eliminations Consolidated (in millions) Successor – Three Months Ended September 30, 2016: Operating revenues $ 455 $ 126 $ (8 ) $ 13 $ 586 $ 2 $ (45 ) $ 543 Segment net income (loss) 27 (4 ) (11 ) 14 26 (22 ) — 4 Predecessor – January 1, 2016 through June 30, 2016: Operating revenues $ 1,575 $ 435 $ (32 ) $ 25 $ 2,003 $ 4 $ (102 ) $ 1,905 Segment EBIT 353 109 (68 ) (6 ) 388 (60 ) — 328 Successor – Total assets at $ 19,453 $ 2,084 $ 1,127 $ 2,211 $ 24,875 $ 11,145 $ (14,167 ) $ 21,853 (*) The revenues for wholesale gas services are netted with costs associated with its energy and risk management activities. A reconciliation of operating revenues and intercompany revenues is shown in the following table. Third Party Gross Revenues Intercompany Revenues Total Gross Revenues Less Gross Gas Costs Operating Revenues (in millions) Successor – Three Months Ended September 30, 2017 $ 1,411 $ 103 $ 1,514 $ 1,538 $ (24 ) Successor – Nine Months Ended September 30, 2017 4,781 362 5,143 5,048 95 Successor – Three Months Ended September 30 , 2016 1,688 77 1,765 1,773 (8 ) Predecessor – January 1, 2016 through June 30, 2016 $ 2,500 $ 143 $ 2,643 $ 2,675 $ (32 )
Financial Data for Products and Services	Products and Services Electric Utilities' Revenues Period Retail Wholesale Other Total (in millions) Three Months Ended September 30, 2017 $ 4,615 $ 718 $ 190 $ 5,523 Three Months Ended September 30, 2016 4,808 613 198 5,619 Nine Months Ended September 30, 2017 $ 11,786 $ 1,867 $ 586 $ 14,239 Nine Months Ended September 30, 2016 11,932 1,455 592 13,979 Southern Company Gas' Revenues Period Gas Gas Other Total (in millions) Three Months Ended September 30, 2017 $ 430 $ 143 $ (8 ) $ 565 Nine Months Ended September 30, 2017 $ 2,119 $ 597 $ 125 $ 2,841 Three and Nine Months Ended September 30, 2016 $ 420 $ 126 $ (3 ) $ 543

Introduction - Narrative (Detai

Introduction - Narrative (Details)	Apr. 04, 2017USD ($)	Apr. 01, 2017	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Sep. 30, 2017USD ($)kWh
GEORGIA POWER CO
Related Party Transaction [Line Items]
Property damage reserve, percentage of balance		0.75
GULF POWER CO
Related Party Transaction [Line Items]
PSC approved annual property damage reserve accrual	$ 3,500,000
Number of agreements for renewable generation	0		$ 39,000,000			$ 39,000,000
Increase (decrease) in recoverable property damage costs	$ 31,000,000
Recovery period for natural disaster reserve costs						60 days
Cumulative costs limit under PSC order						$ 100,000,000
Accrued storm reserve damaged property costs			40,000,000			40,000,000
SOUTHERN Co GAS | GEORGIA POWER CO
Related Party Transaction [Line Items]
Related party transactions				$ 7,000,000		18,000,000
SOUTHERN Co GAS | SOUTHERN POWER CO
Related Party Transaction [Line Items]
Related party transactions				2,000,000		94,000,000
Southern Natural Gas Company, LLC | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Related party transactions				4,000,000		24,000,000
Southern Natural Gas Company, LLC | ALABAMA POWER CO
Related Party Transaction [Line Items]
Related party transactions				1,000,000		8,000,000
Southern Natural Gas Company, LLC | GEORGIA POWER CO
Related Party Transaction [Line Items]
Related party transactions				8,000,000		77,000,000
Southern Natural Gas Company, LLC | SOUTHERN POWER CO
Related Party Transaction [Line Items]
Related party transactions				2,000,000		19,000,000
Maximum | GULF POWER CO
Related Party Transaction [Line Items]
Customer surcharge storm recovery costs						$ 4
Customer surcharge storm recovery capacity | kWh						1,000
Wholesale Gas Services | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Inventory decrement			0			$ 0
Inventory write-down			$ 0	$ 0		$ 0
Predecessor | Wholesale Gas Services | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Inventory write-down					$ 0

Introduction - Goodwill (Detail

Introduction - Goodwill (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill	$ 6,267	$ 6,251
SOUTHERN POWER CO
Goodwill [Line Items]
Goodwill	2	2
SOUTHERN Co GAS
Goodwill [Line Items]
Goodwill	5,967	5,967
SOUTHERN Co GAS | Gas distribution operations
Goodwill [Line Items]
Goodwill	4,702	4,702
SOUTHERN Co GAS | Gas marketing services
Goodwill [Line Items]
Goodwill	$ 1,265	$ 1,265

Introduction - Other Intangible

Introduction - Other Intangibles (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 983	$ 957
Accumulated Amortization	(156)	(62)
Other Intangible Assets, Net	827	895
Total other intangible assets - gross	1,058	1,032
Total other intangible assets - net	902	970
Customer relationships | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	288	268
Accumulated Amortization	(70)	(32)
Other Intangible Assets, Net	218	236
Trade names | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	159	158
Accumulated Amortization	(15)	(5)
Other Intangible Assets, Net	144	153
Storage and transportation contracts | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(27)	(2)
Other Intangible Assets, Net	37	62
PPA fair value adjustments | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(41)	(22)
Other Intangible Assets, Net	415	434
Other | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	16	11
Accumulated Amortization	(3)	(1)
Other Intangible Assets, Net	13	10
Federal Communications Commission licenses | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Other intangible assets not subject to amortization	75	75
SOUTHERN POWER CO
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	(41)	(22)
SOUTHERN POWER CO | PPA fair value adjustments
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(41)	(22)
Other Intangible Assets, Net	415	434
SOUTHERN Co GAS
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	400	400
Accumulated Amortization	(100)	(34)
Other Intangible Assets, Net	300	366
Total other intangible assets - net	300	366
SOUTHERN Co GAS | Customer relationships
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	221	221
Accumulated Amortization	(65)	(30)
Other Intangible Assets, Net	156	191
SOUTHERN Co GAS | Trade names
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	115	115
Accumulated Amortization	(8)	(2)
Other Intangible Assets, Net	107	113
SOUTHERN Co GAS | Storage and transportation contracts
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(27)	(2)
Other Intangible Assets, Net	$ 37	$ 62

Introduction - Intangibles, Amo

Introduction - Intangibles, Amortization (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 29	$ 94
SOUTHERN POWER CO
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	6	19
SOUTHERN Co GAS
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 20	$ 66

