Alabama Power (ALP-PQ) 10-Q 7 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2018shares
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, 2018
Amendment Flag	false
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	1,028,888,684
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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	100

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Revenues:
Total operating revenues	$ 6,159	$ 6,201	$ 18,158	$ 17,403
Operating Expenses:
Other operations and maintenance	1,404	1,341	4,217	4,100
Depreciation and amortization	787	767	2,338	2,236
Taxes other than income taxes	319	303	990	941
Estimated loss on plants under construction	1	34	1,105	3,155
Gain on dispositions, net	(353)	0	(317)	(19)
Impairment charges	36	0	197	0
Goodwill impairment			42
Total operating expenses	3,985	4,210	14,545	15,809
Operating Income (Loss)	2,174	1,991	3,613	1,594
Other Income and (Expense):
Allowance for equity funds used during construction	36	18	99	133
Earnings from equity method investments	36	32	108	100
Interest expense, net of amounts capitalized	(458)	(407)	(1,386)	(1,248)
Other income (expense), net	57	65	195	165
Total other income and (expense)	(329)	(292)	(984)	(850)
Earnings (Loss) Before Income Taxes	1,845	1,699	2,629	744
Income taxes	623	590	598	317
Consolidated net income (loss)	1,222	1,109	2,031	427
Dividends on preferred and preference stock of subsidiaries	4	10	12	32
Net income attributable to noncontrolling interests	54	30	71	48
Net Income (Loss)	$ 1,164	$ 1,069	$ 1,948	$ 347
Earnings per share -
Basic (in dollars per share)	$ 1.14	$ 1.07	$ 1.92	$ 0.35
Diluted (in dollars per share)	$ 1.13	$ 1.06	$ 1.91	$ 0.35
Average number of shares of common stock outstanding (in millions)
Basic (in shares)	1,023	1,003	1,016	998
Diluted (in shares)	1,029	1,010	1,021	1,005
Cash dividends paid per share of common stock (in dollars per share)	$ 0.60	$ 0.58	$ 1.78	$ 1.72
Retail electric revenues
Operating Revenues:
Total operating revenues	$ 4,605	$ 4,615	$ 11,913	$ 11,786
Wholesale electric revenues
Operating Revenues:
Total operating revenues	693	718	1,923	1,867
Other electric revenues
Operating Revenues:
Total operating revenues	170	168	509	510
Natural gas
Operating Revenues:
Total operating revenues	492	532	2,806	2,746
Operating Expenses:
Cost of goods and services sold	104	134	1,053	1,085
Alternative revenue programs
Operating Revenues:
Total operating revenues	5	0	(23)	9
Other
Operating Revenues:
Total operating revenues	199	168	1,007	494
Operating Expenses:
Cost of goods and services sold	120	90	688	293
Fuel
Operating Expenses:
Cost of goods and services sold	1,310	1,285	3,514	3,372
Purchased power
Operating Expenses:
Cost of goods and services sold	257	256	760	646
ALABAMA POWER CO
Operating Revenues:
Total operating revenues	1,740	1,740	4,716	4,606
Operating Expenses:
Other operations and maintenance	401	406	1,191	1,177
Depreciation and amortization	192	185	570	549
Taxes other than income taxes	97	93	289	284
Total operating expenses	1,179	1,139	3,403	3,203
Operating Income (Loss)	561	601	1,313	1,403
Other Income and (Expense):
Allowance for equity funds used during construction	16	11	43	27
Interest expense, net of amounts capitalized	(82)	(76)	(240)	(229)
Other income (expense), net	9	10	24	35
Total other income and (expense)	(57)	(55)	(173)	(167)
Earnings (Loss) Before Income Taxes	504	546	1,140	1,236
Income taxes	127	216	272	493
Consolidated net income (loss)	377	330	868	743
Dividends on preferred and preference stock of subsidiaries	4	5	11	14
Net Income (Loss)	373	325	857	729
ALABAMA POWER CO | Retail electric revenues
Operating Revenues:
Total operating revenues	1,584	1,595	4,208	4,155
ALABAMA POWER CO | Wholesale electric revenues
Operating Revenues:
Total operating revenues	74	77	213	210
ALABAMA POWER CO | Wholesale revenues, affiliates
Operating Revenues:
Total operating revenues	14	18	96	83
ALABAMA POWER CO | Other
Operating Revenues:
Total operating revenues	68	50	199	158
ALABAMA POWER CO | Fuel
Operating Expenses:
Cost of goods and services sold	356	343	1,028	944
ALABAMA POWER CO | Purchased power
Operating Expenses:
Cost of goods and services sold	64	57	176	132
ALABAMA POWER CO | Purchased power, affiliates
Operating Expenses:
Cost of goods and services sold	69	55	149	117
GEORGIA POWER CO
Operating Revenues:
Total operating revenues	2,593	2,546	6,601	6,426
Operating Expenses:
Other operations and maintenance	460	430	1,325	1,248
Depreciation and amortization	232	225	690	669
Taxes other than income taxes	118	112	332	311
Estimated loss on plants under construction	0	0	1,060	0
Total operating expenses	1,602	1,529	5,569	4,305
Operating Income (Loss)	991	1,017	1,032	2,121
Other Income and (Expense):
Allowance for equity funds used during construction			50	29
Interest expense, net of amounts capitalized	(95)	(105)	(303)	(310)
Other income (expense), net	30	22	104	95
Total other income and (expense)	(65)	(83)	(199)	(215)
Earnings (Loss) Before Income Taxes	926	934	833	1,906
Income taxes	262	350	212	705
Consolidated net income (loss)	664	584	621	1,201
Dividends on preferred and preference stock of subsidiaries	0	4	0	13
Net Income (Loss)	664	580	621	1,188
GEORGIA POWER CO | Retail electric revenues
Operating Revenues:
Total operating revenues	2,425	2,402	6,112	5,995
GEORGIA POWER CO | Wholesale electric revenues
Operating Revenues:
Total operating revenues	43	45	123	124
GEORGIA POWER CO | Wholesale revenues, affiliates
Operating Revenues:
Total operating revenues	4	6	17	23
GEORGIA POWER CO | Other
Operating Revenues:
Total operating revenues	121	93	349	284
GEORGIA POWER CO | Fuel
Operating Expenses:
Cost of goods and services sold	480	482	1,269	1,297
GEORGIA POWER CO | Purchased power
Operating Expenses:
Cost of goods and services sold	106	119	338	310
GEORGIA POWER CO | Purchased power, affiliates
Operating Expenses:
Cost of goods and services sold	206	161	555	470
GULF POWER CO
Operating Revenues:
Total operating revenues	414	437	1,106	1,144
Operating Expenses:
Other operations and maintenance	82	84	248	260
Depreciation and amortization	48	42	142	95
Taxes other than income taxes	33	33	91	88
Loss on Plant Scherer Unit 3	0	0	0	33
Total operating expenses	339	324	921	915
Operating Income (Loss)	75	113	185	229
Other Income and (Expense):
Interest expense, net of amounts capitalized	(13)	(13)	(39)	(37)
Other income (expense), net	(3)	3	0	7
Total other income and (expense)	(16)	(10)	(39)	(30)
Earnings (Loss) Before Income Taxes	59	103	146	199
Income taxes	(4)	40	(1)	78
Consolidated net income (loss)	63	63	147	121
Dividends on preferred and preference stock of subsidiaries	0	0	0	4
Net Income (Loss)	63	63	147	117
GULF POWER CO | Retail electric revenues
Operating Revenues:
Total operating revenues	341	375	932	972
GULF POWER CO | Wholesale electric revenues
Operating Revenues:
Total operating revenues	15	14	41	44
GULF POWER CO | Wholesale revenues, affiliates
Operating Revenues:
Total operating revenues	40	28	83	75
GULF POWER CO | Other
Operating Revenues:
Total operating revenues	18	20	50	53
GULF POWER CO | Fuel
Operating Expenses:
Cost of goods and services sold	132	127	305	323
GULF POWER CO | Purchased power
Operating Expenses:
Cost of goods and services sold	44	38	135	116
MISSISSIPPI POWER CO
Operating Revenues:
Total operating revenues	358	341	956	915
Operating Expenses:
Other operations and maintenance	80	68	222	213
Depreciation and amortization	42	39	126	120
Taxes other than income taxes	28	25	83	77
Estimated loss on plants under construction	1	34	45	3,155
Total operating expenses	278	292	815	3,886
Operating Income (Loss)	80	49	141	(2,971)
Other Income and (Expense):
Allowance for equity funds used during construction	0	1	0	72
Interest expense, net of amounts capitalized	(19)	13	(59)	(23)
Other income (expense), net	0	1	28	4
Total other income and (expense)	(19)	15	(31)	53
Earnings (Loss) Before Income Taxes	61	64	110	(2,918)
Income taxes	14	24	23	(885)
Consolidated net income (loss)	47	40	87	(2,033)
Dividends on preferred and preference stock of subsidiaries	0	0	1	1
Net Income (Loss)	47	40	86	(2,034)
MISSISSIPPI POWER CO | Retail electric revenues
Operating Revenues:
Total operating revenues	254	243	660	665
MISSISSIPPI POWER CO | Wholesale electric revenues
Operating Revenues:
Total operating revenues	65	72	184	196
MISSISSIPPI POWER CO | Wholesale revenues, affiliates
Operating Revenues:
Total operating revenues	28	21	81	40
MISSISSIPPI POWER CO | Other
Operating Revenues:
Total operating revenues	11	5	31	14
MISSISSIPPI POWER CO | Fuel
Operating Expenses:
Cost of goods and services sold	116	120	312	301
MISSISSIPPI POWER CO | Purchased power
Operating Expenses:
Cost of goods and services sold	11	6	27	20
SOUTHERN POWER CO
Operating Revenues:
Total operating revenues	635	618	1,699	1,597
Operating Expenses:
Other operations and maintenance	94	83	278	272
Depreciation and amortization	130	131	370	379
Taxes other than income taxes	12	13	36	37
Impairment charges	36	0	155	0
Total operating expenses	499	459	1,487	1,261
Operating Income (Loss)	136	159	212	336
Other Income and (Expense):
Interest expense, net of amounts capitalized	(45)	(47)	(138)	(144)
Other income (expense), net	17	3	22	3
Total other income and (expense)	(28)	(44)	(116)	(141)
Earnings (Loss) Before Income Taxes	108	115	96	195
Income taxes	(38)	(39)	(210)	(129)
Consolidated net income (loss)	146	154	306	324
Net income attributable to noncontrolling interests	54	30	71	48
Net Income (Loss)	92	124	235	276
SOUTHERN POWER CO | Wholesale electric revenues
Operating Revenues:
Total operating revenues	496	510	1,363	1,293
SOUTHERN POWER CO | Wholesale revenues, affiliates
Operating Revenues:
Total operating revenues	134	105	326	295
SOUTHERN POWER CO | Other
Operating Revenues:
Total operating revenues	5	3	10	9
SOUTHERN POWER CO | Fuel
Operating Expenses:
Cost of goods and services sold	190	189	511	460
SOUTHERN POWER CO | Purchased power
Operating Expenses:
Cost of goods and services sold	37	43	137	113
SOUTHERN Co GAS
Operating Revenues:
Total operating revenues	492	565	2,861	2,841
Operating Expenses:
Other operations and maintenance	216	206	730	675
Depreciation and amortization	119	125	374	370
Taxes other than income taxes	32	26	157	140
Gain on dispositions, net	(353)	0	(317)	0
Goodwill impairment	0	0	42	0
Total operating expenses	118	498	2,051	2,290
Operating Income (Loss)	374	67	810	551
Other Income and (Expense):
Earnings from equity method investments	34	32	108	100
Interest expense, net of amounts capitalized	(52)	(51)	(170)	(145)
Other income (expense), net	6	19	21	30
Total other income and (expense)	(12)	0	(41)	(15)
Earnings (Loss) Before Income Taxes	362	67	769	536
Income taxes	316	52	475	233
Consolidated net income (loss)	46	15	294	303
Net Income (Loss)	46	15	294	303
SOUTHERN Co GAS | Natural gas
Operating Revenues:
Total operating revenues	487	532	2,829	2,737
Operating Expenses:
Cost of goods and services sold	104	134	1,053	1,085
SOUTHERN Co GAS | Alternative revenue programs
Operating Revenues:
Total operating revenues	5	0	(23)	9
SOUTHERN Co GAS | Other
Operating Revenues:
Total operating revenues	0	33	55	95
Operating Expenses:
Cost of goods and services sold	$ 0	$ 7	$ 12	$ 20

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Income (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Total operating revenues	$ 6,159	$ 6,201	$ 18,158	$ 17,403
Alternative revenue programs
Total operating revenues	5	0	(23)	9
SOUTHERN Co GAS
Total operating revenues	492	565	2,861	2,841
Excise taxes collected	9	9	83	75
SOUTHERN Co GAS | Alternative revenue programs
Total operating revenues	$ 5	$ 0	$ (23)	$ 9

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income (loss)	$ 1,222	$ 1,109	$ 2,031	$ 427
Qualifying hedges:
Changes in fair value, net of tax	(11)	25	(19)	54
Reclassification adjustment for amounts included in net income, net of tax	14	(17)	60	(59)
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, net of tax	8	1	11	3
Total other comprehensive income (loss)	11	9	52	(2)
Comprehensive Income (Loss)	1,233	1,118	2,083	425
Less:
Dividends on preferred and preference stock of subsidiaries	4	10	12	32
Comprehensive income attributable to noncontrolling interests	54	30	71	48
Comprehensive Income (Loss) Attributable to Parent	1,175	1,078	2,000	345
ALABAMA POWER CO
Net income (loss)	377	330	868	743
Qualifying hedges:
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	3
Comprehensive Income (Loss)	378	331	871	746
Less:
Dividends on preferred and preference stock of subsidiaries	4	5	11	14
GEORGIA POWER CO
Net income (loss)	664	584	621	1,201
Qualifying hedges:
Reclassification adjustment for amounts included in net income, net of tax	1	1	3	2
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	1	1	3	2
Comprehensive Income (Loss)	665	585	624	1,203
Less:
Dividends on preferred and preference stock of subsidiaries	0	4	0	13
GULF POWER CO
Net income (loss)	63	63	147	121
Qualifying hedges:
Changes in fair value, net of tax	0	0	0	(1)
Pension and other postretirement benefit plans:
Total other comprehensive income (loss)	0	0	0	(1)
Comprehensive Income (Loss)	63	63	147	120
Less:
Dividends on preferred and preference stock of subsidiaries	0	0	0	4
MISSISSIPPI POWER CO
Net income (loss)	47	40	87	(2,033)
Qualifying hedges:
Changes in fair value, net of tax	0	(1)	(1)	0
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)	0	(1)	0	1
Comprehensive Income (Loss)	47	39	87	(2,032)
Less:
Dividends on preferred and preference stock of subsidiaries	0	0	1	1
SOUTHERN POWER CO
Net income (loss)	146	154	306	324
Qualifying hedges:
Changes in fair value, net of tax	(11)	25	(19)	58
Reclassification adjustment for amounts included in net income, net of tax	11	(20)	46	(68)
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, net of tax	0	0	1	0
Total other comprehensive income (loss)	0	5	28	(10)
Comprehensive Income (Loss)	146	159	334	314
Less:
Comprehensive income attributable to noncontrolling interests	54	30	71	48
Comprehensive Income (Loss) Attributable to Parent	92	129	263	266
SOUTHERN Co GAS
Net income (loss)	46	15	294	303
Qualifying hedges:
Changes in fair value, net of tax	0	0	2	(3)
Reclassification adjustment for amounts included in net income, net of tax	0	0	2	0
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, net of tax	6	0	5	0
Total other comprehensive income (loss)	6	0	9	(3)
Comprehensive Income (Loss)	$ 52	$ 15	$ 303	$ 300

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Qualifying hedges:
Changes in fair value, tax	$ (4)	$ 15	$ (6)	$ 32
Reclassification adjustment for amounts included in net income, tax	5	(10)	21	(36)
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, tax	3	1	4	2
ALABAMA POWER CO
Qualifying hedges:
Reclassification adjustment for amounts included in net income, tax	0	1	1	2
GEORGIA POWER CO
Qualifying hedges:
Reclassification adjustment for amounts included in net income, tax	0	0	1	1
GULF POWER CO
Qualifying hedges:
Changes in fair value, tax	0	0	0	(1)
MISSISSIPPI POWER CO
Qualifying hedges:
Changes in fair value, tax	0	0	(1)	0
Reclassification adjustment for amounts included in net income, tax	0	0	0	0
SOUTHERN POWER CO
Qualifying hedges:
Changes in fair value, tax	(4)	15	(7)	35
Reclassification adjustment for amounts included in net income, tax	4	(12)	16	(42)
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, tax	0	0	0	0
SOUTHERN Co GAS
Qualifying hedges:
Changes in fair value, tax	0	0	1	(2)
Reclassification adjustment for amounts included in net income, tax	0	0	1	0
Pension and other postretirement benefit plans:
Reclassification adjustment for amounts included in net income, tax	$ 2	$ 0	$ 2	$ (1)