Contingencies and Regulatory 37

Contingencies and Regulatory Matters - Narrative (Details)	Nov. 01, 2017USD ($)	Oct. 23, 2017USD ($)	Oct. 03, 2017USD ($)	Oct. 02, 2017USD ($)	Aug. 21, 2017USD ($)	Aug. 15, 2017	Jul. 01, 2017	Jun. 30, 2017USD ($)	Jun. 09, 2017USD ($)	May 04, 2017USD ($)	Apr. 04, 2017intervenor	Mar. 31, 2017USD ($)	Mar. 27, 2017	Mar. 10, 2017USD ($)	Feb. 21, 2017USD ($)	Dec. 20, 2016USD ($)	May 01, 2016USD ($)	Mar. 31, 2016	Jan. 01, 2016	Sep. 30, 2016USD ($)	Jan. 31, 2016USD ($)	Sep. 30, 2017USD ($)MW	Sep. 30, 2016USD ($)	Jun. 30, 2017USD ($)	Sep. 30, 2017USD ($)clauseMW	Sep. 30, 2016USD ($)	Dec. 31, 2018USD ($)	Aug. 31, 2018	Dec. 31, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2017USD ($)MW	Jan. 01, 2121	Sep. 28, 2017USD ($)	Jul. 06, 2017USD ($)	Jun. 23, 2017USD ($)	May 31, 2017USD ($)	Mar. 07, 2017USD ($)	Feb. 17, 2017USD ($)	Aug. 31, 2016USD ($)
Loss Contingencies [Line Items]
Lawsuit deferral period						30 days							30 days
Maximum borrowing capacity																						$ 8,145,000,000			$ 8,145,000,000															$ 8,145,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																				$ (26,180,000,000)		(25,939,000,000)	$ (26,180,000,000)		(25,939,000,000)	$ (26,180,000,000)					$ (26,612,000,000)	$ (21,982,000,000)							$ (26,612,000,000)	(25,939,000,000)
Property, plant and equipment, gross																						102,014,000,000			102,014,000,000						98,416,000,000								98,416,000,000	102,014,000,000
SOUTHERN POWER CO
Loss Contingencies [Line Items]
Maximum borrowing capacity																						750,000,000			750,000,000															750,000,000					$ 600,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																						(5,843,000,000)			(5,843,000,000)						(5,675,000,000)								(5,675,000,000)	(5,843,000,000)
Wholesale revenues, non-affiliates																						510,000,000	387,000,000		1,293,000,000	866,000,000
Property, plant and equipment, gross																						13,734,000,000			13,734,000,000						12,728,000,000								12,728,000,000	13,734,000,000
GEORGIA POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability																						26,000,000			$ 26,000,000						17,000,000								17,000,000	26,000,000
Cost recovery, new nuclear																																														$ (50,000,000)
Percentage of Proportionate share owed in consortium agreement																																				45.70%
Liquidated damages, percentage																									40.00%											10.00%
Maximum borrowing capacity																						1,750,000,000			$ 1,750,000,000															1,750,000,000
Estimated in-service capital cost																																			$ 4,418,000,000
Financing costs collected, net of tax																									1,500,000,000
Additional construction capital costs																$ 3,300,000,000								$ 542,000,000															3,900,000,000
Estimate of possible loss																$ 240,000,000
Retail rate of return on common equity																																		10.95%
Public utilities, approved return on equity percentage																			10.00%
Financing costs incurred																																								1,500,000,000
Estimated cancellation costs																																								350,000,000
Wholesale revenues, non-affiliates																						45,000,000	49,000,000		124,000,000	131,000,000
Property, plant and equipment, gross																						34,589,000,000			34,589,000,000						33,841,000,000								33,841,000,000	34,589,000,000
GEORGIA POWER CO | Minimum
Loss Contingencies [Line Items]
Estimated financing costs																																								3,400,000,000
GEORGIA POWER CO | Other current liabilities
Loss Contingencies [Line Items]
Over recovered fuel balance																						100,000,000			100,000,000															100,000,000
GEORGIA POWER CO | Other deferred credits and liabilities
Loss Contingencies [Line Items]
Over recovered fuel balance																															84,000,000								84,000,000
GULF POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability																						53,000,000			53,000,000						44,000,000								44,000,000	53,000,000
Accrued storm reserve damaged property costs																						40,000,000			40,000,000															40,000,000
Maximum borrowing capacity																						$ 280,000,000			$ 280,000,000															$ 280,000,000
Retail rate of return on common equity																						10.25%			10.25%															10.25%
Reduction in depreciation expense																								34,000,000							0	$ 20,100,000	$ 8,400,000	$ 62,500,000
Wholesale revenues, non-affiliates																						$ 14,000,000	17,000,000		$ 44,000,000	48,000,000
Retail regulatory equity ratio							0.525
Loss on Plant Scherer Unit 3																						0	0		$ 33,000,000	0
Number of regulatory clauses | clause																									4
Property, plant and equipment, gross																						$ 5,181,000,000			$ 5,181,000,000						5,140,000,000								5,140,000,000	$ 5,181,000,000
GULF POWER CO | Minimum
Loss Contingencies [Line Items]
Retail rate of return on common equity																						9.25%			9.25%															9.25%
GULF POWER CO | Maximum
Loss Contingencies [Line Items]
Retail rate of return on common equity																						11.25%			11.25%															11.25%
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability																						$ 399,000,000			$ 399,000,000						426,000,000								426,000,000	$ 399,000,000
Maximum borrowing capacity																						1,900,000,000			1,900,000,000															1,900,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																						(9,167,000,000)			(9,167,000,000)						(9,109,000,000)								(9,109,000,000)	(9,167,000,000)
Property, plant and equipment, gross																						15,383,000,000			15,383,000,000						14,508,000,000								14,508,000,000	15,383,000,000
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Gain contingency, surcharge revenue																															5,000,000
Increase in base rate under cost based electric tariff due to settlement																	$ 7,000,000
AFUDC cost																									22,000,000
Fuel cost recovery																						2,000,000			2,000,000						37,000,000								37,000,000	2,000,000
Percentage of PSC retail rate increase																																											(0.85%)
Maximum borrowing capacity																						100,000,000			100,000,000															100,000,000
Wholesale revenues, non-affiliates																						72,000,000	78,000,000		196,000,000	198,000,000
PSC retail rate increase (decrease)																																											$ 8,000,000
Maximum annual percentage increase in revenue										2.00%
Maximum annual increase in revenue										$ 18,000,000
Deferred revenue										$ 26,000,000
Property, plant and equipment, gross																						4,836,000,000			4,836,000,000						4,865,000,000								4,865,000,000	4,836,000,000
ALABAMA POWER CO
Loss Contingencies [Line Items]
Maximum borrowing capacity																						1,335,000,000			1,335,000,000															1,335,000,000
Wholesale revenues, non-affiliates																						77,000,000	$ 82,000,000		210,000,000	$ 211,000,000
Property, plant and equipment, gross																						26,619,000,000			26,619,000,000						26,031,000,000								26,031,000,000	26,619,000,000
ALABAMA POWER CO | Regulatory assets, deferred
Loss Contingencies [Line Items]
Over (under) recovered environmental clause																																															$ 36,000,000
Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage															10.75%
Annual proceeds from strategic economic development projects															$ 15,000,000
Approved increase (decrease) in fuel rates amount															$ 20,000,000
Elizabethtown Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount								$ 13,000,000												$ 19,000,000
Test year duration																														12 months
Public utilities, requested return on equity, percentage								9.60%												10.25%
Public utilities, property, plant and equipment, accumulated depreciation								$ 3,000,000																$ 3,000,000													$ 3,000,000
Nicor Gas
Loss Contingencies [Line Items]
Maximum borrowing capacity																						700,000,000			700,000,000															700,000,000					700,000,000
Public utilities, requested rate increase (decrease), amount														$ 208,000,000
Public utilities, requested return on equity, percentage														10.70%
Virginia Natural Gas
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage												10.25%
RE Roserock, LLC | SOUTHERN POWER CO
Loss Contingencies [Line Items]
Ownership percentage by parent																																51.00%
Loss contingency, payments																									26,000,000
Location One | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability																						5,000,000			5,000,000															5,000,000
Natural Gas Storage
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, net																						111,000,000			$ 111,000,000															111,000,000
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, percentage of total assets																									20.00%
Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Estimated Cost Incurred After in Service Date																																													$ 175,000,000
Period of amortization of regulatory assets																		36 months
AFUDC cost																																					$ 494,000,000
Approved increase (decrease) in fuel rates amount					$ 126,000,000
Transmission and Distribution Facilities | GEORGIA POWER CO
Loss Contingencies [Line Items]
Regulatory assets related to storm damage																						360,000,000			$ 360,000,000															360,000,000
Asset recovery damaged property costs																						150,000,000			150,000,000															150,000,000
Accrued storm reserve damaged property costs																						$ 145,000,000			145,000,000															$ 145,000,000
Annual accrual for asset recovery damaged property costs																									30,000,000
Plant Vogtle Units 3 And 4
Loss Contingencies [Line Items]
Guarantor obligations									$ 3,680,000,000
Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Liquidated damages, monetary amount																									$ 1,700,000,000											$ 920,000,000
Liquidated damages, monetary amount Received, based on percentage of ownership																																				$ 420,000,000
Maximum borrowing capacity																					$ 920,000,000
Percentage of ownership interest required for voting for continuing construction																						90.00%			90.00%															90.00%
Public utilities, increase in construction budget																									$ 1,000,000,000
Extension in project schedule																									1 year
Percentage of approval required to change primary construction contractor																						90.00%			90.00%															90.00%
Percentage of approval required for material amendments																						67.00%			67.00%															67.00%
Guarantor obligations									$ 1,700,000,000
Requested rate increase (decrease) amount																									$ (400,000,000)
Plant Scherer Unit 3
Loss Contingencies [Line Items]
Loss on Plant Scherer Unit 3																									$ 33,000,000
Plant Scherer Unit 3 | GULF POWER CO
Loss Contingencies [Line Items]
Number of intervenors | intervenor											3
Plant Smith Units 1 and 2 | GULF POWER CO
Loss Contingencies [Line Items]
Capacity of units included in request for decertification of units | MW																									357
Plant Scherer Unit Three | GULF POWER CO
Loss Contingencies [Line Items]
Loss on Plant Scherer Unit 3																									$ 32,500,000
MRA Revenue | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Fuel cost recovery																						$ 3,000,000			3,000,000						$ 13,000,000								$ 13,000,000	$ 3,000,000
Westinghouse | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Period of notice required in the event letters of credit are not renewed																					60 days
Vogtle Owners | Removal of Subcontractor Liens
Loss Contingencies [Line Items]
Loss contingency, payments																									340,000,000
Estimate of possible loss																						386,000,000			386,000,000															386,000,000
Vogtle Owners | GEORGIA POWER CO | Removal of Subcontractor Liens
Loss Contingencies [Line Items]
Estimate of possible loss																						176,000,000			176,000,000															176,000,000
Vogtle Owners | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Period of notice required in the event letters of credit are not renewed									30 days
Restatement Adjustment | Toshiba
Loss Contingencies [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest																																												$ 4,500,000,000
Scenario, Forecast | GEORGIA POWER CO
Loss Contingencies [Line Items]
Additional construction capital costs																																						$ 5,440,000,000
Percent of basis points																																						0.95%			0.30%
Decrease in basis points																																						$ 8,000,000
Amendment to estimated in-service capital cost																																						$ 5,680,000,000
Scenario, Forecast | GULF POWER CO
Loss Contingencies [Line Items]
Overall net customer impact from rate case settlement agreement																													$ 54,300,000
Increase (decrease) in revenue to be received from base rate change																											$ 63,000,000		62,000,000
Wholesale revenues, non-affiliates																													$ 7,700,000
Scenario, Forecast | Virginia Natural Gas
Loss Contingencies [Line Items]
Test year duration																												12 months
Scenario, Forecast | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Increase in NCCR tariff	$ 90,000,000
FFB Loan | GEORGIA POWER CO
Loss Contingencies [Line Items]
Long-term line of credit																						2,600,000,000			2,600,000,000															2,600,000,000
Maximum borrowing capacity																						$ 3,460,000,000			$ 3,460,000,000															$ 3,460,000,000
Conditional borrowing commitment																																										$ 1,670,000,000
Retirement Of Plant | Scenario, Forecast | GULF POWER CO
Loss Contingencies [Line Items]
Regulatory asset, amortization period																											15 years
Plant Scherer Unit Three | GULF POWER CO
Loss Contingencies [Line Items]
Electric generating units, capacity | MW																						205			205															205
Energy Efficiency Cost Rider | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
PSC retail rate increase (decrease)																																											$ 2,000,000
SAVE Program | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount												$ 44,000,000																		$ 44,000,000
Regulated operation, allowable cost recovery												$ 13,000,000
Approved infrastructure replacement program																						$ 21,000,000			$ 21,000,000															$ 21,000,000
Regulatory Infrastructure Program | Nicor Gas
Loss Contingencies [Line Items]
Program duration period																																	9 years
Property, plant and equipment, gross																						178,000,000			178,000,000															178,000,000
STRIDE | Atlanta Gas Light
Loss Contingencies [Line Items]
Approved infrastructure replacement program																						127,000,000			$ 127,000,000															127,000,000
Integrated System Reinforcement Program | Atlanta Gas Light
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount																																																$ 177,000,000
Integrated System Reinforcement Program | Atlanta Gas Light | Minimum
Loss Contingencies [Line Items]
Program duration period																									4 years
Aging Infrastructure Replacement Program | Elizabethtown Gas
Loss Contingencies [Line Items]
Approved infrastructure replacement program																						16,000,000			$ 16,000,000															16,000,000
Safety, Access, and Facility Enhancement Program | Florida City Gas
Loss Contingencies [Line Items]
Approved infrastructure replacement program																						$ 9,000,000			$ 9,000,000															$ 9,000,000
Subsequent Event | Virginia Natural Gas | Minimum
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			9.00%
Subsequent Event | Virginia Natural Gas | Maximum
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			10.00%
Subsequent Event | Virginia Natural Gas | Midpoint
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			9.50%
Subsequent Event | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 19,000,000
Public utilities, requested return on equity, percentage		11.25%
Regulated operation, allowable cost recovery		$ 3,000,000
Public utilities, interim rate		$ 5,000,000
Subsequent Event | Plant Vogtle Units 3 And 4
Loss Contingencies [Line Items]
Proceeds from guarantor obligations				$ 300,000,000
Subsequent Event | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Proceeds from guarantor obligations				$ 137,000,000
Subsequent Event | SAVE Program | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount			$ 34,000,000
Regulated operation, allowable cost recovery			$ 13,000,000

Contingencies and Regulatory 38

Contingencies and Regulatory Matters - Recovery Balance of Each Regulatory Clause - Alabama Power (Details) - ALABAMA POWER CO - USD ($) $ in Millions	Sep. 30, 2017	Feb. 17, 2017	Dec. 31, 2016
Deferred over recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 9		$ 0
Deferred under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	0		9
Rate CNP PPA	17		142
Other regulatory liabilities, current
Loss Contingencies [Line Items]
Retail Energy Cost Recovery()	0		76
Over (under) recovered environmental clause		$ (11)
Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 51		$ 69
Regulatory assets, deferred
Loss Contingencies [Line Items]
Over (under) recovered environmental clause		$ 36

Contingencies and Regulatory 39

Contingencies and Regulatory Matters - Revised Cost and Schedule - Georgia Power (Details) - USD ($) $ in Millions	9 Months Ended	105 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Construction work in progress	$ 8,026	$ 8,026	$ 8,977
GEORGIA POWER CO
Loss Contingencies [Line Items]
Construction work in progress	5,751	5,751	$ 4,939
Financing costs incurred		1,500
Financing costs collected, net of tax	1,500
Estimated cancellation costs		350
GEORGIA POWER CO | Minimum
Loss Contingencies [Line Items]
Estimated financing costs		3,400
Cancellation Decision | GEORGIA POWER CO
Loss Contingencies [Line Items]
Construction work in progress	4,600	4,600
Guarantee Obligations	(1,700)	(1,700)
Financing costs collected, net of tax		1,500
Estimated cancellation costs		400
Estimated net cancellation cost	4,800	4,800
Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Estimated cost to complete	4,200	4,200
Construction work in progress	4,600	4,600
Guarantee Obligations	(1,700)	(1,700)
Estimated capital costs	7,100	7,100
Vogtle Cost Settlement Agreement Revised Forecast	(5,700)	(5,700)
Estimated net additional capital costs	$ 1,400	$ 1,400

Contingencies and Regulatory 40

Contingencies and Regulatory Matters - Recovery Balance of Each Regulatory Clause - Gulf Power (Details) - GULF POWER CO - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Under recovered regulatory clause revenues, current
Loss Contingencies [Line Items]
Fuel Cost Recovery	$ 13	$ 0
Purchased Power Capacity Recovery	1	0
Environmental Cost Recovery	0	13
Energy Conservation Cost Recovery	1	4
Other regulatory liabilities, current
Loss Contingencies [Line Items]
Fuel Cost Recovery	0	15
Environmental Cost Recovery	$ 1	$ 0