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating Activities:
Net income (loss)	$ 2,031	$ 427
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	2,647	2,564
Deferred income taxes	(286)	15
Allowance for equity funds used during construction	(99)	(133)
Pension, postretirement, and other employee benefits	(60)	(64)
Settlement of asset retirement obligations	(160)	(137)
Estimated loss on Plant Vogtle Units 3 and 4	1,105	3,155
Stock based compensation expense	108	95
Estimated loss on plants under construction	1,081	3,148
Gain on dispositions, net	(324)	(22)
Impairment charges	197	0
Gain on dispositions, net	(317)	(19)
Goodwill impairment	42
Other, net	(21)	(80)
Changes in certain current assets and liabilities —
-Receivables	37	423
-Prepayments	14	(39)
-Natural gas for sale	87	0
-Other current assets	(90)	(66)
-Accounts payable	(248)	(467)
-Accrued taxes	839	157
-Accrued compensation	(138)	(230)
-Retail fuel cost over recovery	36	(211)
-Other current liabilities	(67)	(129)
Net cash provided from operating activities	5,584	5,251
Investing Activities:
Business acquisitions, net of cash acquired	(64)	(1,016)
Property additions	(5,793)	(5,242)
Nuclear decommissioning trust fund purchases	(846)	(585)
Nuclear decommissioning trust fund sales	840	580
Asset dispositions	2,773	66
Cost of removal, net of salvage	(252)	(208)
Change in construction payables, net	91	120
Investment in unconsolidated subsidiaries	(93)	(134)
Payments pursuant to LTSAs	(157)	(189)
Other investing activities	1	(77)
Net cash provided from (used for) investing activities	(3,500)	(6,685)
Financing Activities:
Increase (decrease) in notes payable, net	(1,225)	(515)
Proceeds —
Long-term debt	1,950	4,068
Common stock	878	613
Preferred stock	0	250
Short-term borrowings	3,150	1,263
Redemptions and repurchases —
Long-term debt	(4,498)	(1,981)
Preferred and preference stock	0	(150)
Short-term borrowings	(1,800)	(409)
Distributions to noncontrolling interests	(86)	(89)
Capital contributions from noncontrolling interests	1,333	79
Payment of common stock dividends	(1,805)	(1,716)
Other financing activities	(237)	(113)
Net cash provided from (used for) financing activities	(2,340)	1,300
Net Change in Cash, Cash Equivalents, and Restricted Cash	(256)	(134)
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	2,147	1,992
Cash, Cash Equivalents, and Restricted Cash at End of Period	1,891	1,858
Cash paid (received) during the period for —
Interest, net of amounts capitalized	1,402	1,286
Income taxes, net	137	(187)
Noncash transactions — Accrued property additions at end of period	1,125	805
ALABAMA POWER CO
Operating Activities:
Net income (loss)	868	743
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	683	666
Deferred income taxes	104	260
Allowance for equity funds used during construction	(43)	(27)
Settlement of asset retirement obligations	(31)	(20)
Other, net	(6)	59
Changes in certain current assets and liabilities —
-Receivables	(207)	(163)
-Prepayments	(26)	(28)
-Materials and supplies	(69)	(29)
-Other current assets	66	33
-Accounts payable	(194)	(125)
-Accrued taxes	225	159
-Accrued compensation	(41)	(48)
-Retail fuel cost over recovery	0	(76)
-Other current liabilities	60	7
Net cash provided from operating activities	1,389	1,411
Investing Activities:
Property additions	(1,529)	(1,211)
Nuclear decommissioning trust fund purchases	(207)	(174)
Nuclear decommissioning trust fund sales	207	174
Cost of removal, net of salvage	(78)	(82)
Change in construction payables, net	30	105
Other investing activities	(23)	(29)
Net cash provided from (used for) investing activities	(1,600)	(1,217)
Proceeds —
Preferred stock	0	250
Capital contributions from parent company	495	337
Senior notes	500	550
Redemptions and repurchases —
Senior notes	0	(200)
Pollution control revenue bonds	0	(36)
Payment of common stock dividends	(602)	(536)
Other financing activities	(24)	(26)
Net cash provided from (used for) financing activities	369	339
Net Change in Cash, Cash Equivalents, and Restricted Cash	158	533
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	544	420
Cash, Cash Equivalents, and Restricted Cash at End of Period	702	953
Cash paid (received) during the period for —
Interest, net of amounts capitalized	220	217
Income taxes, net	30	146
Noncash transactions — Accrued property additions at end of period	275	189
GEORGIA POWER CO
Operating Activities:
Net income (loss)	621	1,201
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	854	821
Deferred income taxes	(185)	328
Allowance for equity funds used during construction	(50)	(29)
Pension, postretirement, and other employee benefits	(46)	(42)
Settlement of asset retirement obligations	(82)	(95)
Estimated loss on Plant Vogtle Units 3 and 4	1,060	0
Other, net	9	(51)
Changes in certain current assets and liabilities —
-Receivables	(205)	(254)
-Fossil fuel stock	70	(2)
-Prepaid income taxes	231	(5)
-Other current assets	(36)	(24)
-Accounts payable	109	(161)
-Accrued taxes	26	(52)
-Accrued compensation	(32)	(60)
-Retail fuel cost over recovery	0	(84)
-Other current liabilities	(111)	(11)
Net cash provided from operating activities	2,233	1,480
Investing Activities:
Property additions	(2,276)	(1,907)
Nuclear decommissioning trust fund purchases	(638)	(411)
Nuclear decommissioning trust fund sales	633	406
Asset dispositions	138	63
Cost of removal, net of salvage	(71)	(54)
Change in construction payables, net of joint owner portion	72	180
Payments pursuant to LTSAs	(52)	(59)
Other investing activities	(19)	(52)
Net cash provided from (used for) investing activities	(2,213)	(1,834)
Financing Activities:
Increase (decrease) in notes payable, net	102	(391)
Proceeds —
Short-term borrowings	0	700
Capital contributions from parent company	2,335	412
Senior notes	0	1,350
Other long-term debt	0	370
Redemptions and repurchases —
Senior notes	(1,000)	(450)
Pollution control revenue bonds	(469)	(65)
Other long-term debt	(100)	0
Short-term borrowings	(150)	(300)
Payment of common stock dividends	(1,043)	(961)
Premiums on redemption and repurchases of senior notes	(152)	0
Other financing activities	(15)	(48)
Net cash provided from (used for) financing activities	(492)	617
Net Change in Cash, Cash Equivalents, and Restricted Cash	(472)	263
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	852	3
Cash, Cash Equivalents, and Restricted Cash at End of Period	380	266
Cash paid (received) during the period for —
Interest, net of amounts capitalized	315	284
Income taxes, net	141	369
Noncash transactions — Accrued property additions at end of period	670	470
GULF POWER CO
Operating Activities:
Net income (loss)	147	121
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	147	100
Deferred income taxes	(45)	57
Loss on Plant Scherer Unit 3	0	33
Other, net	(10)	(5)
Changes in certain current assets and liabilities —
-Receivables	(5)	(65)
-Other current assets	9	18
-Accrued taxes	35	21
-Accrued compensation	(9)	(10)
-Over recovered regulatory clause revenues	39	(8)
-Other current liabilities	10	10
Net cash provided from operating activities	318	272
Investing Activities:
Property additions	(207)	(142)
Cost of removal, net of salvage	(18)	(16)
Change in construction payables, net	5	(9)
Other investing activities	(18)	(6)
Net cash provided from (used for) investing activities	(238)	(173)
Financing Activities:
Increase (decrease) in notes payable, net	5	(268)
Proceeds —
Common stock	0	175
Capital contributions from parent company	40	7
Senior notes	0	300
Redemptions and repurchases —
Preferred and preference stock	0	(150)
Senior notes	0	(85)
Payment of common stock dividends	(115)	(94)
Other financing activities	(1)	(3)
Net cash provided from (used for) financing activities	(71)	(118)
Net Change in Cash, Cash Equivalents, and Restricted Cash	9	(19)
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	28	56
Cash, Cash Equivalents, and Restricted Cash at End of Period	37	37
Cash paid (received) during the period for —
Interest, net of amounts capitalized	26	24
Income taxes, net	28	19
Noncash transactions — Accrued property additions at end of period	31	25
MISSISSIPPI POWER CO
Operating Activities:
Net income (loss)	87	(2,033)
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	129	144
Deferred income taxes	420	(1,159)
Allowance for equity funds used during construction	0	(72)
Estimated loss on Plant Vogtle Units 3 and 4	45	3,155
Estimated loss on plants under construction	21	3,148
Other, net	5	(26)
Changes in certain current assets and liabilities —
-Receivables	(46)	438
-Fossil fuel stock	(2)	21
-Other current assets	(5)	(9)
-Accounts payable	(3)	(21)
-Accrued taxes	57	20
-Accrued compensation	(9)	(12)
-Over recovered regulatory clause revenues	20	(47)
-Other current liabilities	(18)	(31)
Net cash provided from operating activities	656	361
Investing Activities:
Property additions	(117)	(411)
Change in construction payables, net	(9)	(47)
Payments pursuant to LTSAs	(28)	(10)
Other investing activities	(16)	(15)
Net cash provided from (used for) investing activities	(170)	(483)
Financing Activities:
Increase (decrease) in notes payable, net	(4)	(23)
Proceeds —
Short-term borrowings	300	113
Capital contributions from parent company	(2)	1,002
Senior notes	600	0
Long-term debt to parent company	0	40
Redemptions and repurchases —
Pollution control revenue bonds	(43)	0
Other long-term debt	(900)	(300)
Short-term borrowings	(300)	(109)
Long-term debt to parent company	0	(591)
Other financing activities	(6)	(3)
Net cash provided from (used for) financing activities	(355)	129
Net Change in Cash, Cash Equivalents, and Restricted Cash	131	7
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	248	224
Cash, Cash Equivalents, and Restricted Cash at End of Period	379	231
Cash paid (received) during the period for —
Interest, net of amounts capitalized	57	45
Income taxes, net	(483)	(209)
Noncash transactions — Accrued property additions at end of period	23	32
SOUTHERN POWER CO
Operating Activities:
Net income (loss)	306	324
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	394	404
Deferred income taxes	(337)	240
Amortization of investment tax credits	(43)	(42)
Income taxes receivable, non-current	(12)	(42)
Impairment charges	155	0
Other, net	10	(4)
Changes in certain current assets and liabilities —
-Receivables	(41)	(77)
-Prepaid income taxes	5	24
-Other current assets	1	14
-Accounts payable	(27)	(31)
-Accrued taxes	256	79
-Other current liabilities	(1)	5
Net cash provided from operating activities	666	894
Investing Activities:
Business acquisitions, net of cash acquired	(64)	(1,016)
Property additions	(226)	(218)
Change in construction payables, net	3	(166)
Payments pursuant to LTSAs	(57)	(99)
Other investing activities	20	7
Net cash provided from (used for) investing activities	(324)	(1,492)
Financing Activities:
Increase (decrease) in notes payable, net	(68)	(89)
Proceeds —
Short-term borrowings	200	0
Other long-term debt	0	43
Redemptions and repurchases —
Senior notes	(350)	0
Other long-term debt	(420)	(4)
Return of capital	(650)	0
Distributions to noncontrolling interests	(86)	(89)
Capital contributions from noncontrolling interests	1,333	79
Payment of common stock dividends	(234)	(238)
Other financing activities	(15)	(27)
Net cash provided from (used for) financing activities	(290)	(325)
Net Change in Cash, Cash Equivalents, and Restricted Cash	52	(923)
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	140	1,112
Cash, Cash Equivalents, and Restricted Cash at End of Period	192	189
Cash paid (received) during the period for —
Interest, net of amounts capitalized	138	144
Income taxes, net	(102)	(343)
Noncash transactions — Accrued property additions at end of period	37	16
SOUTHERN Co GAS
Operating Activities:
Net income (loss)	294	303
Adjustments to reconcile net income (loss) to net cash provided from operating activities —
Depreciation and amortization, total	374	370
Deferred income taxes	(83)	265
Mark-to-market adjustments	23	(32)
Gain on dispositions, net	(317)	0
Goodwill impairment	42	0
Other, net	(41)	(46)
Changes in certain current assets and liabilities —
-Receivables	445	531
-Natural gas for sale	87	0
-Prepaid income taxes	(23)	(7)
-Other current assets	21	(42)
-Accounts payable	(59)	(169)
-Accrued taxes	(64)	(24)
-Accrued compensation	2	(11)
-Other current liabilities	35	8
Net cash provided from operating activities	736	1,146
Investing Activities:
Property additions	(1,029)	(1,093)
Asset dispositions	2,631	0
Cost of removal, net of salvage	(67)	(45)
Change in construction payables, net	(14)	49
Investment in unconsolidated subsidiaries	(90)	(128)
Other investing activities	18	28
Net cash provided from (used for) investing activities	1,449	(1,189)
Financing Activities:
Increase (decrease) in notes payable, net	(1,382)	(323)
Proceeds —
First mortgage bonds	100	200
Capital contributions from parent company	35	79
Senior notes	0	450
Redemptions and repurchases —
Long-term debt	(200)	0
Return of capital	(400)	0
Payment of common stock dividends	(351)	(332)
Other financing activities	(3)	(29)
Net cash provided from (used for) financing activities	(2,201)	45
Net Change in Cash, Cash Equivalents, and Restricted Cash	(16)	2
Cash, Cash Equivalents, and Restricted Cash at Beginning of Period	78	24
Cash, Cash Equivalents, and Restricted Cash at End of Period	62	26
Cash paid (received) during the period for —
Interest, net of amounts capitalized	175	146
Income taxes, net	682	17
Noncash transactions — Accrued property additions at end of period	$ 121	$ 112

Condensed Consolidated Statem_6

Condensed Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Interest capitalized	$ 53	$ 72
ALABAMA POWER CO
Interest capitalized	15	10
GEORGIA POWER CO
Interest capitalized	19	17
GULF POWER CO
Interest capitalized	0	0
MISSISSIPPI POWER CO
Interest paid	57	73
Interest capitalized	0	28
SOUTHERN POWER CO
Interest capitalized	14	7
SOUTHERN Co GAS
Interest capitalized	$ 5	$ 9

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 1,847	$ 2,130
Receivables —
Customer accounts receivable	1,730	1,806
Energy marketing receivables	498	607
Unbilled revenues	738	810
Under recovered fuel clause revenues	105	171
Other accounts and notes receivable	690	698
Accumulated provision for uncollectible accounts	(33)	(44)
Materials and supplies	1,418	1,438
Fossil fuel for generation	390	594
Natural gas for sale	486	595
Prepaid expenses	354	452
Other regulatory assets, current	522	604
Assets held for sale, current	407	12
Other current assets	232	199
Total current assets	9,384	10,072
Property, Plant, and Equipment:
In service	100,672	103,542
Less: Accumulated depreciation	30,739	31,457
Plant in service, net of depreciation	69,933	72,085
Nuclear fuel, at amortized cost	844	883
Construction work in progress	7,655	6,904
Total property, plant, and equipment	78,432	79,872
Other Property and Investments:
Goodwill	5,315	6,268
Equity investments in unconsolidated subsidiaries	1,569	1,513
Other intangible assets, net of amortization	674	873
Nuclear decommissioning trusts, at fair value	1,872	1,832
Leveraged leases	794	775
Miscellaneous property and investments	258	249
Total other property and investments	10,482	11,510
Deferred Charges and Other Assets:
Deferred charges related to income taxes	792	825
Unamortized loss on reacquired debt	328	206
Other regulatory assets, deferred	6,196	6,943
Assets held for sale	4,667	0
Other deferred charges and assets	1,436	1,577
Total deferred charges and other assets	13,419	9,551
Total Assets	111,717	111,005
Current Liabilities:
Securities due within one year	3,013	3,892
Notes payable	2,564	2,439
Energy marketing trade payables	521	546
Accounts payable —
Accounts payable	2,246	2,530
Customer deposits	524	542
Accrued taxes —
Accrued taxes	1,060	636
Accrued interest	422	488
Accrued compensation	800	959
Asset retirement obligations, current	348	351
Other regulatory liabilities, current	349	337
Liabilities held for sale, current	355	0
Other current liabilities	763	874
Total current liabilities	12,965	13,594
Long-term Debt	41,425	44,462
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	6,035	6,842
Deferred credits related to income taxes	6,651	7,256
Accumulated deferred ITCs	2,377	2,267
Employee benefit obligations	2,017	2,256
Asset retirement obligations, deferred	5,817	4,473
Accrued environmental remediation	269	389
Other cost of removal obligations	2,330	2,684
Other regulatory liabilities, deferred	153	239
Liabilities held for sale	2,835	0
Other deferred credits and liabilities	454	691
Total deferred credits and other liabilities	28,938	27,097
Total Liabilities	83,328	85,153
Redeemable Preferred Stock of Subsidiaries	324	324
Common Stockholders' Equity:
Common stock	5,140	5,038
Paid-in capital	10,905	10,469
Treasury, at cost	(39)	(36)
Retained earnings (accumulated deficit)	9,048	8,885
Accumulated other comprehensive income (loss)	(177)	(189)
Total Common Stockholders' Equity	24,877	24,167
Noncontrolling interests	3,188	1,361
Total stockholders' equity	28,065	25,528
Total Liabilities and Stockholders' Equity	111,717	111,005
ALABAMA POWER CO
Current Assets:
Cash and cash equivalents	702	544
Receivables —
Customer accounts receivable	455	355
Unbilled revenues	159	162
Under recovered fuel clause revenues	48	0
Affiliated	68	43
Other accounts and notes receivable	54	55
Accumulated provision for uncollectible accounts	(9)	(9)
Materials and supplies	536	458
Fossil fuel for generation	117	184
Prepaid expenses	59	85
Other regulatory assets, current	141	124
Other current assets	8	5
Total current assets	2,338	2,006
Property, Plant, and Equipment:
In service	29,568	27,326
Less: Accumulated depreciation	9,932	9,563
Plant in service, net of depreciation	19,636	17,763
Nuclear fuel, at amortized cost	316	339
Construction work in progress	1,457	908
Total property, plant, and equipment	21,409	19,010
Other Property and Investments:
Equity investments in unconsolidated subsidiaries	63	67
Nuclear decommissioning trusts, at fair value	938	903
Miscellaneous property and investments	127	124
Total other property and investments	1,128	1,094
Deferred Charges and Other Assets:
Deferred charges related to income taxes	236	239
Deferred under recovered regulatory clause revenues	88	54
Other regulatory assets, deferred	1,209	1,272
Other deferred charges and assets	202	189
Total deferred charges and other assets	1,735	1,754
Total Assets	26,610	23,864
Current Liabilities:
Securities due within one year	321	0
Accounts payable —
Affiliated	341	327
Other	425	585
Customer deposits	96	92
Accrued taxes —
Accrued income taxes	97	9
Other accrued taxes	132	45
Accrued interest	81	77
Accrued compensation	169	205
Asset retirement obligations, current	111	7
Other regulatory liabilities, current	57	1
Other current liabilities	46	52
Total current liabilities	1,876	1,400
Long-term Debt	7,803	7,628
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	2,882	2,760
Deferred credits related to income taxes	2,051	2,082
Accumulated deferred ITCs	107	112
Employee benefit obligations	283	304
Asset retirement obligations, deferred	3,090	1,702
Other cost of removal obligations	542	609
Other regulatory liabilities, deferred	52	84
Other deferred credits and liabilities	48	63
Total deferred credits and other liabilities	9,055	7,716
Total Liabilities	18,734	16,744
Redeemable Preferred Stock	291	291
Common Stockholders' Equity:
Common stock	1,222	1,222
Paid-in capital	3,490	2,986
Retained earnings (accumulated deficit)	2,902	2,647
Accumulated other comprehensive income (loss)	(29)	(26)
Total Common Stockholders' Equity	7,585	6,829
Total Liabilities and Stockholders' Equity	26,610	23,864
GEORGIA POWER CO
Current Assets:
Cash and cash equivalents	380	852
Receivables —
Customer accounts receivable	747	544
Unbilled revenues	245	255
Under recovered fuel clause revenues	105	165
Joint owner accounts receivable	208	262
Affiliated	39	24
Other accounts and notes receivable	96	76
Accumulated provision for uncollectible accounts	(3)	(3)
Materials and supplies	494	504
Fossil fuel for generation	244	314
Prepaid expenses	77	216
Other regulatory assets, current	199	205
Other current assets	91	14
Total current assets	2,922	3,428
Property, Plant, and Equipment:
In service	35,671	34,861
Less: Accumulated depreciation	12,029	11,704
Plant in service, net of depreciation	23,642	23,157
Nuclear fuel, at amortized cost	528	544
Construction work in progress	4,655	4,613
Total property, plant, and equipment	28,825	28,314
Other Property and Investments:
Equity investments in unconsolidated subsidiaries	50	53
Nuclear decommissioning trusts, at fair value	933	929
Miscellaneous property and investments	61	59
Total other property and investments	1,044	1,041
Deferred Charges and Other Assets:
Deferred charges related to income taxes	519	516
Other regulatory assets, deferred	3,041	2,932
Other deferred charges and assets	510	548
Total deferred charges and other assets	4,070	3,996
Total Assets	36,861	36,779
Current Liabilities:
Securities due within one year	511	857
Notes payable	102	150
Accounts payable —
Affiliated	515	493
Other	909	834
Customer deposits	275	270
Accrued taxes —
Accrued taxes	345	344
Accrued interest	108	123
Accrued compensation	185	219
Asset retirement obligations, current	193	270
Other regulatory liabilities, current	151	191
Other current liabilities	180	198
Total current liabilities	3,474	3,949
Long-term Debt	9,863	11,073
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	2,999	3,175
Deferred credits related to income taxes	3,218	3,248
Accumulated deferred ITCs	264	248
Employee benefit obligations	650	659
Asset retirement obligations, deferred	2,401	2,368
Other deferred credits and liabilities	141	128
Total deferred credits and other liabilities	9,673	9,826
Total Liabilities	23,010	24,848
Common Stockholders' Equity:
Common stock	398	398
Paid-in capital	9,670	7,328
Retained earnings (accumulated deficit)	3,792	4,215
Accumulated other comprehensive income (loss)	(9)	(10)
Total Common Stockholders' Equity	13,851	11,931
Total Liabilities and Stockholders' Equity	36,861	36,779
GULF POWER CO
Current Assets:
Cash and cash equivalents	37	28
Receivables —
Customer accounts receivable	100	76
Unbilled revenues	69	67
Under recovered fuel clause revenues	0	27
Affiliated	20	14
Other accounts and notes receivable	5	7
Accumulated provision for uncollectible accounts	(1)	(1)
Materials and supplies	61	57
Fossil fuel for generation	58	63
Other regulatory assets, current	47	56
Other current assets	13	21
Total current assets	409	415
Property, Plant, and Equipment:
In service	5,313	5,196
Less: Accumulated depreciation	1,540	1,461
Plant in service, net of depreciation	3,773	3,735
Construction work in progress	152	91
Total property, plant, and equipment	3,925	3,826
Deferred Charges and Other Assets:
Deferred charges related to income taxes	30	31
Other regulatory assets, deferred	495	502
Other deferred charges and assets	46	23
Total deferred charges and other assets	571	556
Total Assets	4,905	4,797
Current Liabilities:
Notes payable	50	45
Accounts payable —
Affiliated	64	52
Other	67	75
Customer deposits	35	35
Accrued taxes —
Accrued taxes	45	10
Accrued interest	20	9
Accrued compensation	30	39
Deferred capacity expense, current	22	22
Asset retirement obligations, current	43	37
Other regulatory liabilities, current	69	0
Other current liabilities	20	27
Total current liabilities	465	351
Long-term Debt	1,285	1,285
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	542	537
Deferred credits related to income taxes	380	458
Employee benefit obligations	96	102
Deferred capacity expense	81	97
Asset retirement obligations, deferred	121	105
Other cost of removal obligations	218	221
Other regulatory liabilities, deferred	51	43
Other deferred credits and liabilities	62	67
Total deferred credits and other liabilities	1,551	1,630
Total Liabilities	3,301	3,266
Common Stockholders' Equity:
Common stock	678	678
Paid-in capital	636	594
Retained earnings (accumulated deficit)	291	259
Accumulated other comprehensive income (loss)	(1)	0
Total Common Stockholders' Equity	1,604	1,531
Total Liabilities and Stockholders' Equity	4,905	4,797
MISSISSIPPI POWER CO
Current Assets:
Cash and cash equivalents	379	248
Receivables —
Customer accounts receivable	49	36
Unbilled revenues	43	41
Income taxes receivable, current	3	4
Affiliated	35	16
Other accounts and notes receivable	47	12
Materials and supplies	52	44
Fossil fuel for generation	19	17
Other regulatory assets, current	110	125
Other current assets	4	9
Total current assets	741	552
Property, Plant, and Equipment:
In service	4,819	4,773
Less: Accumulated depreciation	1,389	1,325
Plant in service, net of depreciation	3,430	3,448
Construction work in progress	106	84
Total property, plant, and equipment	3,536	3,532
Other Property and Investments:
Total other property and investments	24	30
Deferred Charges and Other Assets:
Deferred charges related to income taxes	34	35
Other regulatory assets, deferred	466	437
Accumulated deferred income taxes	0	247
Other deferred charges and assets	16	33
Total deferred charges and other assets	516	752
Total Assets	4,817	4,866
Current Liabilities:
Securities due within one year	204	989
Notes payable	0	4
Accounts payable —
Affiliated	55	59
Other	90	96
Accrued taxes —
Accrued income taxes	75	40
Other accrued taxes	74	101
Accrued interest	21	16
Accrued compensation	30	39
Accrued plant closure costs	30	35
Asset retirement obligations, current	41	37
Other current liabilities	56	47
Total current liabilities	676	1,463
Long-term Debt	1,532	1,097
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	193	0
Deferred credits related to income taxes	420	372
Employee benefit obligations	111	116
Asset retirement obligations, deferred	136	137
Other cost of removal obligations	181	178
Other regulatory liabilities, deferred	75	79
Other deferred credits and liabilities	17	33
Total deferred credits and other liabilities	1,133	915
Total Liabilities	3,341	3,475
Redeemable Preferred Stock	33	33
Common Stockholders' Equity:
Common stock	38	38
Paid-in capital	4,528	4,529
Retained earnings (accumulated deficit)	(3,119)	(3,205)
Accumulated other comprehensive income (loss)	(4)	(4)
Total Common Stockholders' Equity	1,443	1,358
Total Liabilities and Stockholders' Equity	4,817	4,866
SOUTHERN POWER CO
Current Assets:
Cash and cash equivalents	192	129
Receivables —
Customer accounts receivable	150	117
Affiliated	71	50
Other accounts and notes receivable	62	98
Materials and supplies	214	278
Prepaid income taxes	44	50
Assets held for sale, current	18	1
Other current assets	29	35
Total current assets	780	758
Property, Plant, and Equipment:
In service	13,603	13,755
Less: Accumulated depreciation	2,087	1,910
Plant in service, net of depreciation	11,516	11,845
Construction work in progress	586	511
Total property, plant, and equipment	12,102	12,356
Other Property and Investments:
Intangible assets, net of amortization	391	411
Total other property and investments	391	411
Deferred Charges and Other Assets:
Prepaid LTSAs	106	118
Accumulated deferred income taxes	1,281	925
Income taxes receivable, non-current	84	72
Assets held for sale	185	0
Other deferred charges and assets	426	566
Total deferred charges and other assets	2,082	1,681
Total Assets	15,355	15,206
Current Liabilities:
Securities due within one year	0	770
Notes payable	237	105
Accounts payable —
Affiliated	86	102
Other	88	103
Accrued taxes —
Accrued income taxes	233	0
Liabilities held for sale, current	4	0
Other current liabilities	165	152
Total current liabilities	813	1,232
Long-term Debt	5,029	5,071
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	111	199
Accumulated deferred ITCs	1,842	1,884
Other deferred credits and liabilities	259	322
Total deferred credits and other liabilities	2,212	2,405
Total Liabilities	8,054	8,708
Common Stockholders' Equity:
Common stock	0	0
Paid-in capital	2,604	3,662
Retained earnings (accumulated deficit)	1,478	1,478
Accumulated other comprehensive income (loss)	31	(2)
Total Common Stockholders' Equity	4,113	5,138
Noncontrolling interests	3,188	1,360
Total stockholders' equity	7,301	6,498
Total Liabilities and Stockholders' Equity	15,355	15,206
SOUTHERN Co GAS
Current Assets:
Cash and cash equivalents	56	73
Receivables —
Customer accounts receivable	180	400
Energy marketing receivables	498	607
Unbilled revenues	58	285
Affiliated	23	12
Other accounts and notes receivable	110	91
Accumulated provision for uncollectible accounts	(18)	(28)
Natural gas for sale	486	595
Prepaid expenses	62	53
Assets from risk management activities, net of collateral	87	135
Other regulatory assets, current	72	94
Other current assets	88	78
Total current assets	1,702	2,395
Property, Plant, and Equipment:
In service	14,771	15,833
Less: Accumulated depreciation	4,351	4,596
Plant in service, net of depreciation	10,420	11,237
Construction work in progress	660	491
Total property, plant, and equipment	11,080	11,728
Other Property and Investments:
Goodwill	5,015	5,967
Equity investments in unconsolidated subsidiaries	1,529	1,477
Other intangible assets, net of amortization	113	280
Miscellaneous property and investments	20	21
Total other property and investments	6,677	7,745
Deferred Charges and Other Assets:
Other regulatory assets, deferred	721	901
Other deferred charges and assets	218	218
Total deferred charges and other assets	939	1,119
Total Assets	20,398	22,987
Current Liabilities:
Securities due within one year	515	157
Notes payable	136	1,518
Energy marketing trade payables	521	546
Accounts payable —
Affiliated	37	21
Other	346	425
Customer deposits	136	128
Accrued taxes —
Accrued income taxes	0	40
Other accrued taxes	61	78
Accrued interest	66	51
Accrued compensation	71	74
Liabilities from risk management activities, net of collateral	28	69
Other regulatory liabilities, current	132	135
Other current liabilities	122	159
Total current liabilities	2,171	3,401
Long-term Debt	5,393	5,891
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes	944	1,089
Deferred credits related to income taxes	930	1,063
Employee benefit obligations	412	415
Accrued environmental remediation	269	342
Other cost of removal obligations	1,577	1,646
Other deferred credits and liabilities	79	118
Total deferred credits and other liabilities	4,211	4,673
Total Liabilities	11,775	13,965
Common Stockholders' Equity:
Common stock	0	0
Paid-in capital	8,863	9,214
Retained earnings (accumulated deficit)	(273)	(212)
Accumulated other comprehensive income (loss)	33	20
Total Common Stockholders' Equity	8,623	9,022
Total Liabilities and Stockholders' Equity	$ 20,398	$ 22,987