Contingencies and Regulatory 41

Contingencies and Regulatory Matters - IGCC (Details) $ in Millions

Dec. 31, 2017USD ($)

Aug. 21, 2017USD ($)

Jun. 28, 2017USD ($)

Nov. 17, 2016USD ($)

Jun. 09, 2016USD ($)

Jun. 03, 2016

Dec. 03, 2015USD ($)

Jul. 07, 2015USD ($)

Jun. 03, 2015USD ($)

Jan. 01, 2014USD ($)

Mar. 19, 2013

Jun. 30, 2017USD ($)

Jan. 31, 2013USD ($)

May 31, 2017USD ($)

Sep. 30, 2017USD ($)mi

Sep. 30, 2016USD ($)

May 31, 2017USD ($)

Dec. 31, 2017USD ($)

Jun. 30, 2017USD ($)

Sep. 30, 2017USD ($)miMW

Sep. 30, 2016USD ($)

Dec. 31, 2012USD ($)

Dec. 31, 2010USD ($)

Jun. 30, 2017USD ($)

May 31, 2017USD ($)

May 20, 2015USD ($)

MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Plant capacity under coal gasification combined cycle technology | MW

582

Cost related to grant funding

$ 245

Maximum cap construction cost

2,880

Reduced percentage interest transferred under asset purchase agreement

15.00%

15.00%

Regulatory liabilities

$ 10

$ 10

Public utilities, approved rate increase (decrease), percentage

3.00%

Settlement agreement collection amount to mitigate rate impact, year two

$ 156

Retail rate recovery

$ 342

Carrying costs associated with retail rate recovery

$ 29

AFUDC cost

22

Interest bearing refundable deposit related to assets sale

$ 275

Promissory note

$ 301

Kemper IGCC

Loss Contingencies [Line Items]

Pre-tax charge to income

34

$ 88

3,200

$ 222

After tax charge to income

$ 21

$ 54

$ 2,200

$ 137

Kemper IGCC | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Pipeline infrastructure | mi

61

61

Estimated cost

$ 7,380

$ 68

$ 7,380

2,880

$ 2,970

$ 2,880

Cost related to grant funding

137

137

Maximum cap construction cost

5,950

5,950

$ 2,400

Period of commercial operations established by discovery docket

5 years

Pre-tax charge to income

2,800

$ 196

$ 34

$ 305

$ 3,200

$ 3,070

After tax charge to income

$ 2,000

$ 121

21

$ 188

2,200

$ 1,890

Construction and development costs

$ 1,300

1,300

Average annual increase (decrease) in operations and maintenance expenses

105

Average annual increase (decrease) in maintenance capital

44

Approved increase (decrease) in fuel rates amount

$ 126

Regulatory assets

113

113

Regulatory liabilities

7

$ 7

AFUDC cost

494

Kemper IGCC | MISSISSIPPI POWER CO | Minimum

Loss Contingencies [Line Items]

Amortization period of regulatory assets and liabilities

2 years

Kemper IGCC | MISSISSIPPI POWER CO | Maximum

Loss Contingencies [Line Items]

Amortization period of regulatory assets and liabilities

10 years

Kemper IGCC | MISSISSIPPI POWER CO | Pending Litigation

Loss Contingencies [Line Items]

Loss contingency, value of damages sought

$ 500

$ 100

Kemper IGCC | MISSISSIPPI POWER CO | Property, Plant and Equipment

Loss Contingencies [Line Items]

Construction and development costs

$ 1,100

Kemper IGCC | MISSISSIPPI POWER CO | Property, Plant, and Equipment Accumulated Depreciation

Loss Contingencies [Line Items]

Construction and development costs

80

Kemper IGCC | MISSISSIPPI POWER CO | Materials And Supplies

Loss Contingencies [Line Items]

Construction and development costs

15

Kemper IGCC | MISSISSIPPI POWER CO | Other deferred charges and assets

Loss Contingencies [Line Items]

Construction and development costs

10

Kemper IGCC | MISSISSIPPI POWER CO | Regulatory Assets

Loss Contingencies [Line Items]

Construction and development costs

113

Kemper IGCC | MISSISSIPPI POWER CO | Regulatory Assets Accumulated Depreciation

Loss Contingencies [Line Items]

Construction and development costs

63

Kemper IGCC | MISSISSIPPI POWER CO | Other regulatory assets current

Loss Contingencies [Line Items]

Construction and development costs

21

Kemper IGCC | MISSISSIPPI POWER CO | Other regulatory assets, deferred

Loss Contingencies [Line Items]

Construction and development costs

92

Kemper IGCC | MISSISSIPPI POWER CO | Transmission and Related Regulatory Assets

Loss Contingencies [Line Items]

Construction and development costs

800

Kemper IGCC | MISSISSIPPI POWER CO | Assets, Current

Loss Contingencies [Line Items]

Regulatory assets

$ 21

21

Electricity Generation Plant, Non-Nuclear | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Estimated cost

$ 2,400

$ 2,400

Kemper IGCC - Combined Cycle | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

CIP expenditures incurred but not yet paid

$ 500

Percentage of costs contracted to third party

15.00%

Kemper IGCC - Gasification | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

AFUDC cost

$ 460

Mine | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Term of management fee contract

40 years

Monthly payments of contributed capital

$ 2

Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Additional suspension costs

$ 5

Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO | Minimum

Loss Contingencies [Line Items]

Pre-tax charge to income

$ 100

Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO | Maximum

Loss Contingencies [Line Items]

Pre-tax charge to income

$ 200

In-Service Asset Proposal | Kemper IGCC | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Approved increase (decrease) in fuel rates amount

$ 126

Public utilities, approved equity capital structure, percentage

49.733%

Public utilities, approved return on equity percentage

9.225%

Denbury Onshore, Contractor | Kemper IGCC | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Long term contract for purchase of percentage of carbon dioxide captured from plant

100.00%

70.00%

70.00%

Treetop Midstream Services, LLC, Contractor | Kemper IGCC | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Long term contract for purchase of percentage of carbon dioxide captured from plant

30.00%

30.00%

Denbury Onshore, Contractor | Electricity Generation Plant, Non-Nuclear | MISSISSIPPI POWER CO

Loss Contingencies [Line Items]

Long-term contract for purchase of carbon dioxide, term

16 years

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets:
Fair Value	$ 26
Liabilities:
Collateral already posted, aggregate fair value	76
Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	415
Interest rate derivatives	5
Foreign currency derivatives	103
Nuclear decommissioning trusts	1,782
Cash equivalents	1,271
Other investments	10
Total	3,586
Liabilities:
Energy-related derivatives	411
Interest rate derivatives	24
Foreign currency derivatives	23
Contingent consideration	20
Total	478
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	231
Interest rate derivatives	0
Foreign currency derivatives	0
Nuclear decommissioning trusts	752
Cash equivalents	1,271
Other investments	9
Total	2,263
Liabilities:
Energy-related derivatives	265
Interest rate derivatives	0
Foreign currency derivatives	0
Contingent consideration	0
Total	265
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	184
Interest rate derivatives	5
Foreign currency derivatives	103
Nuclear decommissioning trusts	1,004
Cash equivalents	0
Other investments	0
Total	1,296
Liabilities:
Energy-related derivatives	146
Interest rate derivatives	24
Foreign currency derivatives	23
Contingent consideration	0
Total	193
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Interest rate derivatives	0
Foreign currency derivatives	0
Nuclear decommissioning trusts	0
Cash equivalents	0
Other investments	1
Total	1
Liabilities:
Energy-related derivatives	0
Interest rate derivatives	0
Foreign currency derivatives	0
Contingent consideration	20
Total	20
ALABAMA POWER CO
Assets:
Energy-related derivatives	9	$ 20
Fair Value	26
Liabilities:
Energy-related derivatives	7	9
ALABAMA POWER CO | Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	9
Cash equivalents	808
Total	1,685
Liabilities:
Energy-related derivatives	7
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Domestic equity
Assets:
Nuclear decommissioning trusts	503
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Foreign equity
Assets:
Nuclear decommissioning trusts	117
ALABAMA POWER CO | Fair Value, Measurements, Recurring | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	27
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Corporate bonds
Assets:
Nuclear decommissioning trusts	169
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	18
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Private Equity
Assets:
Nuclear decommissioning trusts	26
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Other
Assets:
Nuclear decommissioning trusts	8
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Cash equivalents	808
Total	1,309
Liabilities:
Energy-related derivatives	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Assets:
Nuclear decommissioning trusts	422
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity
Assets:
Nuclear decommissioning trusts	60
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Assets:
Nuclear decommissioning trusts	19
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Private Equity
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	9
Cash equivalents	0
Total	350
Liabilities:
Energy-related derivatives	7
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Domestic equity
Assets:
Nuclear decommissioning trusts	81
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign equity
Assets:
Nuclear decommissioning trusts	57
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	27
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds
Assets:
Nuclear decommissioning trusts	150
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	18
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Private Equity
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets:
Nuclear decommissioning trusts	8
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Domestic equity
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign equity
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate bonds
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Private Equity
Assets:
Nuclear decommissioning trusts	0
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO
Assets:
Energy-related derivatives	19	46
Nuclear decommissioning trusts	66
Liabilities:
Energy-related derivatives	14	11
GEORGIA POWER CO | Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	18
Interest rate derivatives	1
Cash equivalents	112
Total	1,045
Liabilities:
Energy-related derivatives	11
Interest rate derivatives	3
Total	14
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Domestic equity
Assets:
Nuclear decommissioning trusts	236
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Foreign equity
Assets:
Nuclear decommissioning trusts	156
GEORGIA POWER CO | Fair Value, Measurements, Recurring | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	225
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Corporate bonds
Assets:
Nuclear decommissioning trusts	160
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	38
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Other
Assets:
Nuclear decommissioning trusts	35
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Municipal bonds
Assets:
Nuclear decommissioning trusts	64
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Interest rate derivatives	0
Cash equivalents	112
Total	363
Liabilities:
Energy-related derivatives	0
Interest rate derivatives	0
Total	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Assets:
Nuclear decommissioning trusts	235
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Nuclear decommissioning trusts	16
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	18
Interest rate derivatives	1
Cash equivalents	0
Total	682
Liabilities:
Energy-related derivatives	11
Interest rate derivatives	3
Total	14
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Domestic equity
Assets:
Nuclear decommissioning trusts	1
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign equity
Assets:
Nuclear decommissioning trusts	156
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	225
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds
Assets:
Nuclear decommissioning trusts	160
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	38
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets:
Nuclear decommissioning trusts	19
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets:
Nuclear decommissioning trusts	64
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Interest rate derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Interest rate derivatives	0
Total	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Domestic equity
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign equity
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate bonds
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets:
Nuclear decommissioning trusts	0
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal bonds
Assets:
Nuclear decommissioning trusts	0
GULF POWER CO
Assets:
Energy-related derivatives	0	5
Liabilities:
Energy-related derivatives	22	29
GULF POWER CO | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	21
Liabilities:
Energy-related derivatives	22
GULF POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	21
Liabilities:
Energy-related derivatives	0
GULF POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0
Liabilities:
Energy-related derivatives	22
GULF POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0
Liabilities:
Energy-related derivatives	0
MISSISSIPPI POWER CO
Assets:
Energy-related derivatives	5	7
Liabilities:
Energy-related derivatives	7	11
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	3
Interest rate derivatives	2
Cash equivalents	209
Total	214
Liabilities:
Energy-related derivatives	7
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Interest rate derivatives	0
Cash equivalents	209
Total	209
Liabilities:
Energy-related derivatives	0
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	3
Interest rate derivatives	2
Cash equivalents	0
Total	5
Liabilities:
Energy-related derivatives	7
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Interest rate derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
SOUTHERN POWER CO
Assets:
Energy-related derivatives	112	22
Liabilities:
Energy-related derivatives	27	63
SOUTHERN POWER CO | Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	9
Foreign currency derivatives	103
Cash equivalents	90
Total	202
Liabilities:
Energy-related derivatives	4
Foreign currency derivatives	23
Contingent consideration	20
Total	47
SOUTHERN POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Foreign currency derivatives	0
Cash equivalents	90
Total	90
Liabilities:
Energy-related derivatives	0
Foreign currency derivatives	0
Contingent consideration	0
Total	0
SOUTHERN POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	9
Foreign currency derivatives	103
Cash equivalents	0
Total	112
Liabilities:
Energy-related derivatives	4
Foreign currency derivatives	23
Contingent consideration	0
Total	27
SOUTHERN POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Foreign currency derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Foreign currency derivatives	0
Contingent consideration	20
Total	20
SOUTHERN Co GAS
Assets:
Energy-related derivatives	389	581
Liabilities:
Energy-related derivatives	360	569
Collateral already posted, aggregate fair value	76	$ 62
SOUTHERN Co GAS | Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	376
Liabilities:
Energy-related derivatives	360
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	231
Liabilities:
Energy-related derivatives	265
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	145
Liabilities:
Energy-related derivatives	95
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Liabilities:
Energy-related derivatives	0
Nuclear Decommissioning Trusts
Assets:
Fair Value	26
Nuclear Decommissioning Trusts | ALABAMA POWER CO
Assets:
Fair Value	26
Weather Derivative | Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	$ 13