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Other intangible assets, amortization	$ 225	$ 186
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	1,000,000,000
Treasury shares at cost (in shares)	1,000,000	900,000
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	7,392,717
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, amortization	$ 66	$ 47
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, amortization	$ 133	$ 120
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, 2018
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, 2017 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, 2018 and 2017 . 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. 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 Adopted 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. Revenue 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. ASC 606 became effective on January 1, 2018 and the registrants adopted it using the modified retrospective method applied to open contracts and only to the version of the contracts in effect as of January 1, 2018. In accordance with the modified retrospective method, the registrants' previously issued financial statements have not been restated to comply with ASC 606 and the registrants did not have a cumulative-effect adjustment to retained earnings. The adoption of ASC 606 had no significant impact on the timing of revenue recognition compared to previously reported results; however, it requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenue and the related cash flows arising from contracts with customers, which are included in Note (C). ASC 606 provided additional clarity on financial statement presentation that resulted in reclassifications into other revenues and other operations and maintenance from other income/(expense), net at Alabama Power and Georgia Power related to certain unregulated sales of products and services. In addition, contract assets related to certain fixed retail revenues at Georgia Power and Southern Company's unregulated distributed generation business have been reclassified from unbilled revenue in accordance with the guidance in ASC 606. These reclassifications did not affect the timing or amount of revenues recognized or cash flows. ASC 606 also provided additional guidance on revenue recognized over time, resulting in a change in the timing of revenue recognized from guaranteed and fixed billing arrangements at Southern Company Gas. The changes in natural gas revenues recognized in the third quarter and year-to-date 2018 relate primarily to the seasonal nature of natural gas usage. The net impact of accounting for revenue under ASC 606 decreased Southern Company's and Southern Company Gas' consolidated net income by $4 million for the three months ended September 30, 2018 and increased Southern Company's and Southern Company Gas' consolidated net income by $1 million for the nine months ended September 30, 2018 . The specific impacts of applying ASC 606 to revenues from contracts with customers on the financial statements of Southern Company, Alabama Power, Georgia Power, and Southern Company Gas compared to previously recognized guidance is shown below. For the Three Months Ended For the Nine Months Ended Condensed Statements of Income As Reported Balances Without Adoption of Effect of Change As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) (in millions) Southern Company Natural gas revenues $ 492 $ 497 $ (5 ) $ 2,806 $ 2,805 $ 1 Other revenues 199 198 1 1,007 1,003 4 Other operations and maintenance 1,404 1,387 17 4,217 4,178 39 Operating income 2,174 2,195 (21 ) 3,613 3,647 (34 ) Other income (expense), net 57 41 16 195 160 35 Earnings (loss) before income taxes 1,845 1,850 (5 ) 2,629 2,628 1 Income taxes (benefit) 623 624 (1 ) 598 598 — Consolidated net income (loss) 1,222 1,226 (4 ) 2,031 2,030 1 Consolidated net income (loss) attributable to Southern Company 1,164 1,168 (4 ) 1,948 1,947 1 Alabama Power Other revenues $ 68 $ 59 $ 9 $ 199 $ 173 $ 26 Other operations and maintenance 401 390 11 1,191 1,159 32 Operating income 561 563 (2 ) 1,313 1,319 (6 ) Other income (expense), net 9 7 2 24 18 6 Georgia Power Other revenues $ 121 $ 97 $ 24 $ 349 $ 287 $ 62 Other operations and maintenance 460 437 23 1,325 1,268 57 Operating income (loss) 991 990 1 1,032 1,027 5 Other income (expense), net 30 31 (1 ) 104 109 (5 ) Southern Company Gas Natural gas revenues $ 487 $ 492 $ (5 ) $ 2,829 $ 2,828 $ 1 Operating income 374 379 (5 ) 810 809 1 Earnings before income taxes 362 367 (5 ) 769 768 1 Income taxes 316 317 (1 ) 475 475 — Net income (loss) 46 50 (4 ) 294 293 1 For the Nine Months Ended Condensed Statements of Cash Flows As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) Southern Company Consolidated net income $ 2,031 $ 2,030 $ 1 Changes in certain current assets and liabilities: Receivables 37 27 10 Other current assets (90 ) (80 ) (10 ) Other current liabilities (67 ) (68 ) 1 Georgia Power Changes in certain current assets and liabilities: Receivables $ (205 ) $ (242 ) $ 37 Other current assets (36 ) 1 (37 ) Southern Company Gas Net income $ 294 $ 293 $ 1 Changes in certain current assets and liabilities: Other current liabilities 35 34 1 At September 30, 2018 Condensed Balance Sheets As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) Southern Company Unbilled revenues $ 738 $ 776 $ (38 ) Other accounts and notes receivable 690 691 (1 ) Other current assets 232 193 39 Other current liabilities 763 764 (1 ) Retained earnings 9,048 9,047 1 Georgia Power Unbilled revenues $ 245 $ 310 $ (65 ) Other accounts and notes receivable 96 97 (1 ) Other current assets 91 25 66 Southern Company Gas Other current liabilities 122 123 (1 ) Accumulated deficit (273 ) (274 ) 1 Other In 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (ASU 2016-18). ASU 2016-18 eliminates the need to reflect transfers between cash and restricted cash in operating, investing, and financing activities in the statements of cash flows. In addition, the net change in cash and cash equivalents during the period includes amounts generally described as restricted cash or restricted cash equivalents. The registrants adopted ASU 2016-18 effective January 1, 2018 with no material impact on their financial statements. Southern Company, Southern Power, and Southern Company Gas retrospectively applied ASU 2016-18 effective January 1, 2018 and have restated prior periods in the statements of cash flows by immaterial amounts. The change in restricted cash in the statements of cash flows was previously disclosed in operating activities for Southern Company and Southern Company Gas and in investing activities for Southern Company and Southern Power. See " Restricted Cash " herein for additional information. In March 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 statements of income outside of income from operations. Additionally, only the service cost component is eligible for capitalization, when applicable. The registrants adopted ASU 2017-07 effective January 1, 2018 with no material impact on their financial statements. ASU 2017-07 has been applied retrospectively for the presentation of the service cost component and the other components of net periodic benefit costs in the statements of income for Southern Company, the traditional electric operating companies, and Southern Company Gas. Since Southern Power did not participate in the qualified pension and postretirement benefit plans until December 2017, no retrospective presentation of Southern Power's net periodic benefits costs is required. The requirement to limit capitalization to the service cost component of net periodic benefit costs has been applied on a prospective basis from the date of adoption for all registrants. The presentation changes resulted in a decrease in operating income and an increase in other income for the three and nine months ended September 30, 2018 and 2017 for Southern Company, the traditional electric operating companies, and Southern Company Gas. In August 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities (ASU 2017-12). 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, with early adoption permitted. The registrants adopted ASU 2017-12 effective January 1, 2018 with no material impact on their financial statements. See Note (I) for disclosures required by ASU 2017-12. On February 14, 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (ASU 2018-02) to address the application of ASC 740, Income Taxes (ASC 740) to certain provisions of the Tax Reform Legislation. ASU 2018-02 specifically addresses the ASC 740 requirement that the effect of a change in tax laws or rates on deferred tax assets and liabilities be included in income from continuing operations, even when the tax effects were initially recognized directly in OCI at the previous rate, which strands the income tax rate differential in accumulated OCI. The amendments in ASU 2018-02 allow a reclassification from accumulated OCI to retained earnings for stranded tax effects resulting from the Tax Reform Legislation. The registrants adopted ASU 2018-02 effective January 1, 2018 with no material impact on their financial statements. Asset Retirement Obligations See Note 1 to the financial statements of Southern Company and the traditional electric operating companies under "Asset Retirement Obligations and Other Costs of Removal" in Item 8 of the Form 10-K for additional information regarding each company's AROs and the EPA's Disposal of Coal Combustion Residuals from Electric Utilities final rule (CCR Rule). As of September 30, 2018 , details of the AROs, including those related to the CCR Rule, included in the condensed balance sheets of Southern Company, Alabama Power, Georgia Power, Gulf Power, and Mississippi Power were as follows: Southern Company Alabama Power Georgia Power Gulf Mississippi Power (in millions) Balance at December 31, 2017 $ 4,824 $ 1,709 $ 2,638 $ 142 $ 174 Liabilities incurred 2 — — — — Liabilities settled (160 ) (31 ) (82 ) (23 ) (22 ) Accretion 153 72 70 3 4 Cash flow revisions 1,510 1,451 (32 ) 42 21 Reclassification to held for sale (164 ) — — — — Balance at September 30, 2018 $ 6,165 $ 3,201 $ 2,594 $ 164 $ 177 In June 2018, Alabama Power recorded an increase of approximately $1.2 billion to its AROs related to the CCR Rule. Mississippi Power also recorded an increase of approximately $11 million to its AROs related to an ash pond at Plant Greene County, which is jointly-owned with Alabama Power. The revised cost estimates were based on information from feasibility studies performed on ash ponds in use at plants operated by Alabama Power, including Plant Greene County. During the second quarter 2018, Alabama Power's management completed its analysis of these studies which indicated that additional closure costs, primarily related to increases in estimated ash volume, water management requirements, and design revisions, will be required to close these ash ponds under the planned closure-in-place methodology. As the level of work becomes more defined in the next 12 months, it is likely that these cost estimates will change and the change could be material. Georgia Power continues to perform engineering studies related to its plans to close the ash ponds at all of its generating plants, including Plant Scherer Unit 3, which is jointly owned with Gulf Power, in compliance with federal and state CCR rules. Georgia Power also continues to refine its closure strategy and cost estimates for each ash pond and is preparing permit applications as required by the State of Georgia CCR rule. While Georgia Power and Gulf Power believe their recorded liabilities for ash pond closures appropriately reflect their obligations under the current closure strategies they have elected, changes to such strategies and cost estimates would likely result in additional closure costs which would increase their ARO liabilities. It is not currently possible to quantify the impacts of any increase related to a change in closure strategies and/or ongoing engineering studies for the current closure strategies, and the timing of future cash outflows is indeterminable at this time; however, the impact on Georgia Power's and Gulf Power's ARO liabilities is expected to be material. As permit applications advance, engineering studies continue, and the timing of individual ash pond closures develops further during the fourth quarter 2018, Georgia Power and Gulf Power will record any necessary changes to their ARO liabilities. The traditional electric operating companies expect to continue to periodically update their ARO cost estimates, which could increase further, as additional information becomes available. Absent continued recovery of ARO costs through regulated rates, Southern Company's and the traditional electric operating companies' results of operations, cash flows, and financial condition could be materially impacted. The ultimate outcome of this matter cannot be determined at this time. In June 2018, Alabama Power completed an updated decommissioning cost site study for Plant Farley. The estimated cost of decommissioning based on the study resulted in an increase in Southern Company's and Alabama Power's ARO liability of approximately $300 million . See " Nuclear Decommissioning " below for additional information. Georgia Power expects to complete updated decommissioning cost site studies for Plant Hatch and Plant Vogtle Units 1 and 2 in the fourth quarter 2018, which could result in additional changes to Southern Company's and Georgia Power's ARO liability. The ultimate outcome of these studies cannot be determined at this time. The reclassification of a portion of the ARO liability to liabilities held for sale by Southern Company represents the AROs related to Gulf Power. See Note (J) under " Southern Company's Sale of Gulf Power " and " Assets Held for Sale " for additional information. Nuclear Decommissioning See Note 1 to the financial statements of Southern Company and Alabama Power under "Nuclear Decommissioning" in Item 8 of the Form 10-K for additional information. In June 2018, Alabama Power completed an updated decommissioning cost site study for Plant Farley. The estimated costs of decommissioning based on the 2018 site study are as follows: Decommissioning periods: Beginning year 2037 Completion year 2076 (in millions) Site study costs: Radiated structures $ 1,621 Non-radiated structures 99 Total site study costs $ 1,720 The decommissioning cost estimates are based on prompt dismantlement and removal of the plant from service. The actual decommissioning costs may vary from the above estimates because of changes in the assumed date of decommissioning, changes in NRC requirements, or changes in the assumptions used in making these estimates. For ratemaking purposes, Alabama Power's decommissioning costs are based on the site study. Significant assumptions used to determine these costs for ratemaking were an inflation rate of 4.5% and a trust earnings rate of 7.0% . The next site study is expected to be completed in 2023 . Amounts previously contributed to the external trust funds are currently projected to be adequate to meet the updated decommissioning obligations. Alabama Power will continue to provide site specific estimates of the decommissioning costs and related projections of funds in the external trust to the Alabama PSC and, if necessary, would seek the Alabama PSC's approval to address any changes in a manner consistent with the NRC and other applicable requirements. Goodwill and Other Intangible Assets The following table presents year-to-date changes in goodwill balances for Southern Company and Southern Company Gas: Goodwill Southern Company Southern Company Gas Gas Distribution Operations Gas Marketing Services Total (in millions) Balance at December 31, 2017 $ 6,268 $ 4,702 $ 1,265 $ 5,967 Impairment (a) (42 ) — (42 ) (42 ) Dispositions (b) (910 ) (668 ) (242 ) (910 ) Balance at September 30, 2018 $ 5,315 (c) $ 4,034 $ 981 $ 5,015 (a) On April 11, 2018, Southern Company Gas entered into a stock purchase agreement for the sale of Pivotal Home Solutions. In contemplation of the transaction, a goodwill impairment charge of $42 million was recorded in the first quarter 2018. See Note (J) under " Southern Company Gas " for additional information. (b) Gas distribution operations reflects goodwill allocated to Elizabethtown Gas, Elkton Gas, and Florida City Gas, which were sold during the third quarter 2018. Gas marketing services reflects goodwill associated with Pivotal Home Solutions, which was sold on June 4, 2018. See Note (J) under " Southern Company Gas " for additional information. (c) Total does not add due to rounding. 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, 2018 At December 31, 2017 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 (*) $ 223 $ (87 ) $ 136 $ 288 $ (83 ) $ 205 Trade names (*) 70 (18 ) 52 159 (17 ) 142 Storage and transportation contracts 64 (49 ) 15 64 (34 ) 30 PPA fair value adjustments 456 (66 ) 390 456 (47 ) 409 Other 11 (5 ) 6 17 (5 ) 12 Total other intangible assets subject to amortization $ 824 $ (225 ) $ 599 $ 984 $ (186 ) $ 798 Other intangible assets not subject to amortization: Federal Communications Commission licenses 75 — 75 75 — 75 Total other intangible assets $ 899 $ (225 ) $ 674 $ 1,059 $ (186 ) $ 873 Southern Power Other intangible assets subject to amortization: PPA fair value adjustments $ 456 $ (66 ) $ 390 $ 456 $ (47 ) $ 409 Southern Company Gas Other intangible assets subject to amortization: Gas marketing services (*) Customer relationships $ 156 $ (78 ) $ 78 $ 221 $ (77 ) $ 144 Trade names 26 (6 ) 20 115 (9 ) 106 Wholesale gas services Storage and transportation contracts 64 (49 ) 15 64 (34 ) 30 Total other intangible assets subject to amortization $ 246 $ (133 ) $ 113 $ 400 $ (120 ) $ 280 (*) Balances as of September 30, 2018 reflect Southern Company Gas' sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas – Sale of Pivotal Home Solutions " for additional information. Amortization associated with other intangible assets was as follows: Three Months Ended Nine Months Ended September 30, 2018 (in millions) Southern Company $ 21 $ 70 Southern Power $ 6 $ 19 Southern Company Gas $ 12 $ 42 Restricted Cash The registrants adopted ASU 2016-18 as of January 1, 2018. See " Recently Adopted Accounting Standards – Other " herein for additional information. At December 31, 2017 , Southern Power had restricted cash primarily related to certain acquisitions and construction projects. At both September 30, 2018 and December 31, 2017 , Southern Company Gas had restricted cash held as collateral for worker's compensation, life insurance, and long-term disability insurance. The following tables provide a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that total to the amounts shown in the condensed statements of cash flows for the registrants that had restricted cash at September 30, 2018 and/or December 31, 2017 : Southern Company Southern Company Gas (in millions) At September 30, 2018 Cash and cash equivalents $ 1,847 $ 56 Cash and cash equivalents classified as assets held for sale 37 — Restricted cash: Other accounts and notes receivable 6 6 Total cash, cash equivalents, and restricted cash $ 1,891 (*) $ 62 (*) Total does not add due to rounding. Southern Company Southern Power Southern Company Gas (in millions) At December 31, 2017 Cash and cash equivalents $ 2,130 $ 129 $ 73 Restricted cash: Other accounts and notes receivable 5 — 5 Deferred charges and other assets 12 11 — Total cash, cash equivalents, and restricted cash $ 2,147 $ 140 $ 78 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, 2018 . 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. Hypothetical Liquidation at Book Value Southern Power has consolidated renewable generation projects that are partially funded by a third-party tax equity investor. The related contractual provisions represent profit-sharing arrangements because the allocations of cash distributions and tax benefits are not based on fixed ownership percentages. Therefore, the noncontrolling interest is accounted for under a balance sheet approach utilizing the hypothetical liquidation at book value (HLBV) method. The HLBV method calculates each partner's share of income based on the change in net equity the partner can legally claim in a hypothetical liquidation at the end of the period compared to the beginning of the period.