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 50	$ 49	$ 168	$ 116
Private Equity
Fair Value, Option, Quantitative Disclosures [Line Items]
Liquidations term			10 years
ALABAMA POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term Debt	7,404		$ 7,404
Increase (decrease) in fair value of funds	25	26	87	66
GEORGIA POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term Debt	11,713		11,713
Increase (decrease) in fair value of funds	25	$ 23	81	$ 50
SOUTHERN POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term Debt	5,810		5,810
SOUTHERN Co GAS
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term Debt	$ 5,862		$ 5,862
Minimum | SOUTHERN POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent payment obligations, payment period			10 years
Maximum | SOUTHERN POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent payment obligations, payment period			30 years

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Measurements Of Investments Calculated At Net Asset Value Per Share (Details) $ in Millions	Sep. 30, 2017USD ($)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 26
Private Equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	26
Unfunded Commitments	24
ALABAMA POWER CO
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	26
ALABAMA POWER CO | Private Equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	26
Unfunded Commitments	$ 24

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2017USD ($)
Southern Company
Long-term debt:
Carrying Amount	$ 47,269
Fair Value	49,348
ALABAMA POWER CO
Long-term debt:
Carrying Amount	7,404
Fair Value	8,031
GEORGIA POWER CO
Long-term debt:
Carrying Amount	11,713
Fair Value	12,237
GULF POWER CO
Long-term debt:
Carrying Amount	1,292
Fair Value	1,352
MISSISSIPPI POWER CO
Long-term debt:
Carrying Amount	2,123
Fair Value	2,117
SOUTHERN POWER CO
Long-term debt:
Carrying Amount	5,810
Fair Value	5,916
SOUTHERN Co GAS
Long-term debt:
Carrying Amount	5,862
Fair Value	$ 6,230

Stockholders' Equity - Earnings

Stockholders' Equity - Earnings per Share (Details) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
As reported shares (in shares)	1,003	968	998	940
Effect of options and performance share award units (in shares)	7	7	7	5
Diluted shares (in shares)	1,010	975	1,005	945
Stock options and performance share award units that were not included in the diluted earnings per share calculation (in shares)	0	0	0	0

Stockholders' Equity - Changes

Stockholders' Equity - Changes in Stockholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)				(800)
Beginning balance				$ 26,612	$ 21,982
Consolidated net income attributable to Southern Company		$ 1,069	$ 1,139	347	2,251
Other comprehensive income (loss)				(2)	(95)
Stock issued				613	3,265
Stock-based compensation				97	94
Cash dividends on common stock				(1,716)	(1,553)
Preference stock redemption				(150)
Contributions from noncontrolling interests				77	357
Distributions to noncontrolling interests				(87)	(21)
Purchase of membership interests from noncontrolling interests					(129)
Net income attributable to noncontrolling interests				45	36
Reclassification from redeemable noncontrolling interests				114
Other				$ (11)	(7)
Ending balance, treasury shares (in shares)		(900)		(900)
Ending balance		$ 25,939	$ 26,180	$ 25,939	$ 26,180
Number of Common Shares Issued
Changes in Stockholders' Equity
Beginning balance (in shares)				991,213	915,073
Stock issued (in shares)				13,308	65,725
Other (in shares)				0
Ending balance (in shares)		1,004,521	980,798	1,004,521	980,798
Number of Common Shares Treasury Issued
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)				(819)	(3,352)
Stock issued (in shares)				0	2,599
Other (in shares)				(75)	(46)
Ending balance, treasury shares (in shares)		(894)	(799)	(894)	(799)
Common Stockholders' Equity
Changes in Stockholders' Equity
Beginning balance				$ 24,758	$ 20,592
Consolidated net income attributable to Southern Company				347	2,251
Other comprehensive income (loss)				(2)	(95)
Stock issued				613	3,265
Stock-based compensation				97	94
Cash dividends on common stock				(1,716)	(1,553)
Other				(15)	(7)
Ending balance		$ 24,082	$ 24,547	24,082	24,547
Preferred and Preference Stock of Subsidiaries
Changes in Stockholders' Equity
Beginning balance				609	609
Preference stock redemption				(150)
Other				3
Ending balance		462	609	462	609
Noncontrolling Interest
Changes in Stockholders' Equity
Beginning balance				1,245	781
Contributions from noncontrolling interests				77	357
Distributions to noncontrolling interests				(87)	(21)
Purchase of membership interests from noncontrolling interests					(129)
Net income attributable to noncontrolling interests				45	36
Reclassification from redeemable noncontrolling interests				114
Other				1
Ending balance		$ 1,395	$ 1,024	$ 1,395	$ 1,024
SunPower Corp. | Noncontrolling Interest
Changes in Stockholders' Equity
Beginning balance	$ 114

Financing - Narrative (Details)