Contingencies and Regulatory Ma

Contingencies and Regulatory Matters	9 Months Ended
	Sep. 30, 2018
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 laws and regulations governing air, water, land, and protection of natural resources. Litigation over environmental issues and claims of various types, including property damage, personal injury, common law nuisance, and citizen enforcement of environmental laws and regulations 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 In January 2017, a putative 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 County energy facility 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. In June 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. In July 2017, the defendants filed a motion to dismiss the plaintiffs' amended complaint with prejudice, to which the plaintiffs filed an opposition in September 2017. On March 29, 2018, the U.S. District Court for the Northern District of Georgia, Atlanta Division, issued an order granting, in part, the defendants' motion to dismiss. The court dismissed certain claims against certain officers of Southern Company and Mississippi Power and dismissed the allegations related to a number of the statements that plaintiffs challenged as being false or misleading. On April 26, 2018, the defendants filed a motion for reconsideration of the court's order, seeking dismissal of the remaining claims in the lawsuit. On August 10, 2018, the court denied the motion for reconsideration and denied a motion to certify the issue for interlocutory appeal. In February 2017, Jean Vineyard filed a shareholder derivative lawsuit and, in May 2017, Judy Mesirov filed a shareholder derivative lawsuit, each 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. In August 2017, these two shareholder derivative lawsuits were consolidated in the U.S. District Court for the Northern District of Georgia. The complaints allege that the defendants caused Southern Company to make false or misleading statements regarding the Kemper County energy facility cost and schedule. Further, the complaints allege that the defendants were unjustly enriched and caused the waste of corporate assets and also allege that the individual defendants violated their fiduciary duties. Each plaintiff seeks to recover, on behalf of Southern Company, unspecified actual damages and, on each plaintiff's own behalf, attorneys' fees and costs in bringing the lawsuit. Each plaintiff also seeks certain changes to Southern Company's corporate governance and internal processes. On April 25, 2018, the court entered an order staying this lawsuit until 30 days after the resolution of any dispositive motions or any settlement, whichever is earlier, in the putative securities class action. In May 2017, Helen E. Piper Survivor's Trust filed a shareholder derivative lawsuit in the Superior Court of Gwinnett County, State 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 individual defendants, among other things, breached their fiduciary duties in connection with schedule delays and cost overruns associated with the construction of the Kemper County energy facility. The complaint further alleges that the individual defendants authorized or failed to correct false and misleading statements regarding the Kemper County energy facility schedule and cost and failed to implement necessary internal controls to prevent harm to Southern Company. The 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. The plaintiff also seeks certain unspecified changes to Southern Company's corporate governance and internal processes. On May 4, 2018, the court entered an order staying this lawsuit until 30 days after the resolution of any dispositive motions or any settlement, whichever is earlier, in the putative securities class action. 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. Alabama Power On March 2, 2018, the Alabama Department of Environmental Management (ADEM) issued proposed administrative orders assessing a penalty of $1.25 million to Alabama Power for unpermitted discharge of fluids and/or pollutants to groundwater at five electric generating plants. The orders were finalized and Alabama Power paid the penalty on September 27, 2018. This matter is now concluded. 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 2016, the Georgia Court of Appeals reversed the trial court's previous dismissal of the case and remanded the case to the trial court. Georgia Power filed a petition for writ of certiorari with the Georgia Supreme Court, which was granted in August 2017. On June 18, 2018, the Georgia Supreme Court affirmed the judgment of the Georgia Court of Appeals and remanded the case to the trial court for further proceedings. On August 27, 2018, Georgia Power filed a motion to stay the case and requested the trial court refer the case to the Georgia PSC for a declaratory ruling. Georgia Power believes the plaintiffs' claims have no merit and will continue to vigorously defend itself in this matter. The amount of any possible losses cannot be calculated at this time because, among other factors, it is unknown whether any class will ultimately be certified; the scope of such a class, if certified; and whether any losses would be subject to recovery from any municipalities. The ultimate outcome of this matter cannot be determined at this time. Mississippi Power In 2016, a complaint against Mississippi Power was filed in Harrison County Circuit Court (Circuit Court) by Biloxi Freezing & Processing Inc., Gulfside Casino Partnership, and John Carlton Dean, which was amended and refiled to include, among other things, Southern Company as a defendant. The individual plaintiff alleged that Mississippi Power and Southern Company violated the Mississippi Unfair Trade Practices Act. All plaintiffs alleged that Mississippi Power and Southern Company concealed, falsely represented, and failed to fully disclose important facts concerning the cost and schedule of the Kemper County energy facility and that these alleged breaches unjustly enriched Mississippi Power and Southern Company. The plaintiffs sought unspecified actual damages and punitive damages; asked the Circuit Court to appoint a receiver to oversee, operate, manage, and otherwise control all affairs relating to the Kemper County energy facility; asked the Circuit Court to revoke any licenses or certificates authorizing Mississippi Power or Southern Company to engage in any business related to the Kemper County energy facility in Mississippi; and sought attorney's fees, costs, and interest. The plaintiffs also sought an injunction to prevent any Kemper County energy facility costs from being charged to customers through electric rates. In June 2017, the Circuit Court ruled in favor of motions by Southern Company and Mississippi Power and dismissed the case. In July 2017, the plaintiffs filed notice of an appeal. On July 13, 2018, Mississippi Power and Southern Company reached a settlement agreement with the plaintiffs and the plaintiffs' appeal was dismissed with prejudice. The settlement had no material impact on Southern Company's or Mississippi Power's financial statements. On May 18, 2018, Southern Company and Mississippi Power received a notice of dispute and arbitration demand filed by Martin Product Sales, LLC (Martin) based on two agreements, both related to Kemper IGCC byproducts for which Mississippi Power provided termination notices in September 2017. Martin alleges breach of contract, breach of good faith and fair dealing, fraud and misrepresentation, and civil conspiracy and makes a claim for damages in the amount of approximately $143 million , as well as additional unspecified damages, attorney's fees, costs, and interest. Southern Company and Mississippi Power believe this legal challenge has no merit; however, an adverse outcome in this proceeding could have a material impact on Southern Company's and Mississippi Power's results of operations, financial condition, and liquidity. Southern Company and Mississippi Power will vigorously defend themselves in this matter, the ultimate outcome of which cannot be determined at this time. On May 14, 2018, Mississippi Power's claim for lost revenue resulting from the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 was settled. The settlement proceeds of $18 million , net of expenses and income tax, are included in Southern Company's and Mississippi Power's earnings for the nine months ended September 30, 2018. Southern Power Southern Power indirectly owns a 51% membership interest in RE Roserock LLC (Roserock), the owner of the Roserock facility in Pecos County, Texas. Prior to the facility being placed in service in November 2016, certain solar panels were damaged during installation by the construction contractor, McCarthy Building Companies, Inc. (McCarthy), and certain solar panels were damaged by a hail event that also occurred during construction. In May 2017, Roserock filed a lawsuit in the state district court in Pecos County, Texas, (State Court lawsuit) against XL Insurance America, Inc. (XL) and North American Elite Insurance Company (North American Elite) seeking recovery from an insurance policy for damages resulting from the hail storm and McCarthy's installation practices. On June 1, 2018, the court in the State Court lawsuit granted Roserock's motion for partial summary judgment, finding that the insurers were in breach of contract and in violation of the Texas Insurance Code for failing to pay any monies owed for the hail claim. In addition to the State Court lawsuit, lawsuits were filed between Roserock and McCarthy, as well as other parties, and that litigation has been consolidated in the U.S. District Court for the Western District of Texas. 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 Energy Services Company, doing business as Pivotal Home Solutions, formerly a wholly-owned subsidiary of Southern Company Gas, was a defendant in a putative class action initially filed in 2017 in the state court in Indiana. The plaintiffs purported to represent a class of the customers who purchased products from Nicor Energy Services Company and alleged that the marketing, sale, and billing of the products violated the Indiana Consumer Fraud and Deceptive Business Practices Act, constituting common law fraud and resulting in unjust enrichment of these entities. In 2018, Nicor Energy Services Company was named in a second class action filed in the state court of Ohio asserting nearly identical allegations and legal claims. The plaintiffs sought, on behalf of the classes they purported to represent, actual and punitive damages, interest costs, attorney fees, and injunctive relief. To facilitate the sale of Pivotal Home Solutions, Southern Company Gas retained most of the financial responsibility for these lawsuits following the completion of the sale. On June 12, 2018, the parties settled these claims and Southern Company Gas recorded an $11 million charge, which is included in other operations and maintenance expenses for the nine months ended September 30, 2018. Environmental Matters Environmental Remediation The Southern Company system must comply with environmental laws and regulations governing 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 and Georgia have all 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 $25 million and $22 million as of September 30, 2018 and December 31, 2017 , 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 $48 million and $52 million as of September 30, 2018 and December 31, 2017 , 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. At September 30, 2018 , Southern Company Gas' environmental remediation liability was $294 million based on the estimated cost of environmental investigation and remediation associated with known current and former manufactured gas plant operating sites. At December 31, 2017 , Southern Company Gas' total environmental remediation liability was $388 million , of which $85 million related to Elizabethtown Gas, which was sold on July 1, 2018. 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 $2 million of the total accrued remediation costs. See Note (J) under " Southern Company Gas " for information regarding Southern Company Gas' sale of Elizabethtown Gas. The ultimate outcome of these matters cannot be determined at this time; however, as a result of the regulatory treatment for environmental remediation expenses described above, 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. FERC Matters Market-Based Rate Authority See Note 3 to the financial statements of Southern Company, the traditional electric operating companies, and Southern Power under "FERC Matters" in Item 8 of the Form 10-K for additional information regarding proceedings related to the traditional electric operating companies' and Southern Power's 2014 and 2017 triennial market power analyses. On May 4, 2018, the FERC issued an order terminating both proceedings, finding that the traditional electric operating companies and Southern Power satisfy the FERC's standards for market-based rates. On May 9, 2018, the traditional electric operating companies and Southern Power made the compliance filing required by the order. These proceedings are concluded. Open Access Transmission Tariff On May 10, 2018, the Alabama Municipal Electric Authority and Cooperative Energy filed with the FERC a complaint against SCS and the traditional electric operating companies claiming that the current 11.25% base ROE used in calculating the annual transmission revenue requirements of the traditional electric operating companies' open access transmission tariff is unjust and unreasonable as measured by the applicable FERC standards. The complaint requests that the base ROE be set no higher than 8.65% and that the FERC order refunds for the difference in revenue requirements that results from applying a just and reasonable ROE established in this proceeding upon determining the current ROE is unjust and unreasonable. On June 18, 2018, SCS and the traditional electric operating companies filed their response challenging the adequacy of the showing presented by the complainants and offering support for the current ROE. On September 6, 2018, the FERC issued an order establishing a refund effective date of May 10, 2018 in the event a refund is due and initiating an investigation and settlement procedures regarding the current base ROE. Through September 30, 2018, the estimated maximum potential refund is not expected to be material to Southern Company's or the traditional electric operating companies' results of operations. The ultimate outcome of this matter cannot be determined at this time. Fuel Cost Recovery 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. Mississippi Power has a wholesale MRA and a Market Based (MB) fuel cost recovery factor. At September 30, 2018 , the amount of over-recovered wholesale MRA fuel costs included in other regulatory liabilities, current on the condensed balance sheet was approximately $7 million compared to an immaterial amount at December 31, 2017. Under-recovered wholesale MB fuel costs included in the balance sheets were immaterial at September 30, 2018 and December 31, 2017. Cooperative Energy Power Supply Agreement See Note 3 to the financial statements of Mississippi Power under "FERC Matters – Cooperative Energy Power Supply Agreement" in Item 8 of the Form 10-K for additional information regarding Cooperative Energy's network integration transmission service agreement (NITSA) with SCS. On March 23, 2018, the FERC accepted the amendment to the NITSA between Cooperative Energy and SCS, effective April 1, 2018. 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 under recovered regulatory clause revenues $ — $ 17 Under recovered regulatory clause revenues 7 — Rate CNP PPA Deferred under recovered regulatory clause revenues 30 12 Retail Energy Cost Recovery Deferred under recovered regulatory clause revenues 58 25 Under recovered regulatory clause revenues 41 — Natural Disaster Reserve Other regulatory liabilities, deferred 24 38 On May 1, 2018, the Alabama PSC approved modifications to Rate RSE and other commitments designed to position Alabama Power to address the growing pressure on its credit quality resulting from the Tax Reform Legislation, without increasing retail rates under Rate RSE in the near term. Alabama Power plans to reduce growth in total debt by increasing equity, with corresponding reductions in debt issuances, thereby de-leveraging its capital structure. Alabama Power's goal is to achieve an equity ratio of approximately 55% by the end of 2025. At September 30, 2018, Alabama Power's equity ratio was approximately 47% . Rate RSE The approved modifications to Rate RSE became effective June 2018 and are applicable for January 2019 billings and thereafter. The modifications include reducing the top of the allowed weighted common equity return (WCER) range from 6.21% to 6.15% and modifications to the refund mechanism applicable to prior year actual results. The modifications to the refund mechanism allow Alabama Power to retain a portion of the revenue that causes the actual WCER for a given year to exceed the allowed range. Generally, if Alabama Power's actual WCER range is between 6.15% and 7.65% , customers will receive 25% of the amount between 6.15% and 6.65% , 40% of the amount between 6.65% and 7.15% , and 75% of the amount between 7.15% and 7.65% . Customers will receive all amounts in excess of an actual WCER of 7.65% . In conjunction with these modifications to Rate RSE, on May 8, 2018, Alabama Power consented to a moratorium on any upward adjustments under Rate RSE for 2019 and 2020. Additionally, Alabama Power will return $50 million to customers through bill credits in 2019. In accordance with an established retail tariff that provides for an interim adjustment to customer billings to recognize the impact of a change in the statutory income tax rate, Alabama Power has returned $151 million through September 30, 2018 and anticipates returning a total of approximately $257 million to retail customers through bill credits by December 31, 2018 as a result of the change in the federal income tax rate under the Tax Reform Legislation. Rate ECR On May 1, 2018, the Alabama PSC approved an increase to Rate ECR from 2.015 cents per KWH to 2.353 cents per KWH effective July 2018 which is expected to result in additional collections of approximately $100 million through December 31, 2018. The approved increase in the Rate ECR factor will have no significant effect on Alabama Power's net income, but will increase operating cash flows related to fuel cost recovery in 2018. Absent any further order from the Alabama PSC, in January 2019, the rate will return to the originally authorized 5.910 cents per KWH. Accounting Order On May 1, 2018, the Alabama PSC approved an accounting order that authorizes Alabama Power to defer the benefits of federal excess deferred income taxes associated with the Tax Reform Legislation for the year ending December 31, 2018 as a regulatory liability and to use up to $30 million of such deferrals to offset under recovered amounts under Rate ECR. Any remaining amounts will be used for the benefit of customers as determined by the Alabama PSC. As of September 30, 2018, Alabama Power had applied the full $30 million to offset the under recovered balance under Rate ECR and expects the total deferrals for the year ending December 31, 2018 to be approximately $50 million . See Note 5 to the financial statements of Southern Company and Alabama Power under "Federal Tax Reform Legislation" and of Alabama Power under "Current and Deferred Income Taxes" in Item 8 of the Form 10-K for additional information. Plant Greene County Alabama Power jointly owns Plant Greene County with an affiliate, Mississippi Power. See Note 4 to the financial statements of Alabama Power in Item 8 of the Form 10-K for additional information regarding the joint ownership agreement. On August 6, 2018, Mississippi Power filed its proposed Reserve Margin Plan (RMP) with the Mississippi PSC, which proposes a four -year acceleration of the retirement of Plant Greene County Units 1 and 2 to the third quarter 2021 and the third quarter 2022, respectively. Mississippi Power's proposed Plant Greene County unit retirements would require the completion of proposed transmission and system reliability improvements, as well as agreement by Alabama Power. Alabama Power will monitor Mississippi Power's proposed RMP and associated regulatory process as well as the proposed transmission and system reliability improvements. Alabama Power will review all the facts and circumstances and will evaluate all its alternatives prior to reaching a final determination on the ongoing operations of Plant Greene County. The ultimate outcome of this matter cannot be determined at this time. Georgia Power 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 certified construction costs 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 "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. 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 regarding Georgia Power's 2013 ARP and the Georgia PSC's 2018 order related to the Tax Reform Legislation. On April 3, 2018, the Georgia PSC approved a settlement agreement between Georgia Power and the staff of the Georgia PSC regarding the retail rate impact of the Tax Reform Legislation (Georgia Power Tax Reform Settlement Agreement). Pursuant to the Georgia Power Tax Reform Settlement Agreement, to reflect the federal income tax rate reduction impact of the Tax Reform Legislation, Georgia Power will refund to customers a total of $330 million through bill credits. Georgia Power issued bill credits of approximately $130 million in October 2018 and will issue bill credits of approximately $95 million in June 2019 and $105 million in February 2020. In addition, Georgia Power is deferring as a regulatory liability (i) the revenue equivalent of the tax expense reduction resulting from legislation lowering the Georgia state income tax rate from 6.00% to 5.75% in 2019 and (ii) the entire benefit of approximately $700 million in federal and state excess accumulated deferred income taxes. At September 30, 2018 , Georgia Power's related regulatory liability balance totaled $655 million . The amortization of these regulatory liabilities is expected to be addressed in Georgia Power's next base rate case, which is scheduled to be filed by July 1, 2019. If there is not a base rate case in 2019, customers will receive $185 million in annual bill credits beginning in 2020, with any additional federal and state income tax savings deferred as a regulatory liability, until Georgia Power's next base rate case. To address the negative cash flow and credit metric impacts of the Tax Reform Legislation, the Georgia PSC also approved an increase in Georgia Power's retail equity ratio to the lower of (i) Georgia Power's actual common equity weight in its capital structure or (ii) 55% , until Georgia Power's next base rate case. At September 30, 2018 , Georgia Power's actual retail common equity ratio (on a 13 -month average basis) was approximately 53% . Benefits from reduced federal income tax rates in excess of the amounts refunded to customers will be retained by Georgia Power to cover the carrying costs of the incremental equity in 2018 and 2019. Fuel Cost Recovery As of September 30, 2018 and December 31, 2017 , Georgia Power's under recovered fuel balance totaled $105 million and $165 million , respectively, and is included as under recovered fuel clause revenues on Southern Company's and Georgia Power's condensed balance sheets. On August 16, 2018, the Georgia PSC approved the deferral of Georgia Power's next fuel case to no later than March 16, 2020, with rates to be effective June 1, 2020. Georgia Power continues to be allowed to adjust its fuel cost recovery rates under an interim fuel rider prior to the next fuel case if the under or over recovered fuel balance exceeds $200 million . 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 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. Georgia Power is accruing $30 million annually through December 31, 2019, as provided in the 2013 ARP, for incremental operations and maintenance costs of damage from major storms to its transmission and distribution facilities. As of September 30, 2018, the total balance in Georgia Power's regulatory asset related to storm damage was $311 million . During October 2018, Hurricane Michael caused significant damage to Georgia Power's transmission and distribution facilities. Georgia Power currently estimates the costs of repairing the damage will total approximately $125 million to $150 million , which will be charged to Georgia Power's storm damage reserve or capitalized . The rate of storm damage cost recovery is expected to be adjusted as part of Georgia Power's next base rate case, which is scheduled to be filed by July 1, 2019. The ultimate outcome of this matter cannot be determined at this time. Gulf Power See Note 3 to the financial statements of Gulf Power under "Retail Regulatory Matters" in Item 8 of the Form 10-K for additional information regarding Gulf Power's rates and charges for service to retail customers. Storm Damage Cost Recovery See Note 1 to the financial statements of Gulf Power under "Property Damage Reserve" in Item 8 of the Form 10-K for information on how Gulf Power maintains a reserve for property damage to cover the cost of damages from major storms to its transmission and distribution lines and the cost of uninsured damages to its generating facilities and other property. On October 10, 2018, Hurricane Michael made landfall on the Gulf Coa