Financing - Narrative (Details) $ / shares in Units, shares in Thousands	Jun. 03, 2015USD ($)	Feb. 20, 2014	Oct. 31, 2017USD ($)shares	Sep. 30, 2017USD ($)$ / sharesshares	Aug. 31, 2017USD ($)	Jun. 30, 2017USD ($)agreement	May 31, 2017USD ($)shares	Feb. 28, 2017USD ($)	Jun. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)$ / shares	Sep. 30, 2016USD ($)	Nov. 30, 2017USD ($)	Sep. 29, 2017USD ($)	Sep. 28, 2017USD ($)	Aug. 10, 2017USD ($)	Jul. 31, 2017USD ($)	Apr. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Short-term debt				$ 2,579,000,000							$ 2,579,000,000									$ 2,241,000,000
Variable rate pollution control revenue bonds outstanding				1,500,000,000							1,500,000,000
Maximum borrowing capacity				8,145,000,000							8,145,000,000
Value of stock redeemed											150,000,000
Line Of Credit Expire Year Four				530,000,000							530,000,000
Southern Company
Debt Instrument [Line Items]
Maximum borrowing capacity				2,000,000,000			$ 1,250,000,000				2,000,000,000
Line Of Credit Expire Year Four				0							0
Southern Company | Line of Credit | Uncommitted Bank Credit Arrangement
Debt Instrument [Line Items]
Increase in debt instrument					$ 250,000,000
Payable to bank by demand, term					30 days
Southern Company | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount						$ 500,000,000			$ 500,000,000
Debt stated interest rate						5.325%			5.325%
Southern Company | Senior Notes | Series 2017A Floating Rate Senior Notes
Debt Instrument [Line Items]
Debt face amount						$ 300,000,000			$ 300,000,000
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Debt Instrument [Line Items]
Number of loan agreements | agreement						2
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 4
Debt Instrument [Line Items]
Debt face amount						$ 100,000,000			100,000,000
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 5
Debt Instrument [Line Items]
Debt face amount						100,000,000			100,000,000
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Working capital				769,000,000							769,000,000
Senior notes, current				4,000,000							4,000,000
Short-term debt				4,000,000							4,000,000									23,000,000
Long-term pollution control bond				50,000,000							50,000,000
Variable rate pollution control revenue bonds outstanding				40,000,000							40,000,000
Fixed rate pollution control revenue bonds outstanding				50,000,000							50,000,000
Maximum borrowing capacity				100,000,000							100,000,000
Promissory note	$ 301,000,000
Capital contributions from parent company						1,000,000,000					1,002,000,000	$ 227,000,000
Line Of Credit Expire Year Four				0							0
MISSISSIPPI POWER CO | Unsecured Debt
Debt Instrument [Line Items]
Loans payable to bank				935,000,000							935,000,000
MISSISSIPPI POWER CO | Tax-Exempt Variable Rate Demand Obligations
Debt Instrument [Line Items]
Short-term debt				40,000,000							40,000,000
MISSISSIPPI POWER CO | Short Term Note
Debt Instrument [Line Items]
Debt face amount																			$ 9,000,000
Debt stated interest rate																			5.00%
MISSISSIPPI POWER CO | Bank Loans | Loans Payable
Debt Instrument [Line Items]
Repayments of debt						10,000,000
MISSISSIPPI POWER CO | Notes Payable to Banks | Notes Payable to Banks
Debt Instrument [Line Items]
Repayments of debt					$ 12,500,000
MISSISSIPPI POWER CO | Promissory Notes | Promissory Notes
Debt Instrument [Line Items]
Debt face amount				150,000,000							150,000,000
Increase in debt instrument									40,000,000
Repayments of debt				109,000,000		591,000,000
MISSISSIPPI POWER CO | Unsecured Debt | Term Loan
Debt Instrument [Line Items]
Debt face amount						1,200,000,000			1,200,000,000
Repayments of debt						300,000,000
GEORGIA POWER CO
Debt Instrument [Line Items]
Short-term debt				400,000,000							400,000,000									391,000,000
Debt instrument, basis spread on variable rate		0.375%
Amortization period for line of credit facility		5 years
Variable rate pollution control revenue bonds outstanding				550,000,000							550,000,000
Variable rate pollution control revenue bonds outstanding, amount remarketed				318,000,000							318,000,000
Fixed rate pollution control revenue bonds outstanding				509,000,000							509,000,000
Maximum borrowing capacity				1,750,000,000							1,750,000,000
Capital contributions from parent company											412,000,000	294,000,000
Line Of Credit Expire Year Four				0							0
GEORGIA POWER CO | Loan Guarantee Agreement
Debt Instrument [Line Items]
Conditional borrowing commitment															$ 1,670,000,000
GEORGIA POWER CO | Line of Credit | Uncommitted Bank Credit Arrangement
Debt Instrument [Line Items]
Increase in debt instrument						$ 500,000,000
Repayments of debt					250,000,000
GEORGIA POWER CO | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount																			$ 450,000,000
Debt stated interest rate																			2.00%
GEORGIA POWER CO | Senior Notes | Series 2017B
Debt Instrument [Line Items]
Debt face amount																			$ 400,000,000
Debt stated interest rate																			3.25%
GEORGIA POWER CO | Senior Notes | Series 2017C
Debt Instrument [Line Items]
Debt face amount					$ 500,000,000
Debt stated interest rate					2.00%
GEORGIA POWER CO | Municipal Bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fifth Series 1995
Debt Instrument [Line Items]
Debt face amount																		$ 27,000,000
GEORGIA POWER CO | Municipal Bonds | Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 1997
Debt Instrument [Line Items]
Debt face amount					$ 38,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Debt Instrument [Line Items]
Number of loan agreements | agreement						3
Payable to bank by demand, term						30 days
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 1
Debt Instrument [Line Items]
Debt face amount					50,000,000	$ 50,000,000			50,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 2
Debt Instrument [Line Items]
Debt face amount						150,000,000			150,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 3
Debt Instrument [Line Items]
Debt face amount					$ 100,000,000	$ 100,000,000			$ 100,000,000
GEORGIA POWER CO | Junior Subordinated Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount				$ 270,000,000							$ 270,000,000
Debt stated interest rate				5.00%							5.00%
ALABAMA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding				$ 854,000,000							$ 854,000,000
Maximum borrowing capacity				1,335,000,000							1,335,000,000
Capital contributions from parent company											337,000,000	253,000,000
Line Of Credit Expire Year Four				$ 500,000,000							$ 500,000,000
ALABAMA POWER CO | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount																			$ 550,000,000
Debt stated interest rate																			2.45%
ALABAMA POWER CO | Senior Notes | 5.00% Class A Preferred Stock
Debt Instrument [Line Items]
Preferred stock, shares issued (in shares) | shares				10,000
Preferred stock, value of shares issued				$ 250,000,000
Preferred stock, par value (in dollars per share) | $ / shares				$ 1							$ 1
Share price (in dollars per share) | $ / shares				$ 25							$ 25
Preferred stock, dividend rate				5.00%
GULF POWER CO
Debt Instrument [Line Items]
Short-term debt				$ 0							$ 0									268,000,000
Variable rate pollution control revenue bonds outstanding				82,000,000							82,000,000
Fixed rate pollution control revenue bonds outstanding				140,000,000							140,000,000
Maximum borrowing capacity				280,000,000							280,000,000
Capital contributions from parent company											7,000,000	10,000,000
Line Of Credit Expire Year Four				30,000,000							30,000,000
GULF POWER CO | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount							$ 300,000,000
Debt stated interest rate							3.30%
GULF POWER CO | Senior Notes | Series 2007A Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares							450
Value of stock redeemed							$ 45,000,000
Preferred stock, dividend rate							6.45%
GULF POWER CO | Senior Notes | Series 2007A
Debt Instrument [Line Items]
Debt stated interest rate							5.90%
Repayments of debt							$ 85,000,000
GULF POWER CO | Senior Notes | Short-term Floating Rate Bank Loan
Debt Instrument [Line Items]
Repayments of debt							$ 100,000,000
GULF POWER CO | Senior Notes | Series Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares							550
Value of stock redeemed							$ 55,000,000
Preferred stock, dividend rate							6.00%
GULF POWER CO | Senior Notes | Series 2013A Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares							500
Value of stock redeemed							$ 50,000,000
Preferred stock, dividend rate							5.60%
GULF POWER CO | Bank Loans
Debt Instrument [Line Items]
Debt face amount																			$ 100,000,000
Traditional Electric Operating Companies
Debt Instrument [Line Items]
Fixed rate pollution control revenue bonds outstanding				699,000,000							699,000,000
SOUTHERN POWER CO
Debt Instrument [Line Items]
Short-term debt				120,000,000							120,000,000									209,000,000
Maximum borrowing capacity				750,000,000			$ 600,000,000				750,000,000
Repayments of debt				40,000,000
Capital contributions from parent company											0	$ 800,000,000
Line Of Credit Expire Year Four				0							0
SOUTHERN POWER CO | Notes Payable to Banks | Floating Rate Bank Loan
Debt Instrument [Line Items]
Debt face amount				100,000,000							100,000,000			$ 60,000,000
Southern Company Gas Capital
Debt Instrument [Line Items]
Maximum borrowing capacity				1,200,000,000			1,200,000,000				1,200,000,000
Nicor Gas
Debt Instrument [Line Items]
Maximum borrowing capacity				700,000,000			700,000,000				700,000,000
Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount																	$ 400,000,000
Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds 3.03% Series
Debt Instrument [Line Items]
Debt face amount																$ 100,000,000
Debt stated interest rate																3.03%
Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds 3.62% Series
Debt Instrument [Line Items]
Debt face amount																$ 100,000,000
Debt stated interest rate																3.62%
SOUTHERN Co GAS
Debt Instrument [Line Items]
Short-term debt				934,000,000							934,000,000									$ 1,257,000,000
Maximum borrowing capacity				1,900,000,000							1,900,000,000
Capital contributions from parent company										$ 1,089,000,000	79,000,000
Line Of Credit Expire Year Four				$ 0							$ 0
SOUTHERN Co GAS | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount							$ 450,000,000
Debt stated interest rate							4.40%
Southern Company | MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Promissory note								$ 551,000,000
Line of Credit | Southern Company | Line of Credit
Debt Instrument [Line Items]
Extinguishment of debt, amount							$ 1,000,000,000
Line of Credit | Southern Company Gas Capital
Debt Instrument [Line Items]
Termination of credit arrangement							1,300,000,000
Line of Credit | Nicor Gas | Line of Credit
Debt Instrument [Line Items]
Termination of credit arrangement							$ 700,000,000
Scenario, Forecast | Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount													$ 200,000,000
Subsequent Event | GEORGIA POWER CO
Debt Instrument [Line Items]
Fixed rate pollution control revenue bonds outstanding, amount remarketed			$ 40,000,000
Subsequent Event | GEORGIA POWER CO | Senior Notes | Preferred Class A
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			1,800
Value of stock redeemed			$ 45,000,000
Preferred stock, dividend rate			6.125%
Subsequent Event | GEORGIA POWER CO | Senior Notes | Series 2007A Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			2,250
Value of stock redeemed			$ 225,000,000
Preferred stock, dividend rate			6.50%
Subsequent Event | ALABAMA POWER CO | Senior Notes | 6.50% Series Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			2,000
Value of stock redeemed			$ 50,000,000
Preferred stock, dividend rate			6.50%
Subsequent Event | ALABAMA POWER CO | Senior Notes | 6.45% Series Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			6,000
Value of stock redeemed			$ 150,000,000
Preferred stock, dividend rate			6.45%
Subsequent Event | ALABAMA POWER CO | Senior Notes | 5.83% Class A Preferred Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			1,520
Value of stock redeemed			$ 38,000,000
Preferred stock, dividend rate			5.83%

Financing - Schedule of Credit

Financing - Schedule of Credit Arrangements (Details) - USD ($)	Sep. 30, 2017	May 31, 2017
Line of Credit Facility [Line Items]
Expires, 2017	$ 130,000,000
Expires, 2018	260,000,000
Expires, 2019	25,000,000
Expires, 2020	530,000,000
Expires, 2022	7,200,000,000
Total	8,145,000,000
Unused	8,066,000,000
Executable Term Loans, One Year	65,000,000
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	20,000,000
Expires Within One Year, No Term Out	185,000,000
Southern Company
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	2,000,000,000
Total	2,000,000,000	$ 1,250,000,000
Unused	2,000,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
ALABAMA POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	35,000,000
Expires, 2019	0
Expires, 2020	500,000,000
Expires, 2022	800,000,000
Total	1,335,000,000
Unused	1,335,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	35,000,000
GEORGIA POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,750,000,000
Total	1,750,000,000
Unused	1,732,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
GULF POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	30,000,000
Expires, 2018	195,000,000
Expires, 2019	25,000,000
Expires, 2020	30,000,000
Expires, 2022	0
Total	280,000,000
Unused	280,000,000
Executable Term Loans, One Year	45,000,000
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	40,000,000
MISSISSIPPI POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	100,000,000
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	0
Total	100,000,000
Unused	100,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	100,000,000
SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	750,000,000
Total	750,000,000	600,000,000
Unused	728,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
SOUTHERN Co GAS
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,900,000,000
Total	1,900,000,000
Unused	1,861,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
Other
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	30,000,000
Expires, 2019	0
Expires, 2020	0
Expires, 2022	0
Total	30,000,000
Unused	30,000,000
Executable Term Loans, One Year	20,000,000
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	20,000,000
Expires Within One Year, No Term Out	10,000,000
Southern Company Gas Capital
Line of Credit Facility [Line Items]
Total	1,200,000,000	1,200,000,000
Nicor Gas
Line of Credit Facility [Line Items]
Total	700,000,000	$ 700,000,000
Standby Letters of Credit | SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Total	120,000,000
Unused	9,000,000
Long-term line of credit	$ 111,000,000

Financing - Schedule of Long-Te

Financing - Schedule of Long-Term Debt Financing Activities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)
Debt Instrument [Line Items]
Senior Note Issuances	$ 2,950
Senior Note Maturities and Redemptions	1,135
Revenue Bond Maturities, Redemptions, and Repurchases	101
Other Long-Term Debt Issuances	1,119
Other Long-Term Debt Redemptions and Maturities	745
Southern Company
Debt Instrument [Line Items]
Senior Note Issuances	300
Senior Note Maturities and Redemptions	400
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	500
Other Long-Term Debt Redemptions and Maturities	400
ALABAMA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	550
Senior Note Maturities and Redemptions	200
Revenue Bond Maturities, Redemptions, and Repurchases	36
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	0
GEORGIA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	1,350
Senior Note Maturities and Redemptions	450
Revenue Bond Maturities, Redemptions, and Repurchases	65
Other Long-Term Debt Issuances	370
Other Long-Term Debt Redemptions and Maturities	13
GULF POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	300
Senior Note Maturities and Redemptions	85
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	6
Other Long-Term Debt Redemptions and Maturities	0
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities and Redemptions	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	40
Other Long-Term Debt Redemptions and Maturities	893
SOUTHERN POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities and Redemptions	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	43
Other Long-Term Debt Redemptions and Maturities	4
SOUTHERN Co GAS
Debt Instrument [Line Items]
Senior Note Issuances	450
Senior Note Maturities and Redemptions	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	200
Other Long-Term Debt Redemptions and Maturities	22
Other
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities and Redemptions	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	12
Elimination
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities and Redemptions	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	(40)
Other Long-Term Debt Redemptions and Maturities	$ (599)

Retirement Benefits (Details)