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	REVENUE FROM CONTRACTS WITH CUSTOMERS The registrants generate revenues from a variety of sources, some of which are excluded from the scope of ASC 606, such as leases, derivatives, and certain cost recovery mechanisms. See Note (A) under " Recently Adopted Accounting Standards – Revenue " for additional information on the adoption of ASC 606 for revenue from contracts with customers. The majority of the revenues of the traditional electric operating companies and Southern Company Gas are generated from contracts with retail electric and natural gas distribution customers. Revenues from this integrated service to deliver electricity or gas when and if called upon by the customer is recognized as a single performance obligation satisfied over time and is recognized at a tariff rate as electricity or gas is delivered to the customer during the month. The traditional electric operating companies and Southern Company Gas exclude taxes imposed on the customer and collected on behalf of governmental agencies to be remitted to these agencies from the transaction price in determining the revenue related to contracts with a customer. The traditional electric operating companies and Southern Power also have contracts with multiple performance obligations, such as capacity and energy in a wholesale PPA, where the contract's total transaction price is allocated to each performance obligation based on the standalone selling price. The standalone selling price is primarily determined by the price charged to customers for the specific goods or services transferred with the performance obligations. Generally, the registrants recognize revenue as the performance obligations are satisfied over time as electricity or natural gas is delivered to the customer or as generation capacity is available to the customer. At Southern Company Gas, the performance obligations related to wholesale gas services are satisfied, and revenue is recognized, at a point in time when natural gas is delivered to the customer. The registrants generally have a right to consideration in an amount that corresponds directly with the value to the customer of the entity's performance completed to date and may recognize revenue in the amount to which the entity has a right to invoice and has elected to recognize revenue for its sales of electricity, capacity, and natural gas using the invoice practical expedient. In addition, payment for goods and services rendered is typically due in the subsequent month following satisfaction of the registrants' performance obligation. The following tables disaggregate revenue sources for the three and nine months ended September 30, 2018 : For the Three Months Ended September 30, 2018 For the Nine Months Ended September 30, 2018 (in millions) Southern Company Operating revenues Retail electric revenues (a) Residential $ 2,148 $ 5,266 Commercial 1,527 4,084 Industrial 901 2,471 Other 29 92 Natural gas distribution revenues 433 2,299 Alternative revenue programs (b) 5 (23 ) Total retail electric and gas distribution revenues $ 5,043 $ 14,189 Wholesale energy revenues (c)(d) 516 1,444 Wholesale capacity revenues (d) 177 479 Other natural gas revenues (e) 54 530 Other revenues (f) 369 1,516 Total operating revenues $ 6,159 $ 18,158 (a) Retail electric revenues include $17 million and $54 million of leases for the three and nine months ended September 30, 2018 , respectively, and a (net reduction) or net increase of $(98) million and $4 million for the three and nine months ended September 30, 2018 , respectively, from certain cost recovery mechanisms that are not accounted for as revenue under ASC 606. See Note 3 to the financial statements of Southern Company under "Regulatory Matters" in Item 8 of the Form 10-K for additional information on cost recovery mechanisms. (b) See Note 1 to the financial statements of Southern Company under "Revenues" in Item 8 of the Form 10-K for additional information on alternative revenue programs at the natural gas distribution utilities. Alternative revenue program revenues are presented net of any previously recognized program amounts billed to customers during the same accounting period. (c) Wholesale energy revenues include $63 million and $217 million for the three and nine months ended September 30, 2018 , respectively, of revenues accounted for as derivatives, primarily related to physical energy sales in the wholesale electricity market. See Note (I) for additional information on energy-related derivative contracts. (d) Wholesale energy and wholesale capacity revenues include $130 million and $31 million , respectively, for the three months ended September 30, 2018 and $318 million and $92 million , respectively, for the nine months ended September 30, 2018 of PPA contracts accounted for as leases. (e) Other natural gas revenues related to Southern Company Gas' energy and risk management activities are presented net of the related costs of those activities and include gross third-party revenues of $1.6 billion and $4.8 billion for the three and nine months ended September 30, 2018 , respectively, of which $0.9 billion and $2.7 billion , respectively, relates to contracts that are accounted for as derivatives. See Note (L) under " Southern Company Gas " for additional information on the components of wholesale gas services operating revenues. (f) Other revenues include $92 million and $274 million for the three and nine months ended September 30, 2018 , respectively, of revenues not accounted for under ASC 606. Alabama Power Georgia Power Gulf Power Mississippi Power (in millions) For the Three Months Ended September 30, 2018 Operating revenues Retail revenues (a)(b) Residential $ 721 $ 1,142 $ 200 $ 85 Commercial 464 877 103 82 Industrial 392 385 37 86 Other 7 21 1 1 Total retail electric revenues $ 1,584 $ 2,425 $ 341 $ 254 Wholesale energy revenues (c) 62 33 48 92 Wholesale capacity revenues 26 14 7 1 Other revenues (b)(d) 68 121 18 11 Total operating revenues $ 1,740 $ 2,593 $ 414 $ 358 For the Nine Months Ended September 30, 2018 Operating revenues Retail revenues (a)(b) Residential $ 1,848 $ 2,671 $ 537 $ 209 Commercial 1,238 2,343 291 212 Industrial 1,103 1,036 100 233 Other 19 62 4 6 Total retail electric revenues $ 4,208 $ 6,112 $ 932 $ 660 Wholesale energy revenues (c) 234 99 104 259 Wholesale capacity revenues 75 41 20 6 Other revenues (b)(d) 199 349 50 31 Total operating revenues $ 4,716 $ 6,601 $ 1,106 $ 956 (a) Retail revenues at Alabama Power, Georgia Power, Gulf Power, and Mississippi Power include a net increase or (net reduction) of $(12) million , $(47) million , $(36) million , and $(3) million , respectively, for the three months ended September 30, 2018 and $113 million , $(35) million , $(63) million , and $(11) million , respectively, for the nine months ended September 30, 2018 related to certain cost recovery mechanisms that are not accounted for as revenue under ASC 606. See Note 3 to the financial statements of Alabama Power, Georgia Power, Gulf Power, and Mississippi Power under "Retail Regulatory Matters" in Item 8 of the Form 10-K for additional information on cost recovery mechanisms. (b) Retail revenues and other revenues at Georgia Power include $17 million and $34 million , respectively, for the three months ended September 30, 2018 and $54 million and $100 million , respectively, for the nine months ended September 30, 2018 of revenues accounted for as leases. (c) Wholesale energy revenues at Alabama Power and Georgia Power include $6 million and $8 million , respectively, for the three months ended September 30, 2018 and $14 million and $21 million , respectively, for the nine months ended September 30, 2018 accounted for as derivatives primarily related to physical energy sales in the wholesale electricity market. See Note (I) for additional information on energy-related derivative contracts. (d) Other revenues at Alabama Power, Georgia Power, and Gulf Power include $27 million , $28 million , and $2 million , respectively, for the three months ended September 30, 2018 and $79 million , $80 million , and $5 million , respectively, for the nine months ended September 30, 2018 of revenues not accounted for under ASC 606. For the Three Months Ended September 30, 2018 For the Nine Months Ended September 30, 2018 (in millions) Southern Power PPA capacity revenues (a) $ 168 $ 450 PPA energy revenues (a) 336 892 Non-PPA revenues (b) 126 347 Other revenues 5 10 Total operating revenues $ 635 $ 1,699 (a) PPA capacity revenues and PPA energy revenues include $47 million and $139 million , respectively, for the three months ended September 30, 2018 and $141 million and $342 million , respectively, for the nine months ended September 30, 2018 related to PPAs accounted for as leases. See Note 1 to the financial statements of Southern Power under "Revenues" in Item 8 of the Form 10-K for additional information on capacity revenues accounted for as leases. (b) Non-PPA revenues include $47 million and $176 million for the three and nine months ended September 30, 2018 , respectively, of revenues from short-term sales related to physical energy sales from uncovered capacity in the wholesale electricity market. See Note 1 to the financial statements of Southern Power under "Revenues" in Item 8 of the Form 10-K and Note (I) for additional information on energy-related derivative contracts. For the Three Months Ended September 30, 2018 For the Nine Months Ended September 30, 2018 (in millions) Southern Company Gas Operating revenues Natural gas distribution revenues Residential $ 149 $ 1,082 Commercial 45 313 Transportation 203 708 Industrial 4 28 Other 32 168 Alternative revenue programs (a) 5 (23 ) Total natural gas distribution revenues $ 438 $ 2,276 Gas marketing services (b) 44 403 Wholesale gas services (c) (10 ) 121 Gas midstream operations 20 60 Other revenues — 1 Total operating revenues $ 492 $ 2,861 (a) See Note 1 to the financial statements of Southern Company Gas under "Revenues" in Item 8 of the Form 10-K for additional information on alternative revenue programs at the natural gas distribution utilities. Alternative revenue program revenues are presented net of any previously recognized program amounts billed to customers during the same accounting period. (b) Gas marketing services includes $4 million for the nine months ended September 30, 2018 of revenues not accounted for under ASC 606. (c) Wholesale gas services revenues are presented net of the related costs associated with its energy trading and risk management activities. Operating revenues, as presented, include gross third-party revenues of $1.6 billion and $4.8 billion for the three and nine months ended September 30, 2018 , respectively, of which $0.9 billion and $2.7 billion , respectively, relates to contracts that are accounted for as derivatives. See Note (L) under " Southern Company Gas " for additional information on the components of wholesale gas services operating revenues and Note (I) for additional information on energy-related derivative contracts. Contract Balances The following table reflects the closing balances of receivables, contract assets, and contract liabilities related to revenues from contracts with customers as of September 30, 2018 : Receivables Contract Assets Contract Liabilities (in millions) Southern Company $ 2,778 $ 99 $ 34 Alabama Power 649 1 14 Georgia Power 924 70 3 Gulf Power 186 — — Mississippi Power 96 — — Southern Power 142 — 17 Southern Company Gas 523 — 1 As of September 30, 2018 , Alabama Power had contract liabilities for outstanding performance obligations primarily related to extended service agreements. Georgia Power had contract assets primarily related to fixed retail customer bill programs where the payment is contingent upon Georgia Power's continued performance and the customer's continued participation in the program over the one-year contract term. Southern Power's contract liabilities relate to collections recognized in advance of revenue for certain levelized PPAs with Georgia Power. Southern Company's unregulated distributed generation business had $27 million and $17 million of contract assets and contract liabilities, respectively, remaining for outstanding performance obligations. Remaining Performance Obligations The traditional electric operating companies and Southern Power have long-term contracts with customers in which revenues are recognized as performance obligations are satisfied over the contract term. These contracts primarily relate to PPAs whereby the traditional electric operating companies and Southern Power provide electricity and generation capacity to a customer. The revenue recognized for the delivery of electricity is variable; however, certain PPAs include a fixed payment for fixed generation capacity over the term of the contract. Southern Company's unregulated distributed generation business also has partially satisfied performance obligations related to certain fixed price contracts. Revenues from contracts with customers related to these performance obligations remaining at September 30, 2018 are expected to be recognized as follows: 2018 2019 2020 2021 2022 2023 and Thereafter (in millions) Southern Company (*) $ 168 $ 406 $ 322 $ 322 $ 310 $ 2,112 Alabama Power 6 22 22 26 23 161 Georgia Power 10 41 38 40 30 113 Gulf Power 5 22 — — — — Mississippi Power 1 3 3 1 — — Southern Power (*) 75 310 283 277 276 2,005 (*) Excludes amounts related to held for sale assets. See Note (J) under " Southern Company's Sale of Gulf Power " and " Southern Power – Sale of Florida Plants " for additional information.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS As of September 30, 2018 , 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, 2018: 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) $ 271 $ 150 $ — $ — $ 421 Foreign currency derivatives — 122 — — 122 Nuclear decommissioning trusts (c) 828 1,007 — 37 1,872 Non-qualified deferred compensation trusts: Domestic equity — 11 — — 11 Foreign equity — 6 — — 6 Pooled funds – fixed income — 13 — — 13 Cash equivalents 15 — — — 15 Other 9 — — — 9 Cash equivalents 1,309 — — — 1,309 Total $ 2,432 $ 1,309 $ — $ 37 $ 3,778 Liabilities: Energy-related derivatives (a)(b) $ 416 $ 159 $ — $ — $ 575 Interest rate derivatives — 72 — — 72 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 22 — 22 Total $ 416 $ 254 $ 22 $ — $ 692 Fair Value Measurements Using: As of September 30, 2018: 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) Alabama Power Assets: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Nuclear decommissioning trusts: (d) Domestic equity 469 89 — — 558 Foreign equity 60 56 — — 116 U.S. Treasury and government agency securities — 18 — — 18 Corporate bonds 26 154 — — 180 Mortgage and asset backed securities — 22 — — 22 Private equity — — — 37 37 Other 7 — — — 7 Cash equivalents 513 — — — 513 Total $ 1,075 $ 346 $ — $ 37 $ 1,458 Liabilities: Energy-related derivatives $ — $ 10 $ — $ — $ 10 Georgia Power Assets: Energy-related derivatives $ — $ 8 $ — $ — $ 8 Nuclear decommissioning trusts: (d)(e) Domestic equity 250 1 — — 251 Foreign equity — 134 — — 134 U.S. Treasury and government agency securities — 236 — — 236 Municipal bonds — 82 — — 82 Corporate bonds — 163 — — 163 Mortgage and asset backed securities — 42 — — 42 Other 16 9 — — 25 Cash equivalents 350 — — — 350 Total $ 616 $ 675 $ — $ — $ 1,291 Liabilities: Energy-related derivatives $ — $ 22 $ — $ — $ 22 Interest rate derivatives — 6 — — 6 Total $ — $ 28 $ — $ — $ 28 Gulf Power Assets: Cash equivalents $ 27 $ — $ — $ — $ 27 Liabilities: Energy-related derivatives $ — $ 8 $ — $ — $ 8 Fair Value Measurements Using: As of September 30, 2018: 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) Mississippi Power Assets: Energy-related derivatives $ — $ 3 $ — $ — $ 3 Cash equivalents 346 — — — 346 Total $ 346 $ 3 $ — $ — $ 349 Liabilities: Energy-related derivatives $ — $ 9 $ — $ — $ 9 Southern Power Assets: Energy-related derivatives $ — $ 3 $ — $ — $ 3 Foreign currency derivatives — 122 — — 122 Total $ — $ 125 $ — $ — $ 125 Liabilities: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 22 — 22 Total $ — $ 30 $ 22 $ — $ 52 Southern Company Gas Assets: Energy-related derivatives (a)(b) $ 271 $ 129 $ — $ — $ 400 Non-qualified deferred compensation trusts: Domestic equity — 11 — — 11 Foreign equity — 6 — — 6 Pooled funds – fixed income — 13 — — 13 Cash equivalents 4 — — — 4 Cash equivalents 26 — — — 26 Total $ 301 $ 159 $ — $ — $ 460 Liabilities: Energy-related derivatives (a)(b) $ 416 $ 101 $ — $ — $ 517 (a) Excludes $5 million associated with premiums and certain weather derivatives accounted for based on intrinsic value rather than fair value. (b) Excludes cash collateral of $189 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, 2018 , approximately $37 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, including reinvested interest and dividends and excluding the funds' expenses, increased for the three and nine months ended September 30, 2018 and 2017 by the amounts shown in the table below. The increases were recorded as a change to the regulatory assets and liabilities related to AROs for Georgia Power and Alabama Power, respectively. Three Months Ended September 30, 2018 Three Months Ended September 30, 2017 Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 (in millions) Southern Company $ 58 $ 50 $ 68 $ 168 Alabama Power 39 25 49 87 Georgia Power 19 25 19 81 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 (I) for additional information on how these derivatives are used. For fair value measurements of the investments within the nuclear decommissioning trusts and the non-qualified deferred compensation 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. The NRC requires licensees of commissioned nuclear power reactors to establish a plan for providing reasonable assurance of funds for future decommissioning. 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 primarily obligated to make generation-based payments to the seller, which commenced at the commercial operation date of the respective facility and continue through 2026. 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. 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, 2018 , the fair value measurements of private equity investments held in the nuclear decommissioning trusts 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, 2018: Fair Value Unfunded Commitments (in millions) Southern Company $ 37 $ 47 Alabama Power $ 37 $ 47 Private equity funds include funds-of-funds that invest 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, 2018 , 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 $ 45,524 $ 45,500 Alabama Power 8,120 8,321 Georgia Power 10,227 10,159 Gulf Power 1,285 1,290 Mississippi Power 1,736 1,702 Southern Power 5,029 5,058 Southern Company Gas 5,908 5,935 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, 2018
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 stock-based compensation plans. See Note 8 to the financial statements of Southern Company in Item 8 of the Form 10-K for information on stock-based compensation plans. The effect of stock-based compensation plans was determined using the treasury stock method. Shares used to compute diluted earnings per share were as follows: Three Months Ended Three Months Ended Nine Months Ended Nine Months Ended (in millions) As reported shares 1,023 1,003 1,016 998 Effect of stock-based compensation 6 7 5 7 Diluted shares 1,029 1,010 1,021 1,005 Stock-based compensation awards 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, 2018 and 2017 . 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 (a) (in thousands) (in millions) Balance at December 31, 2017 1,008,532 (929 ) $ 24,167 $ — $ 1,361 $ 25,528 Consolidated net income attributable to Southern Company — — 1,948 — — 1,948 Other comprehensive income — — 52 — — 52 Stock issued 21,342 — 878 — — 878 Stock-based compensation — — 74 — — 74 Cash dividends on common stock — — (1,805 ) — — (1,805 ) Contributions from noncontrolling interests — — — — 154 154 Distributions to noncontrolling interests — — — — (87 ) (87 ) Net income attributable to noncontrolling interests — — — — 71 71 Sale of noncontrolling interests (b) — — (410 ) — 1,690 1,280 Other — (57 ) (27 ) — (1 ) (28 ) Balance at September 30, 2018 1,029,874 (986 ) $ 24,877 $ — $ 3,188 $ 28,065 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 (a) Primarily related to Southern Power and excludes redeemable noncontrolling interests. See Note 10 to the financial statements of Southern Power in Item 8 of the Form 10-K for additional information. (b) See Note (J) under "Southern Power – Sale of Solar Facility Interests " for additional information.