Retirement Benefits (Details) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future employer contributions, remainder of fiscal year	$ 0			$ 0
Pension Plans | Southern Company
Pension Plans and Postretirement Plans
Service cost	73,000,000	$ 68,000,000		220,000,000	$ 192,000,000
Interest cost	114,000,000	110,000,000		341,000,000	311,000,000
Expected return on plan assets	(224,000,000)	(203,000,000)		(673,000,000)	(577,000,000)
Amortization:
Prior service costs	3,000,000	3,000,000		9,000,000	10,000,000
Net (gain)/loss	41,000,000	45,000,000		122,000,000	120,000,000
Net periodic pension cost (income)	7,000,000	23,000,000		19,000,000	56,000,000
Pension Plans | SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future employer contributions, remainder of fiscal year	0			0
Pension Plans and Postretirement Plans
Service cost	6,000,000	7,000,000		17,000,000
Interest cost	10,000,000	10,000,000		30,000,000
Expected return on plan assets	(18,000,000)	(17,000,000)		(53,000,000)
Amortization of regulatory asset		6,000,000
Amortization:
Prior service costs				(1,000,000)
Net (gain)/loss	5,000,000			15,000,000
Net periodic pension cost (income)	3,000,000	6,000,000		8,000,000
Pension Plans | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	15,000,000	14,000,000		47,000,000	43,000,000
Interest cost	25,000,000	23,000,000		73,000,000	71,000,000
Expected return on plan assets	(49,000,000)	(46,000,000)		(147,000,000)	(138,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		2,000,000	2,000,000
Net (gain)/loss	10,000,000	10,000,000		31,000,000	30,000,000
Net periodic pension cost (income)	2,000,000	2,000,000		6,000,000	8,000,000
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	19,000,000	17,000,000		56,000,000	52,000,000
Interest cost	34,000,000	34,000,000		103,000,000	102,000,000
Expected return on plan assets	(71,000,000)	(64,000,000)		(212,000,000)	(193,000,000)
Amortization:
Prior service costs	0	1,000,000		2,000,000	4,000,000
Net (gain)/loss	15,000,000	14,000,000		43,000,000	41,000,000
Net periodic pension cost (income)	(3,000,000)	2,000,000		(8,000,000)	6,000,000
Pension Plans | GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	3,000,000	3,000,000		10,000,000	9,000,000
Interest cost	5,000,000	5,000,000		15,000,000	14,000,000
Expected return on plan assets	(10,000,000)	(9,000,000)		(29,000,000)	(26,000,000)
Amortization:
Prior service costs	0	0		0	1,000,000
Net (gain)/loss	2,000,000	2,000,000		5,000,000	5,000,000
Net periodic pension cost (income)	0	1,000,000		1,000,000	3,000,000
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	3,000,000		11,000,000	9,000,000
Interest cost	5,000,000	4,000,000		15,000,000	14,000,000
Expected return on plan assets	(9,000,000)	(9,000,000)		(29,000,000)	(26,000,000)
Amortization:
Prior service costs	0	1,000,000		1,000,000	1,000,000
Net (gain)/loss	1,000,000	2,000,000		5,000,000	5,000,000
Net periodic pension cost (income)	1,000,000	1,000,000		3,000,000	3,000,000
Postretirement Benefits | Southern Company
Pension Plans and Postretirement Plans
Service cost	6,000,000	6,000,000		18,000,000	17,000,000
Interest cost	19,000,000	20,000,000		59,000,000	55,000,000
Expected return on plan assets	(16,000,000)	(16,000,000)		(49,000,000)	(44,000,000)
Amortization:
Prior service costs	2,000,000	1,000,000		5,000,000	4,000,000
Net (gain)/loss	3,000,000	5,000,000		10,000,000	12,000,000
Net periodic pension cost (income)	14,000,000	16,000,000		43,000,000	44,000,000
Postretirement Benefits | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000		2,000,000
Interest cost	3,000,000	2,000,000		8,000,000
Expected return on plan assets	(2,000,000)	(2,000,000)		(5,000,000)
Amortization of regulatory asset		1,000,000
Amortization:
Prior service costs	(1,000,000)			(2,000,000)
Net (gain)/loss	1,000,000			3,000,000
Net periodic pension cost (income)	2,000,000	2,000,000		6,000,000
Postretirement Benefits | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000		4,000,000	4,000,000
Interest cost	4,000,000	5,000,000		13,000,000	14,000,000
Expected return on plan assets	(5,000,000)	(6,000,000)		(19,000,000)	(19,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		3,000,000	3,000,000
Net (gain)/loss	0	0		1,000,000	1,000,000
Net periodic pension cost (income)	1,000,000	1,000,000		2,000,000	3,000,000
Postretirement Benefits | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	2,000,000		5,000,000	5,000,000
Interest cost	6,000,000	7,000,000		21,000,000	22,000,000
Expected return on plan assets	(6,000,000)	(6,000,000)		(18,000,000)	(17,000,000)
Amortization:
Prior service costs	0	0		1,000,000	1,000,000
Net (gain)/loss	3,000,000	3,000,000		6,000,000	7,000,000
Net periodic pension cost (income)	5,000,000	6,000,000		15,000,000	18,000,000
Postretirement Benefits | GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0		1,000,000	1,000,000
Interest cost	1,000,000	1,000,000		2,000,000	2,000,000
Expected return on plan assets	0	0		(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0		0	0
Net (gain)/loss	0	0		0	0
Net periodic pension cost (income)	1,000,000	1,000,000		2,000,000	2,000,000
Postretirement Benefits | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0		1,000,000	1,000,000
Interest cost	1,000,000	0		3,000,000	2,000,000
Expected return on plan assets	0	0		(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0		0	0
Net (gain)/loss	0	1,000,000		0	1,000,000
Net periodic pension cost (income)	$ 1,000,000	$ 1,000,000		$ 3,000,000	$ 3,000,000
Predecessor | Pension Plans | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost			$ 13,000,000
Interest cost			21,000,000
Expected return on plan assets			(33,000,000)
Amortization:
Prior service costs			(1,000,000)
Net (gain)/loss			13,000,000
Net periodic pension cost (income)			13,000,000
Predecessor | Postretirement Benefits | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost			1,000,000
Interest cost			5,000,000
Expected return on plan assets			(3,000,000)
Amortization:
Prior service costs			(1,000,000)
Net (gain)/loss			2,000,000
Net periodic pension cost (income)			$ 4,000,000

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	Sep. 08, 2017	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								$ 82	$ 21
Effective tax rate						42.60%	28.30%
Income benefit			$ (590)	$ (439)		$ (317)	$ (917)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016		$ 17				484
Tax positions from current periods						9
Tax positions from prior periods						186
Reductions due to settlements						290
Balance as of September 30, 2017			17			17
Tax positions impacting the effective tax rate								17	20
Tax positions not impacting the effective tax rate								0	464
Balance of unrecognized tax benefits		17	17			$ 484		17	484
Significantly increase (decrease) in amount of unrecognized tax benefits						12 months
Kemper IGCC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Tax positions from prior periods	$ 36
Balance as of September 30, 2017	176
Balance of unrecognized tax benefits	$ 176
MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								46
Effective tax rate						(30.30%)	(282.80%)
Income benefit			(24)	$ 2		$ 885	$ 29
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016		2				465
Tax positions from current periods						2
Tax positions from prior periods						175
Reductions due to settlements						290
Balance as of September 30, 2017			2			2
Tax positions impacting the effective tax rate								2
Tax positions not impacting the effective tax rate								0
Balance of unrecognized tax benefits		2	2			465		2	465
MISSISSIPPI POWER CO | Kemper IGCC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016
Balance as of September 30, 2017
Balance of unrecognized tax benefits
Interest on accrued income taxes
SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								19
Effective tax rate				60.30%		43.40%
Income benefit			(52)	$ (7)		$ (233)
SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Effective tax rate						(66.50%)	(88.90%)
Income benefit			39	$ 102		$ 129	$ 167
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016		0				17
Tax positions from current periods						0
Tax positions from prior periods						17
Reductions due to settlements						0
Balance as of September 30, 2017			0			0
Balance of unrecognized tax benefits		0	$ 0			$ 17		0	17
Investment Tax Credit Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward, amount								1,900	$ 1,800
Federal
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								18
Federal | MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								0
Federal | SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								18
State (net of federal benefit)
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								64
State (net of federal benefit) | MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								46
State (net of federal benefit) | SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								$ 1
Scenario, Forecast | SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Income benefit		$ 40
Predecessor | SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Effective tax rate					37.60%
Income benefit					$ (87)

Derivatives - Energy-Related an

Derivatives - Energy-Related and Interest Rate Derivatives (Details) MMBTU in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)MMBTU
Interest rate derivatives
Interest rate derivatives
Notional Amount	$ 3,650
Fair Value Gain (Loss) at September 30, 2017	$ (19)
Southern Company | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	463
Longest Hedge Date	2,021
Longest Non-Hedge Date	2,024
Southern Company | Interest rate derivatives | 2020-06-01 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 300
Interest Rate Received	2.75%
Fair Value Gain (Loss) at September 30, 2017	$ 0
Southern Company | Interest rate derivatives | 2020-06-01 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.92%
Southern Company | Interest rate derivatives | 7/31/2021 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 1,500
Interest Rate Received	2.35%
Fair Value Gain (Loss) at September 30, 2017	$ (19)
Southern Company | Interest rate derivatives | 7/31/2021 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.87%
ALABAMA POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	66
Longest Hedge Date	2,020
GEORGIA POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	159
Longest Hedge Date	2,021
GEORGIA POWER CO | Interest rate derivatives | 6/1/2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 250
Interest Rate Received	5.40%
Fair Value Gain (Loss) at September 30, 2017	$ 0
GEORGIA POWER CO | Interest rate derivatives | 6/1/2018 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	4.02%
GEORGIA POWER CO | Interest rate derivatives | 12/1/2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 500
Interest Rate Received	1.95%
Fair Value Gain (Loss) at September 30, 2017	$ (2)
GEORGIA POWER CO | Interest rate derivatives | 12/1/2018 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.76%
GEORGIA POWER CO | Interest rate derivatives | 12/1/2019 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 200
Interest Rate Received	4.25%
Fair Value Gain (Loss) at September 30, 2017	$ 0
GEORGIA POWER CO | Interest rate derivatives | 12/1/2019 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	2.46%
GULF POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	28
Longest Hedge Date	2,020
MISSISSIPPI POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	44
Longest Hedge Date	2,021
MISSISSIPPI POWER CO | Interest rate derivatives | 3/1/2018 | Cash Flow Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 900
Weighted Average Interest Rate Paid	0.79%
Fair Value Gain (Loss) at September 30, 2017	$ 2
SOUTHERN POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	13
Longest Hedge Date	2,018
SOUTHERN Co GAS | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	153
Longest Hedge Date	2,020
Longest Non-Hedge Date	2,024
SOUTHERN Co GAS | Energy-related, Natural Gas | Long | Derivatives not designated as hedging instruments
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,300
SOUTHERN Co GAS | Energy-related, Natural Gas | Short | Derivatives not designated as hedging instruments
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,100

Derivatives - Narrative (Detail

Derivatives - Narrative (Details) MMBTU in Millions	9 Months Ended
	Sep. 30, 2017USD ($)MMBTU	Dec. 31, 2016USD ($)
Derivative [Line Items]
Collateral already posted, aggregate fair value	$ 76,000,000
Southern Company
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	34
Cash flow hedge gain (loss) to be reclassified within twelve months	$ (19,000,000)
GEORGIA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	11
ALABAMA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	5
Gulf Power and Mississippi Power
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	3
MISSISSIPPI POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	4
SOUTHERN POWER CO
Derivative [Line Items]
Cash flow hedge gain (loss) to be reclassified within twelve months	$ 5,000,000
Parent Company and Southern Power
Derivative [Line Items]
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months	(23,000,000)
SOUTHERN Co GAS
Derivative [Line Items]
Collateral already posted, aggregate fair value	76,000,000	$ 62,000,000
Derivative Counterparties | Registrants
Derivative [Line Items]
Collateral already posted, aggregate fair value	$ 0

Derivatives - Foreign Currency

Derivatives - Foreign Currency Derivatives (Details) - 9 months ended Sep. 30, 2017 - Cash Flow Hedges of Existing Debt - Foreign Exchange Contract € in Millions, $ in Millions	USD ($)	EUR (€)
Derivative [Line Items]
Pay Notional	$ 1,241
Receive Notional | €		€ 1,100
Fair Value Gain (Loss) at September 30, 2017	80
SOUTHERN POWER CO | June 2022
Derivative [Line Items]
Pay Notional	$ 677
Pay Rate	2.95%
Receive Notional | €		600
Receive Rate	1.00%
Fair Value Gain (Loss) at September 30, 2017	$ 42
SOUTHERN POWER CO | June 2026
Derivative [Line Items]
Pay Notional	$ 564
Pay Rate	3.78%
Receive Notional | €		€ 500
Receive Rate	1.85%
Fair Value Gain (Loss) at September 30, 2017	$ 38

Derivatives - Derivative Financ

Derivatives - Derivative Financial Statement Presentation and Amounts With Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	$ 76
Southern Company
Derivatives, Fair Value [Line Items]
Derivatives, assets	536	$ 690
Derivatives, liabilities	458	718
Derivative asset amount offset	(275)	(462)
Derivative liability amount offset	(351)	(524)
Derivative asset	261	228
Derivative liability	107	194
Southern Company | Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivatives, assets	386	556
Derivatives, liabilities	357	564
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	271	489
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	115	66
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	103	81
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	254	483
Southern Company | Derivatives not designated as hedging instruments | Interest rate derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
Southern Company | Derivatives not designated as hedging instruments | Interest rate derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	0
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Derivatives, assets	34	98
Derivatives, liabilities	48	60
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	21	73
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	13	25
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	23	33
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	25	27
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	116	36
Derivatives, liabilities	53	94
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	8	23
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	6	7
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	5	12
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	23	28
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	1
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	103	0
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	33
Southern Company | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	23	25
ALABAMA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	9	20
Derivatives, liabilities	7	9
Derivative asset amount offset	(5)	(8)
Derivative liability amount offset	(5)	(8)
Derivative asset	4	12
Derivative liability	2	1
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	9	20
Derivatives, liabilities	7	9
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	6	13
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	7
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	3	4
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	4	5
GEORGIA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	19	46
Derivatives, liabilities	14	11
Derivative asset amount offset	(10)	(8)
Derivative liability amount offset	(10)	(8)
Derivative asset	9	38
Derivative liability	4	3
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	18	44
Derivatives, liabilities	11	8
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	10	30
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	8	14
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	8	7
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	3	1
GEORGIA POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	2
Derivatives, liabilities	3	3
GEORGIA POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	2
GEORGIA POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
GEORGIA POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	2	3
GEORGIA POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	0
GULF POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	5
Derivatives, liabilities	22	29
Derivative asset amount offset	0	(4)
Derivative liability amount offset	0	(4)
Derivative asset	0	1
Derivative liability	22	25
GULF POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	5
Derivatives, liabilities	22	29
GULF POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	4
GULF POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
GULF POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	9	17
GULF POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	13	12
MISSISSIPPI POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	5	7
Derivatives, liabilities	7	11
Derivative asset amount offset	(3)	(3)
Derivative liability amount offset	(3)	(3)
Derivative asset	2	4
Derivative liability	4	8
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	4
Derivatives, liabilities	7	11
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	2
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	2
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	3	5
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	4	6
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	3
Derivatives, liabilities	0	0
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	2
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	0
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	0
SOUTHERN POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	112	22
Derivatives, liabilities	27	63
Derivative asset amount offset	(1)	(5)
Derivative liability amount offset	(1)	(5)
Derivative asset	111	17
Derivative liability	26	58
SOUTHERN POWER CO | Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	4
Derivatives, liabilities	0	1
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	3
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	1
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Interest rate derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Interest rate derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	0
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	111	18
Derivatives, liabilities	27	62
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	8	18
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	4	4
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	103	0
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	33
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	23	25
SOUTHERN Co GAS
Derivatives, Fair Value [Line Items]
Derivatives, assets	389	581
Derivatives, liabilities	360	569
Derivative asset amount offset	(251)	(435)
Derivative liability amount offset	(327)	(497)
Derivative asset	138	146
Derivative liability	33	72
Collateral already posted, aggregate fair value	76	62
SOUTHERN Co GAS | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	1	8
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Derivatives, assets	385	552
Derivatives, liabilities	357	563
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	115	66
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	103	81
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	270	486
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	254	482
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Derivatives, assets	4	25
Derivatives, liabilities	1	3
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	0
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	4	24
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	3
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	4
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	$ 2	$ 3

Derivatives - Pre-tax Effects o

Derivatives - Pre-tax Effects of Derivatives Designated as Cash Flow Hedging Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	$ 76			$ 76
Southern Company
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	39	$ 31		86	$ (191)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	27	(4)		95	(32)
Southern Company | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(14)			(14)		$ 33
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(6)	0		(26)	(1)
Southern Company | Energy-related derivatives | Depreciation and amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(6)	1		(12)	1
Southern Company | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(18)			(18)		(16)
Southern Company | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(12)			(12)		(19)
Southern Company | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	14			14		56
Southern Company | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	2			2		12
Southern Company | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(1)	(6)		(2)	(189)
Southern Company | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5)	(6)		(15)	(13)
Southern Company | Foreign Exchange Contract
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	46	37		114	(1)
Southern Company | Foreign Exchange Contract | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5)	(6)		(17)	(7)
Southern Company | Foreign Exchange Contract | Other income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	43	7		139	(13)
ALABAMA POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	2			2		11
ALABAMA POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		(1)
ALABAMA POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		0
ALABAMA POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	3			3		8
ALABAMA POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	1			1		4
ALABAMA POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	0	0		0	(3)
ALABAMA POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(2)	(2)		(5)	(5)
GEORGIA POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	7			7		36
GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		0
GEORGIA POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	7			7		29
GEORGIA POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	1			1		7
GEORGIA POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	0	0		0	0
GEORGIA POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1)	(1)		(3)	(3)
GULF POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				(2)	(7)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	0
GULF POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(22)			(22)		(24)
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				(1)	0
GULF POWER CO | Energy-related derivatives | Depreciation and amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	0
GULF POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(13)			(13)		(9)
GULF POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(9)			(9)		(16)
GULF POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		1
GULF POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
GULF POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				(1)	(7)
GULF POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	0
MISSISSIPPI POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(4)			(4)		(7)
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(3)			(3)		(5)
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		(3)
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		1
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
MISSISSIPPI POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(1)	(1)		0	(1)
MISSISSIPPI POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0	0		(1)	(1)
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	76			76		62
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		(5)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)		0
SOUTHERN Co GAS | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	3			3		17
Collateral already posted, aggregate fair value	1			1		8
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		0		(4)
SOUTHERN Co GAS | Energy-related derivatives | Cost of natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)		0		0
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		(1)
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	4			4		17
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		$ 1
SOUTHERN Co GAS | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	0	(5)
SOUTHERN Co GAS | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0	0
SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	40	37		93	(2)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	32	2		110	(20)
SOUTHERN POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(6)	0		(21)	(1)
SOUTHERN POWER CO | Energy-related derivatives | Depreciation and amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(6)	1		(12)	1
SOUTHERN POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				0	0
SOUTHERN POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	(1)
SOUTHERN POWER CO | Foreign Exchange Contract
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	46	37		114	(1)
SOUTHERN POWER CO | Foreign Exchange Contract | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5)	(6)		(17)	(7)
SOUTHERN POWER CO | Foreign Exchange Contract | Other income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 43	$ 7		$ 139	$ (13)
Predecessor | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)			$ (64)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			(1)
Predecessor | SOUTHERN Co GAS | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)			0
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Cost of natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			(1)
Predecessor | SOUTHERN Co GAS | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)			(64)
Predecessor | SOUTHERN Co GAS | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			$ 0

Derivatives - Pre-tax Effects58

Derivatives - Pre-tax Effects of Derivatives Not Designated as Hedging (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Energy-related derivatives | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	$ (15)	$ 6		$ 46	$ 6
Energy-related derivatives | Natural gas revenues | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(17)	0		48	0
Energy-related derivatives | Cost of natural gas | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	2	6		(2)	$ 6
Southern Company | Weather Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net				15
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net		(17)		32
SOUTHERN Co GAS | Energy-related derivatives | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(15)	6		46
SOUTHERN Co GAS | Energy-related derivatives | Natural gas revenues | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(17)	0		48
SOUTHERN Co GAS | Energy-related derivatives | Cost of natural gas | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	$ 2	$ 6		(2)
SOUTHERN Co GAS | Weather Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net				$ 15
SOUTHERN Co GAS | Predecessor
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net			$ (162)
SOUTHERN Co GAS | Predecessor | Energy-related derivatives | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net			(63)
SOUTHERN Co GAS | Predecessor | Energy-related derivatives | Natural gas revenues | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net			(1)
SOUTHERN Co GAS | Predecessor | Energy-related derivatives | Cost of natural gas | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net			$ (62)

Derivatives - Pre-tax Effects59

Derivatives - Pre-tax Effects of Derivatives as Fair Value Hedging Relationships (Details) - Interest rate derivatives - Interest expense, net of amounts capitalized - Fair Value Hedging - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Southern Company
Derivative Instruments, Gain (Loss) [Line Items]
Derivative assets, fair value, gross asset	$ (5)	$ (9)	$ (6)	$ 15
GEORGIA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Derivative assets, fair value, gross asset	$ 0	$ (5)	$ (1)	$ 10

Acquisitions and Dispositions -

Acquisitions and Dispositions - Narrative (Details) $ / shares in Units, MW in Thousands, $ in Millions	Jul. 01, 2016USD ($)	May 09, 2016USD ($)$ / shares	Oct. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2020	Jan. 06, 2017	Dec. 31, 2016USD ($)MW
Business Acquisition [Line Items]
Goodwill				$ 6,267		$ 6,267		$ 6,267				$ 6,251
Revenues						6,201	$ 6,264	17,403	$ 14,715
Consolidated net income attributable to Southern Company						1,069	1,139	347	2,251
Construction work in progress				8,026		8,026		8,026				8,977
SOUTHERN Co GAS
Business Acquisition [Line Items]
Goodwill				5,967		5,967		5,967				5,967
Revenues						565	543	2,841
Consolidated net income attributable to Southern Company						15	4	303
Construction work in progress				596		596		596				496
SOUTHERN POWER CO
Business Acquisition [Line Items]
Goodwill				2		2		2				2
Revenues						618	500	1,597	1,189
Consolidated net income attributable to Southern Company						124	176	276	315
Total costs of construction								494
Construction work in progress				425		425		425				$ 398
Repayments of debt				40
SOUTHERN Co GAS
Business Acquisition [Line Items]
Aggregate purchase price	$ 8,000
Goodwill				5,967		5,967		$ 5,967
SOUTHERN Co GAS | Minimum
Business Acquisition [Line Items]
Estimated lives of intangible assets								1 year
SOUTHERN Co GAS | Maximum
Business Acquisition [Line Items]
Estimated lives of intangible assets								28 years
PowerSecure International, Inc.
Business Acquisition [Line Items]
Aggregate purchase price		$ 429
Goodwill				284		284		$ 284
Acquisition, share price (in dollars per share) | $ / shares		$ 18.75
PowerSecure International, Inc. | Minimum
Business Acquisition [Line Items]
Estimated lives of intangible assets								1 year
PowerSecure International, Inc. | Maximum
Business Acquisition [Line Items]
Estimated lives of intangible assets								26 years
Bethel | SOUTHERN POWER CO
Business Acquisition [Line Items]
Ownership percentage by parent											100.00%
Series of Construction Projects | SOUTHERN POWER CO
Business Acquisition [Line Items]
Construction work in progress				122		122		$ 122
Series of Construction Projects | Minimum | SOUTHERN POWER CO
Business Acquisition [Line Items]
Estimated future construction payments				360		360		360
Series of Construction Projects | Maximum | SOUTHERN POWER CO
Business Acquisition [Line Items]
Estimated future construction payments				$ 415		415		415
Scenario, Forecast | Elizabethtown Gas and Elkton Gas
Business Acquisition [Line Items]
Aggregate purchase price					$ 1,700
Renewable Energy Systems Americas, Inc. | SOUTHERN POWER CO
Business Acquisition [Line Items]
Approximate nameplate capacity, wind (MW) | MW												3
Wind Generating Facility | Scenario, Forecast | SOUTHERN POWER CO
Business Acquisition [Line Items]
Project qualification for production tax credits, percentage										100.00%
Production tax credits, project qualification period										10 years
Operating Segments | SOUTHERN Co GAS
Business Acquisition [Line Items]
Revenues						607	586	3,000
Consolidated net income attributable to Southern Company						44		325
SOUTHERN Co GAS | Operating Segments
Business Acquisition [Line Items]
Revenues						565	543	2,841	543
Consolidated net income attributable to Southern Company						$ 15	$ 4	$ 303	$ 4
Subsequent Event | Southern Turner Renewable Energy, LLC [Member] | SOUTHERN POWER CO
Business Acquisition [Line Items]
Ownership percentage by parent			10.00%
Repayments of debt			$ 14