Financing

Financing	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
FINANCING	FINANCING Bank Credit Arrangements Bank credit arrangements provide liquidity support to the registrants' commercial paper borrowings and the traditional electric operating companies' revenue bonds. The amount of variable rate revenue bonds of the traditional electric operating companies outstanding requiring liquidity support as of September 30, 2018 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 addition, at September 30, 2018 , the traditional electric operating companies had approximately $573 million (comprised of approximately $120 million at Alabama Power, $345 million at Georgia Power, $58 million at Gulf Power, and $50 million at Mississippi Power) of revenue bonds outstanding that were required to be remarketed within the next 12 months. Subsequent to September 30, 2018, Alabama Power purchased and held its approximately $120 million of outstanding pollution control revenue bonds required to be remarketed. 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, 2018 : Expires Executable Term Loans Expires Within One Year Company 2018 2019 2020 2022 Total Unused One Year Term Out No Term Out (in millions) Southern Company (a) $ — $ — $ — $ 2,000 $ 2,000 $ 1,999 $ — $ — $ — Alabama Power — 33 500 800 1,333 1,333 — — 33 Georgia Power — — — 1,750 1,750 1,736 — — — Gulf Power 20 25 235 — 280 280 45 45 — Mississippi Power — 100 — — 100 100 — — — Southern Power Company (b) — — — 750 750 728 — — — Southern Company Gas (c) — — — 1,900 1,900 1,895 — — — Other — 30 — — 30 30 — — 30 Southern Company Consolidated $ 20 $ 188 $ 735 $ 7,200 $ 8,143 $ 8,101 $ 45 $ 45 $ 63 (a) Represents the Southern Company parent entity. (b) Does not include Southern Power Company's $120 million continuing letter of credit facility for standby letters of credit expiring in 2019, of which $22 million remains unused at September 30, 2018 . (c) Southern Company Gas, as the parent entity, guarantees the obligations of Southern Company Gas Capital, which is the borrower of $1.4 billion of these arrangements. Southern Company Gas' committed credit arrangements also include $500 million for which Nicor Gas is the borrower and which is restricted for working capital needs of Nicor Gas. 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. DOE Loan Guarantee Borrowings See Note 6 to the financial statements of Southern Company and Georgia Power under "DOE Loan Guarantee Borrowings" in Item 8 of the Form 10-K for additional information regarding Georgia Power's Loan Guarantee Agreement. 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 Vogtle 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 certain conditions are satisfied, including (i) receipt of the DOE's approval of the Bechtel Agreement (together with the Vogtle Services Agreement and the IP Licenses, the Replacement EPC Arrangements) and (ii) Georgia Power's entry into a further amendment to the Loan Guarantee Agreement with the DOE to reflect the Replacement EPC Arrangements. In September 2017, the DOE issued a conditional commitment to Georgia Power for up to approximately $1.67 billion in additional guaranteed loans under the Loan Guarantee Agreement. In September 2018, the DOE extended the conditional commitment to March 31, 2019 . Any further extension must be approved by the DOE. 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. As of September 30, 2018 , Georgia Power had $2.6 billion of borrowings outstanding under the multi-advance term loan facility (FFB Credit Facility) among Georgia Power, the DOE, and the FFB. 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 (including any decision not to continue construction of Plant Vogtle Units 3 and 4) 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 Vogtle Services Agreement or rejection of the Vogtle 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) cancellation of Plant Vogtle Units 3 and 4 by the Georgia PSC, or by Georgia Power if authorized by the Georgia PSC; and (iv) 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, if Georgia Power discontinues construction of Plant Vogtle Units 3 and 4, Georgia Power would be obligated to immediately repay a portion of the outstanding borrowings under the FFB Credit Facility to the extent such outstanding borrowings exceed 70% of Eligible Project Costs, net of the proceeds received by Georgia Power under the Guarantee Settlement 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. Financing Activities The following table outlines the long-term debt financing activities for Southern Company and its subsidiaries for the first nine months of 2018 : Company Senior Note Issuances Senior Note Maturities, Redemptions, and Repurchases Revenue Bond Other Long-Term Debt Issuances Other Long-Term Debt Redemptions and Maturities (a) (in millions) Southern Company (b) $ 750 $ 1,000 $ — $ — $ — Alabama Power 500 — — — — Georgia Power — 1,000 469 — 107 Mississippi Power 600 — 43 — 900 Southern Power — 350 — — 420 Southern Company Gas — — 200 100 — Other — — — — 10 Elimination (c) — — — — (1 ) Southern Company Consolidated $ 1,850 $ 2,350 $ 712 $ 100 $ 1,436 (a) Includes reductions in capital lease obligations resulting from cash payments under capital leases. (b) Represents the Southern Company parent entity. (c) Represents reductions in affiliate capital lease obligations at Georgia Power, which are eliminated in Southern Company's Consolidated Financial Statements. Except as otherwise described herein, Southern Company and its subsidiaries used the proceeds of debt issuances for their redemptions and maturities shown in the table above, to repay short-term indebtedness, and for general corporate purposes, including working capital. The subsidiaries also used the proceeds for their construction programs. Southern Company In March 2018, Southern Company entered into a $900 million short-term floating rate bank loan bearing interest based on one -month LIBOR, which was repaid in August 2018. In April 2018, Southern Company borrowed $250 million pursuant to a short-term uncommitted bank credit arrangement, bearing interest at a rate agreed upon by Southern Company and the bank from time to time and payable on no less than 30 days' demand by the bank. In June 2018, Southern Company repaid at maturity two $100 million short-term floating rate bank term loans. In August 2018, Southern Company issued $750 million aggregate principal amount of Series 2018A Floating Rate Senior Notes due February 14, 2020 bearing interest based on three-month LIBOR, entered into a $1.5 billion short-term floating rate bank loan bearing interest based on one -month LIBOR, and repaid $250 million borrowed in August 2017 pursuant to a short-term uncommitted bank credit arrangement. Alabama Power In June 2018, Alabama Power issued $500 million aggregate principal amount of Series 2018A 4.30% Senior Notes due July 15, 2048. Georgia Power In January 2018, Georgia Power repaid its outstanding $150 million and $100 million floating rate bank loans due May 31, 2018 and October 26, 2018, respectively. In April 2018, Georgia Power redeemed all $250 million aggregate principal amount of its Series 2008B 5.40% Senior Notes due June 1, 2018. In May 2018, through cash tender offers, Georgia Power repurchased and retired $89 million of the $250 million aggregate principal amount outstanding of its Series 2007A 5.65% Senior Notes due March 1, 2037, $326 million of the $500 million aggregate principal amount outstanding of its Series 2009A 5.95% Senior Notes due February 1, 2039, and $335 million of the $600 million aggregate principal amount outstanding of its Series 2010B 5.40% Senior Notes due June 1, 2040, for an aggregate purchase price, excluding accrued and unpaid interest, of $902 million . During 2018, Georgia Power purchased and held the following pollution control revenue bonds, which may be reoffered to the public at a later date: • $104.6 million aggregate principal amount of Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), First Series 2013 • $173 million aggregate principal amount of Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 2009 • $55 million aggregate principal amount of Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fifth Series 1994 • $65 million aggregate principal amount of Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Second Series 2008 • $71.735 million aggregate principal amount of Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 2013 Mississippi Power In March 2018, Mississippi Power issued $300 million aggregate principal amount of Series 2018A Floating Rate Senior Notes due March 27, 2020 bearing interest based on three -month LIBOR and $300 million aggregate principal amount of Series 2018B 3.95% Senior Notes due March 30, 2028. In March 2018, Mississippi Power also entered into a $300 million short-term floating rate bank loan bearing interest based on one -month LIBOR, of which $200 million was repaid in the second quarter 2018 and $100 million was repaid in the third quarter 2018. Mississippi Power used the proceeds from these financings to repay a $900 million unsecured term loan. In July 2018, Mississippi Power purchased and held approximately $43 million aggregate principal amount of Mississippi Business Finance Corporation Pollution Control Revenue Refunding Bonds, Series 2002. Mississippi Power may reoffer these bonds to the public at a later date. Subsequent to September 30, 2018, Mississippi Power completed the redemption of all 8,867 outstanding shares ( $886,700 aggregate par value) of its 4.40% Series Preferred Stock, all 8,643 outstanding shares ( $864,300 aggregate par value) of its 4.60% Series Preferred Stock, all 16,700 outstanding shares ( $1.67 million aggregate par value) of its 4.72% Series Preferred Stock, all 1,200,000 outstanding depositary shares ( $30 million aggregate stated value) each representing a 1 / 4 th interest in a share of its 5.25% Series Preferred Stock, all $30 million aggregate principal amount outstanding of its Series G 5.40% Senior Notes due July 1, 2035, and all $125 million aggregate principal amount outstanding of its Series 2009A 5.55% Senior Notes due March 1, 2019. Southern Power In May 2018, Southern Power entered into two short-term floating rate bank loans, each for an aggregate principal amount of $100 million , which bear interest based on one -month LIBOR. In the second quarter 2018, Southern Power repaid $420 million aggregate principal amount of long-term floating rate bank loans and $350 million aggregate principal amount of Series 2015A 1.50% Senior Notes due June 1, 2018. During the nine months ended September 30, 2018 , Southern Power received approximately $148 million of third-party tax equity related to the Gaskell West 1 and Cactus Flats facilities. See Note (J) under "Southern Power" for additional information. Southern Company Gas On January 4, 2018, Southern Company Gas issued a floating rate promissory note to Southern Company in an aggregate principal amount of $100 million bearing interest based on one -month LIBOR. On March 28, 2018, Southern Company Gas repaid this promissory note. Prior to its sale, in the second quarter 2018, Pivotal Utility Holdings caused $200 million aggregate principal amount of gas facility revenue bonds to be redeemed. In May 2018, Southern Company Gas Capital borrowed $95 million pursuant to a short-term uncommitted bank credit arrangement, guaranteed by Southern Company Gas, bearing interest at a rate agreed upon by Southern Company Gas Capital and the bank from time to time and payable on no less than 30 days' demand by the bank. The proceeds of the loan were used to repay short-term debt. In July 2018, Southern Company Gas Capital repaid this loan. In July 2018, Nicor Gas agreed to issue $300 million aggregate principal amount of first mortgage bonds in a private placement, $100 million of which was issued in August 2018 and $200 million of which was issued in November 2018.