Acquisitions and Dispositions61

Acquisitions and Dispositions - Schedule of Acquisitions (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill	$ 6,267	$ 6,251
SOUTHERN Co GAS
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Current assets	1,557
Property, plant, and equipment	10,108
Goodwill	5,967
Intangible assets	400
Regulatory assets	1,118
Other assets	229
Current liabilities	(2,201)
Other liabilities	(4,742)
Long-term debt	(4,261)
Noncontrolling interest	(174)
Total purchase price	8,001
PowerSecure International, Inc.
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Current assets	172
Property, plant, and equipment	46
Goodwill	284
Intangible assets	106
Other assets	4
Current liabilities	(121)
Long-term debt, including current portion	(48)
Other liabilities	(14)
Total purchase price	429
SOUTHERN POWER CO
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill	$ 2	$ 2

Acquisitions and Dispositions62

Acquisitions and Dispositions - Schedule of Pro Forma Consolidated Information (Details) - SOUTHERN Co GAS $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2016USD ($)$ / shares
Business Acquisition [Line Items]
Operating revenues | $	$ 16,609
Net income attributable to Southern Company | $	$ 2,394
Basic EPS (in dollars per share) | $ / shares	$ 2.52
Diluted EPS (in dollars per share) | $ / shares	$ 2.51

Acquisitions and Dispositions63

Acquisitions and Dispositions - Schedule of Construction Projects (Details) - SOUTHERN POWER CO - MW	Jan. 06, 2017	Apr. 30, 2017	Mar. 31, 2017	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Jul. 31, 2017
Bethel
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	276
Southern Power Percentage Ownership	100.00%
PPA Contract Period	12 years
East Pecos
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)						120
PPA Contract Period			15 years
Lamesa
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)						102
PPA Contract Period		15 years
Cactus Flats
Business Acquisition [Line Items]
Southern Power Percentage Ownership							100.00%
Scenario, Forecast | Cactus Flats
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)					148
Scenario, Forecast | Mankato
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)				345
PPA Contract Period				20 years
Minimum | Scenario, Forecast | Cactus Flats
Business Acquisition [Line Items]
PPA Contract Period					12 years
Maximum | Scenario, Forecast | Cactus Flats
Business Acquisition [Line Items]
PPA Contract Period					15 years

Joint Ownership Agreements - Ba

Joint Ownership Agreements - Balance Sheet Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,637	$ 1,549
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,609	1,541
SNG | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,385	1,394
Atlantic Coast Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	61	33
PennEast Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	49	22
Triton | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	43	44
Pivotal JAX LNG, LLC | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	40	16
Horizon Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	30	30
Other | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1	$ 2

Joint Ownership Agreements - In

Joint Ownership Agreements - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 32	$ 29		$ 100	$ 28
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	32	29		100
SOUTHERN Co GAS | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 2
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	28	27		86
SOUTHERN Co GAS | SNG | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
SOUTHERN Co GAS | Triton
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		0		5
SOUTHERN Co GAS | Triton | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	1		4
SOUTHERN Co GAS | PennEast Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	1		3
SOUTHERN Co GAS | Atlantic Coast Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1
SOUTHERN Co GAS | Horizon Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 1	$ 0		$ 2
SOUTHERN Co GAS | Horizon Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 0

Joint Ownership Agreements - Na

Joint Ownership Agreements - Narrative (Details) - SNG - SOUTHERN Co GAS - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%
Capital contributions from parent company	$ 50

Joint Ownership Agreements - 67

Joint Ownership Agreements - Investment in SNG (Details) - SNG - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Revenues	$ 82	$ 146	$ 445
Operating income	60	71	218
Net income	$ 55	$ 57	$ 172

Segment and Related Informati68

Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017USD ($)statesegment	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)statesegment	Sep. 30, 2016USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates | state	4		4
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates | state	7		7
Number of natural gas distribution utilities | segment	7		7
Number of reportable segments | segment			4
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Wholesale revenues, affiliates | $	$ 105	$ 110	$ 295	$ 313

Segment and Related Informati69

Segment and Related Information - Financial Data for Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 6,201	$ 6,264		$ 17,403	$ 14,715
Segment net income	1,069	1,139		347	2,251
Total assets	110,315			110,315		$ 109,697
Kemper IGCC
Segment Reporting Information [Line Items]
Pre-tax charge to income	34	88		3,200	222
After tax charge to income	21	54		2,200	137
Plant Scherer Unit 3
Segment Reporting Information [Line Items]
After tax charge to income				20
Loss on Plant Scherer Unit 3				33
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(40)	(37)		(119)	(118)
Segment net income	2	(1)		0	(8)
Total assets	(1,532)			(1,532)		(1,624)
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,523	5,619		14,239	13,979
Segment net income	1,132	1,198		276	2,401
Total assets	87,382			87,382		86,994
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	565	543		2,841	543
Segment net income	15	4		303	4
Total assets	22,190			22,190		21,853
All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	153	139		442	311
Segment net income	(80)	(62)		(232)	(146)
Total assets	2,275			2,275		2,474
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	618	500		1,597	1,189
Segment net income	124	176		276	315
Total assets	14,648			14,648		15,169
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	565	543		2,841
Segment net income	15	4		303
Total assets	22,190			22,190		21,853
EBIT		4
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	607	586		3,000
Segment net income	44			325
Total assets	24,052			24,052		24,875
EBIT		26
SOUTHERN Co GAS | Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(44)	(45)		(166)
Segment net income	0			0
Total assets	(13,262)			(13,262)		(14,167)
EBIT		0
SOUTHERN Co GAS | All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2	2		7
Segment net income	(29)			(22)
Total assets	11,400			11,400		11,145
EBIT		(22)
SOUTHERN Co GAS | Gas distribution operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	472	455		2,255
Segment net income	52			223
Total assets	18,711			18,711		19,453
EBIT		27
SOUTHERN Co GAS | Gas marketing services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	143	126		597
Segment net income	1			36
Total assets	2,089			2,089		2,084
EBIT		(4)
SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	(24)	(8)		95
Segment net income	(23)			28
Total assets	893			893		1,127
EBIT		(11)
Less Gross Gas Costs	1,538	1,773		5,048
SOUTHERN Co GAS | Gas Midstream Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	16	13		53
Segment net income	14			38
Total assets	2,359			2,359		2,211
EBIT		14
Predecessor | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues			$ 1,905
Segment net income			131
EBIT			328
Predecessor | SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			2,003
EBIT			388
Predecessor | SOUTHERN Co GAS | Eliminations
Segment Reporting Information [Line Items]
Operating revenues			(102)
EBIT			0
Predecessor | SOUTHERN Co GAS | All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			4
EBIT			(60)
Predecessor | SOUTHERN Co GAS | Gas distribution operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			1,575
EBIT			353
Predecessor | SOUTHERN Co GAS | Gas marketing services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			435
EBIT			109
Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			(32)
EBIT			(68)
Less Gross Gas Costs			2,675
Predecessor | SOUTHERN Co GAS | Gas Midstream Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			25
EBIT			(6)
Third Party Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,411	1,688		4,781
Third Party Gross Revenues | Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			2,500
Intercompany Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	103	77		362
Intercompany Revenues | Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			143
Total Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,514	1,765		5,143
Total Gross Revenues | Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			$ 2,643
Reportable Subsegments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,017	5,236		12,960	13,120
Segment net income	1,008	1,022		0	2,086
Total assets	73,056			73,056		72,141
Reportable Subsegments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	618	500		1,597	1,189
Segment net income	124	176		276	315
Total assets	14,648			14,648		15,169
Intersubsegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(112)	(117)		(318)	(330)
Segment net income	0	$ 0		0	$ 0
Total assets	$ (322)			$ (322)		$ (316)

Segment and Related Informati70

Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 5,523	$ 5,619	$ 14,239	$ 13,979
Gas Revenue	565	543	2,841	543
Retail
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	4,615	4,808	11,786	11,932
Wholesale
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	718	613	1,867	1,455
Other
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	190	198	586	592
Gas distribution operations
Revenue from External Customer [Line Items]
Gas Revenue	430	420	2,119	420
Gas marketing services
Revenue from External Customer [Line Items]
Gas Revenue	143	126	597	126
Other
Revenue from External Customer [Line Items]
Gas Revenue	$ (8)	$ (3)	$ 125	$ (3)

Uncategorized Items - so-201709

Label	Element	Value
Southern Company Gas [Member]
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	$ (8,000,000)
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	(1,000,000)
Payments to Acquire Equity Method Investments	us-gaap_PaymentsToAcquireEquityMethodInvestments	1,421,000,000
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	(30,000,000)
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	300,000,000
Depreciation, Amortization and Accretion, Net	us-gaap_DepreciationAmortizationAndAccretionNet	116,000,000
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	us-gaap_PensionAndOtherPostretirementBenefitExpense	(123,000,000)
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	78,000,000
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	(11,000,000)
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	11,000,000
Interest Paid, Net	us-gaap_InterestPaidNet	86,000,000
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	0
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	36,000,000
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	21,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	0
Proceeds from Equity Method Investment, Distribution, Return of Capital	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	2,000,000
Cash and Cash Equivalents, Period Increase (Decrease)	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease	33,000,000
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	2,090,000,000
Income Taxes Paid, Net	us-gaap_IncomeTaxesPaidNet	54,000,000
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	18,000,000
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	900,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	(342,000,000)
Hedge Settlements	so_HedgeSettlements	35,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	47,000,000
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	222,000,000
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	287,000,000
Change in Construction Payables	so_ChangeInConstructionPayables	9,000,000
Repayments of Long-term Debt	us-gaap_RepaymentsOfLongTermDebt	0
Payments to Noncontrolling Interests	us-gaap_PaymentsToMinorityShareholders	0
Increase (Decrease) in Other Operating Liabilities	us-gaap_IncreaseDecreaseInOtherOperatingLiabilities	(11,000,000)
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	63,000,000
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	472,000,000
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(36,000,000)
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	(3,000,000)
Capital Expenditures Incurred but Not yet Paid	us-gaap_CapitalExpendituresIncurredButNotYetPaid	50,000,000
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	(1,715,000,000)
Southern Company Gas [Member] | Predecessor [Member]
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	10,000,000
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	(151,000,000)
Payments to Acquire Equity Method Investments	us-gaap_PaymentsToAcquireEquityMethodInvestments	14,000,000
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	8,000,000
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	0
Depreciation, Amortization and Accretion, Net	us-gaap_DepreciationAmortizationAndAccretionNet	206,000,000
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	us-gaap_PensionAndOtherPostretirementBenefitExpense	5,000,000
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	43,000,000
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	41,000,000
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	20,000,000
Interest Paid, Net	us-gaap_InterestPaidNet	119,000,000
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	125,000,000
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	(37,000,000)
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	32,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	250,000,000
Proceeds from Equity Method Investment, Distribution, Return of Capital	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	3,000,000
Cash and Cash Equivalents, Period Increase (Decrease)	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease	(4,000,000)
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	(558,000,000)
Income Taxes Paid, Net	us-gaap_IncomeTaxesPaidNet	(100,000,000)
Proceeds from Contributions from Parent	us-gaap_ProceedsFromContributionsFromParent	0
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	(181,000,000)
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	350,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	1,113,000,000
Cash and Cash Equivalents, at Carrying Value	us-gaap_CashAndCashEquivalentsAtCarryingValue	15,000,000
Hedge Settlements	so_HedgeSettlements	26,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	82,000,000
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	(273,000,000)
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	509,000,000
Change in Construction Payables	so_ChangeInConstructionPayables	(7,000,000)
Repayments of Long-term Debt	us-gaap_RepaymentsOfLongTermDebt	0
Payments to Noncontrolling Interests	us-gaap_PaymentsToMinorityShareholders	19,000,000
Increase (Decrease) in Other Operating Liabilities	us-gaap_IncreaseDecreaseInOtherOperatingLiabilities	(30,000,000)
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	128,000,000
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	(896,000,000)
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(21,000,000)
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	0
Capital Expenditures Incurred but Not yet Paid	us-gaap_CapitalExpendituresIncurredButNotYetPaid	41,000,000
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	$ (559,000,000)