Retirement Benefits

Retirement Benefits	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS On January 1, 2018, the qualified defined benefit pension plan of Southern Company Gas was merged into the qualified defined benefit pension plan of Southern Company. Following the plan merger, Southern Company has a qualified defined benefit, trusteed, pension plan covering substantially all employees, with the exception of employees at PowerSecure. The Southern Company qualified defined benefit 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 defined benefit pension plan are anticipated for the year ending December 31, 2018 . In addition, the Southern Company Gas non-qualified retirement plans were merged into the Southern Company non-qualified retirement plan (defined benefit and defined contribution). Following the non-qualified retirement plan mergers, Southern Company continues to provide certain non-qualified defined benefits for a select group of management and highly compensated employees, which are funded on a cash basis. Furthermore, 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. Southern Company Gas also provides certain medical care and life insurance benefits for eligible retired employees through a postretirement benefit plan. Southern Company Gas has a separate unfunded supplemental retirement health care plan that provides medical care and life insurance benefits to employees of discontinued businesses. As indicated in Note (A), the registrants adopted ASU 2017-07 as of January 1, 2018. ASU 2017-07 requires that an employer report the service cost component of net periodic benefit costs in the same line item or items as other compensation costs and requires the other components of net periodic benefit costs to be separately presented in the statements of income outside of income from operations. The presentation requirements of ASU 2017-07 have been applied retrospectively with the service cost component of net periodic benefit costs included in operations and maintenance and all other components of net periodic benefit costs included in other income (expense), net in the statements of income for the three and nine months ended September 30, 2017. With respect to the presentation requirements, the registrants have used the practical expedient provided by ASU 2017-07, which permits an employer to use the amounts disclosed in its retirement benefits footnote for prior comparative periods as the estimation basis for applying the retrospective presentation requirements to those periods. The amounts of the other components of net periodic benefit costs reclassified for the prior period are presented in the following tables. See Note 2 to the financial statements of each registrant 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, 2018 and 2017 are presented in the following tables. Three Months Ended September 30, 2018 Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Power Southern Company Gas (in millions) Pension Plans Service cost $ 90 $ 19 $ 22 $ 4 $ 5 $ 3 $ 8 Interest cost 116 26 34 5 5 1 10 Expected return on plan assets (236 ) (51 ) (74 ) (10 ) (11 ) (3 ) (18 ) Amortization: Prior service costs 1 — 1 — — — (1 ) Regulatory asset — — — — — — 4 Net (gain)/loss 53 13 18 2 3 — 3 Net periodic pension cost (income) $ 24 $ 7 $ 1 $ 1 $ 2 $ 1 $ 6 Postretirement Benefits Service cost $ 6 $ 1 $ 2 $ — $ — $ 1 $ — Interest cost 19 5 7 1 — — 2 Expected return on plan assets (17 ) (7 ) (6 ) — — — (1 ) Amortization: Prior service costs 2 1 — — — — — Regulatory asset — — — — — — 2 Net (gain)/loss 3 — 2 — — — — Net periodic postretirement benefit cost $ 13 $ — $ 5 $ 1 $ — $ 1 $ 3 Nine Months Ended September 30, 2018 Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Power Southern Company Gas (in millions) Pension Plans Service cost $ 269 $ 58 $ 65 $ 12 $ 13 $ 7 $ 24 Interest cost 348 76 104 15 15 4 29 Expected return on plan assets (707 ) (155 ) (222 ) (30 ) (31 ) (8 ) (53 ) Amortization: Prior service costs 3 1 2 — — — (2 ) Regulatory asset — — — — — — 11 Net (gain)/loss 160 40 52 7 8 1 9 Net periodic pension cost (income) $ 73 $ 20 $ 1 $ 4 $ 5 $ 4 $ 18 Postretirement Benefits Service cost $ 18 $ 4 $ 5 $ 1 $ 1 $ 1 $ 1 Interest cost 56 13 21 2 2 — 7 Expected return on plan assets (51 ) (20 ) (19 ) (1 ) (1 ) — (5 ) Amortization: Prior service costs 5 3 1 — — — — Regulatory asset — — — — — — 5 Net (gain)/loss 10 1 6 — — — — Net periodic postretirement benefit cost $ 38 $ 1 $ 14 $ 2 $ 2 $ 1 $ 8 Three Months Ended September 30, 2017 (*) Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (in millions) Pension Plans Service cost $ 73 $ 15 $ 19 $ 3 $ 4 $ 6 Interest cost 114 25 34 5 5 10 Expected return on plan assets (224 ) (49 ) (71 ) (10 ) (9 ) (18 ) Amortization: Prior service costs 3 1 — — — — Net (gain)/loss 41 10 15 2 1 5 Net periodic pension cost (income) $ 7 $ 2 $ (3 ) $ — $ 1 $ 3 Postretirement Benefits Service cost $ 6 $ 1 $ 2 $ — $ — $ 1 Interest cost 19 4 6 1 1 3 Expected return on plan assets (16 ) (5 ) (6 ) — — (2 ) Amortization: Prior service costs 2 1 — — — (1 ) Net (gain)/loss 3 — 3 — — 1 Net periodic postretirement benefit cost $ 14 $ 1 $ 5 $ 1 $ 1 $ 2 (*) Excludes Southern Power since Southern Power did not participate in the qualified pension and postretirement benefit plans until December 2017. Nine Months Ended September 30, 2017 (*) Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (in millions) Pension Plans Service cost $ 220 $ 47 $ 56 $ 10 $ 11 $ 17 Interest cost 341 73 103 15 15 30 Expected return on plan assets (673 ) (147 ) (212 ) (29 ) (29 ) (53 ) Amortization: Prior service costs 9 2 2 — 1 (1 ) Net (gain)/loss 122 31 43 5 5 15 Net periodic pension cost (income) $ 19 $ 6 $ (8 ) $ 1 $ 3 $ 8 Postretirement Benefits Service cost $ 18 $ 4 $ 5 $ 1 $ 1 $ 2 Interest cost 59 13 21 2 3 8 Expected return on plan assets (49 ) (19 ) (18 ) (1 ) (1 ) (5 ) Amortization: Prior service costs 5 3 1 — — (2 ) Net (gain)/loss 10 1 6 — — 3 Net periodic postretirement benefit cost $ 43 $ 2 $ 15 $ 2 $ 3 $ 6 (*) Excludes Southern Power since Southern Power did not participate in the qualified pension and postretirement benefit plans until December 2017.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
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. Federal Tax Reform Legislation Following the enactment of the Tax Reform Legislation, the SEC staff issued Staff Accounting Bulletin 118 – "Income Tax Accounting Implications of the Tax Cuts and Jobs Act" (SAB 118), which provides for a measurement period of up to one year from the enactment date to complete accounting under GAAP for the tax effects of the legislation. Due to the complex and comprehensive nature of the enacted tax law changes, and their application under GAAP, the registrants consider all amounts recorded in the financial statements as a result of the Tax Reform Legislation to be "provisional" as discussed in SAB 118 and subject to revision. Each of the registrants is awaiting additional guidance from industry and income tax authorities in order to finalize its accounting. The ultimate impact of the Tax Reform Legislation on deferred income tax assets and liabilities and the related regulatory assets and liabilities cannot be determined at this time. See Note (B) under " Regulatory Matters " for additional information. Current and Deferred Income Taxes Tax Credit Carryforwards Southern Company had federal ITC and PTC carryforwards (primarily related to Southern Power) totaling $2.4 billion as of September 30, 2018 compared to $2.1 billion as of December 31, 2017 . The federal ITC and PTC carryforwards begin expiring in 2034 and 2032, respectively, but are expected to be fully utilized by 2023. The estimated tax credit utilization reflects the 2018 abandonment loss related to certain Kemper County energy facility expenditures as well as the projected taxable gains on the various sale transactions described in Note (J) and " Legal Entity Reorganizations " herein. The expected utilization of tax credit carryforwards could be further delayed by numerous factors, including the acquisition of additional renewable projects, increased generation at existing wind facilities, the purchase of rights to additional PTCs during construction of Plant Vogtle Units 3 and 4 pursuant to the MEAG Term Sheet, and changes in taxable income projections. See Note (B) under "Nuclear Construction" for additional information on Plant Vogtle Units 3 and 4. The ultimate outcome of these matters cannot be determined at this time. Valuation Allowances Georgia Power Mississippi Power Southern Company Gas Southern Company (in millions) Federal $ 6 $ — $ 11 $ 19 State (net of federal benefit) 33 124 1 171 Balance at September 30, 2018 $ 39 $ 124 $ 12 $ 190 Southern Company had valuation allowances, net of related federal benefits, of $ 190 million at September 30, 2018 compared to $ 148 million at December 31, 2017. The increase was primarily due to Georgia Power's projected inability to utilize certain state tax credit carryforwards. Effective Tax Rate Each registrant's effective tax rate for the nine months ended September 30, 2018 varied significantly as compared to the corresponding period in 2017 due to the 14% lower 2018 federal tax rate resulting from the Tax Reform Legislation. 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 22.7% for the nine months ended September 30, 2018 compared to 42.6% for the corresponding period in 2017 . The effective tax rate decrease was primarily due to the reduction in the federal corporate income tax rate and the benefit from the flowback of excess deferred income taxes as a result of the Tax Reform Legislation, the net state income tax benefits related to changes in state apportionment rates arising from the reorganization of Southern Power's legal entities as discussed further herein, and the $3.1 billion pre-tax loss on the Kemper IGCC, net of the non-deductible AFUDC equity portion, recorded in 2017, partially offset by the $1.1 billion pre-tax loss related to Plant Vogtle Units 3 and 4 and the income taxes recorded related to the Southern Company Gas Dispositions in 2018. See Note 3 to the financial statements of Southern Company under "Kemper County Energy Facility" in Item 8 of the Form 10-K and Note (B) under " Kemper County Energy Facility " for additional information regarding the Kemper IGCC and Note (B) under " Nuclear Construction " for additional information regarding Plant Vogtle Units 3 and 4. See Note (B) under "Regulatory Matters" for additional information on the flowback of excess deferred income taxes and Note (J) under "Southern Company Gas" for additional information on the Southern Company Gas Dispositions. 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. Alabama Power Alabama Power's effective tax rate was 23.9% for the nine months ended September 30, 2018 compared to 39.9% for the corresponding period in 2017 . The effective tax rate decrease was primarily due to the reduction in the federal corporate income tax rate and the benefit from the flowback of excess deferred income taxes as a result of the Tax Reform Legislation. See Note (B) under "Regulatory Matters – Alabama Power" for additional information. Georgia Power Georgia Power's effective tax rate was 25.5% for the nine months ended September 30, 2018 compared to 37.0% for the corresponding period in 2017 . The effective tax rate decrease was primarily due to the reduction in the federal corporate income tax rate and the $1.1 billion pre-tax loss related to the estimated probable loss on Plant Vogtle Units 3 and 4 recorded in 2018, partially offset by the valuation allowance on certain state tax credit carryforwards. See Note (B) under " Nuclear Construction " for additional information. Gulf Power Gulf Power's effective tax benefit rate was (0.5)% for the nine months ended September 30, 2018 compared to an effective tax rate of 39.4% for the corresponding period in 2017 . The effective tax rate decrease was primarily due to the reduction in the federal corporate income tax rate and the benefit from the flowback of excess deferred income taxes as a result of the Tax Reform Legislation. See Note (B) under "Regulatory Matters – Gulf Power" for additional information. Mississippi Power Mississippi Power's effective tax rate was 20.8% for the nine months ended September 30, 2018 compared to a benefit rate of (30.3)% for the corresponding period in 2017 . The effective tax rate increase was primarily due to the $3.1 billion pre-tax loss on the Kemper IGCC, net of the non-deductible AFUDC equity portion, recorded in 2017, partially offset by the reduction in the federal corporate income tax rate as a result of the Tax Reform Legislation. See Note (B) under "Regulatory Matters – Mississippi Power" for additional information. Southern Power Southern Power's effective tax benefit rate was (220.3)% for the nine months ended September 30, 2018 compared to (66.5)% for the corresponding period in 2017 . The effective tax rate decrease was primarily due to lower earnings before income taxes resulting from a $119 million asset impairment charge as a result of the pending sale of Plant Oleander and Plant Stanton Unit A (together, the Florida Plants) and a $36 million asset impairment charge on wind turbine equipment held for development projects, as well as the reduction in the federal corporate income tax rate and the net state income tax benefits related to certain changes in apportionment rates arising from the reorganization of Southern Power's legal entities as described below. See Note (J) under "Southern Power" for additional information. 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. Southern Company Gas Southern Company Gas' effective tax rate was 61.8% for the nine months ended September 30, 2018 compared to 43.4% for the corresponding period in 2017 . This increase was primarily related to income taxes recorded related to the Southern Company Gas Dispositions, partially offset by the reduction in the federal corporate income tax rate and the benefit from the flowback of excess deferred income taxes as a result of the Tax Reform Legislation, as well as the 2017 increases in deferred tax expense related to the enactment of the State of Illinois income tax legislation and new income tax apportionment factors in several states. See Note (B) under "Regulatory Matters – Southern Company Gas" and Note (J) under "Southern Company Gas" for additional information. Legal Entity Reorganizations In April 2018, Southern Power completed the final stage of a legal entity reorganization of various direct and indirect subsidiaries that own and operate substantially all of its solar facilities, including certain subsidiaries owned in partnership with various third parties. The reorganization resulted in net state tax benefits related to certain changes in apportionment rates totaling approximately $54 million , which were recorded in the first half of 2018. In September 2018, Southern Power also completed a legal reorganization of eight operating wind facilities under a new holding company, SP Wind, which resulted in net state tax benefits totaling approximately $11 million related to certain changes in apportionment rates. 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. The registrants had no unrecognized tax benefits as of September 30, 2018 . 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 income tax returns through 2016, as well as the pre-Merger Southern Company Gas tax returns. Southern Company is a participant in the Compliance Assurance Process of the IRS. 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 2012.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 2018
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 (D) 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. The registrants adopted ASU 2017-12 as of January 1, 2018. See Note (A) under " Recently Adopted Accounting Standards – Other " for additional information. 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 approved a moratorium on Gulf Power's fuel-hedging program until January 1, 2021. 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 operating revenues. 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 and in the same income statement line item as the earnings effect of the hedged transactions. • 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, 2018 , 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 (*) 595 2022 2029 Alabama Power 82 2022 — Georgia Power 165 2022 — Gulf Power 9 2020 — Mississippi Power 69 2022 — Southern Power 15 2020 — Southern Company Gas (*) 255 2021 2029 (*) 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 4.3 billion mmBtu and short natural gas positions of 4 billion mmBtu as of September 30, 2018 , 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 13 million mmBtu for Southern Company, 2 million mmBtu for Alabama Power, 4 million mmBtu for Georgia Power, 1 million mmBtu for Gulf Power, 2 million mmBtu for Mississippi Power, and 4 million mmBtu for Southern 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, 2019 are immaterial for all 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 derivatives' fair value gains or losses are recorded in OCI and are reclassified into earnings at the same time and presented on the same income statement line item as the earnings effect of the hedged transactions. 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 on the same income statement line item. 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, 2018 , 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, 2018 (in millions) (in millions) Fair Value Hedges of Existing Debt Southern Company (*) $ 300 2.75% 3-month June 2020 $ (6 ) Southern Company (*) 1,500 2.35% 1-month July 2021 (60 ) Georgia Power 500 1.95% 3-month December 2018 (3 ) Georgia Power 200 4.25% 3-month December 2019 (3 ) Southern Company Consolidated $ 2,500 $ (72 ) (*) 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, 2019 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 derivatives' fair value gains or losses are recorded in OCI and are reclassified into earnings at the same time and on the same income statement line as the earnings effect of the hedged transactions, including foreign currency gains or losses arising from changes in the U.S. currency exchange rates. The derivatives employed as hedging instruments are structured to minimize ineffectiveness. At September 30, 2018 , the following foreign currency derivatives were outstanding: Pay Notional Pay Rate Receive Notional Receive Rate Hedge Fair Value Gain (Loss) at September 30, 2018 (in millions) (in millions) (in millions) Cash Flow Hedges of Existing Debt Southern Power $ 677 2.95% € 600 1.00% June 2022 $ 48 Southern Power 564 3.78% 500 1.85% June 2026 51 Total $ 1,241 € 1,100 $ 99 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, 2019 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, 2018 As of December 31, 2017 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 $ 15 $ 17 $ 10 $ 43 Other deferred charges and assets/Other deferred credits and liabilities 5 26 7 24 Assets held for sale, current/Liabilities held for sale, current — 6 — — Assets held for sale/Liabilities held for sale — 2 — — Total derivatives designated as hedging instruments for regulatory purposes $ 20 $ 51 $ 17 $ 67 Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 3 $ 6 $ 3 $ 14 Other deferred charges and assets/Other deferred credits and liabilities 1 1 — — Interest rate derivatives: Other current assets/Other current liabilities — 22 1 4 Other deferred charges and assets/Other deferred credits and liabilities — 50 — 34 Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 23 Other deferred charges and assets/Other deferred credits and liabilities 122 — 129 — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 126 $ 102 $ 133 $ 75 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ 263 $ 317 $ 380 $ 437 Other deferred charges and assets/Other deferred credits and liabilities 134 198 170 215 Total derivatives not designated as hedging instruments $ 397 $ 515 $ 550 $ 652 Gross amounts recognized $ 543 $ 668 $ 700 $ 794 Gross amounts offset (a) $ (303 ) $ (491 ) $ (405 ) $ (598 ) Net amounts recognized in the Balance Sheets (b) $ 240 $ 177 $ 295 $ 196 As of September 30, 2018 As of December 31, 2017 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 $ 5 $ 4 $ 2 $ 6 Other deferred charges and assets/Other deferred credits and liabilities 2 6 2 4 Total derivatives designated as hedging instruments for regulatory purposes $ 7 $ 10 $ 4 $ 10 Gross amounts recognized $ 7 $ 10 $ 4 $ 10 Gross amounts offset $ (4 ) $ (4 ) $ (4 ) $ (4 ) Net amounts recognized in the Balance Sheets $ 3 $ 6 $ — $ 6 Georgia Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 5 $ 9 $ 2 $ 9 Other deferred charges and assets/Other deferred credits and liabilities 2 13 4 10 Total derivatives designated as hedging instruments for regulatory purposes $ 7 $ 22 $ 6 $ 19 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ — $ 5 $ — $ 4 Other deferred charges and assets/Other deferred credits and liabilities — 1 — 1 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ — $ 6 $ — $ 5 Gross amounts recognized $ 7 $ 28 $ 6 $ 24 Gross amounts offset $ (7 ) $ (7 ) $ (6 ) $ (6 ) Net amounts recognized in the Balance Sheets $ — $ 21 $ — $ 18 Gulf Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ — $ 6 $ — $ 14 Other deferred charges and assets/Other deferred credits and liabilities — 2 — 7 Total derivatives designated as hedging instruments for regulatory purposes $ — $ 8 $ — $ 21 Gross amounts recognized $ — $ 8 $ — $ 21 Net amounts recognized in the Balance Sheets $ — $ 8 $ — $ 21 As of September 30, 2018 As of December 31, 2017 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 $ 2 $ 3 $ 1 $ 6 Other deferred charges and assets/Other deferred credits and liabilities 1 6 1 3 Total derivatives designated as hedging instruments for regulatory purposes $ 3 $ 9 $ 2 $ 9 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ — $ — $ 1 $ — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ — $ — $ 1 $ — Gross amounts recognized $ 3 $ 9 $ 3 $ 9 Gross amounts offset $ (3 ) $ (3 ) $ (2 ) $ (2 ) Net amounts recognized in the Balance Sheets $ — $ 6 $ 1 $ 7 Southern Power Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 2 $ 6 $ 3 $ 11 Other deferred charges and assets/Other deferred credits and liabilities 1 1 — — Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 23 Other deferred charges and assets/Other deferred credits and liabilities 122 — 129 — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 125 $ 30 $ 132 $ 34 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ — $ — $ — $ 2 Gross amounts recognized $ 125 $ 30 $ 132 $ 36 Gross amounts offset $ (2 ) $ (2 ) $ (3 ) $ (3 ) Net amounts recognized in the Balance Sheets $ 123 $ 28 $ 129 $ 33 As of September 30, 2018 As of December 31, 2017 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 $ 3 $ 1 $ 5 $ 8 Other deferred charges and assets/Other deferred credits and liabilities — 1 — — Total derivatives designated as hedging instruments for regulatory purposes $ 3 $ 2 $ 5 $ 8 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 $ 1 $ — $ — $ 3 Derivatives not designated as hedging instruments Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ 262 $ 316 $ 379 $ 434 Other deferred charges and assets/Other deferred credits and liabilities 134 198 170 215 Total derivatives not designated as hedging instruments $ 396 $ 514 $ 549 $ 649 Gross amounts of recognized $ 400 $ 516 $ 554 $ 660 Gross amounts offset (a) $ (287 ) $ (475 ) $ (390 ) $ (583 ) Net amounts recognized in the Balance Sheets (b) $ 113 $ 41 $ 164 $ 77 (a) Gross amounts offset include cash collateral held on deposit in broker margin accounts of $189 million and $193 million as of September 30, 2018 and December 31, 2017 , respectively. (b) Net amounts of derivative instruments outstanding exclude premium and intrinsic value associated with weather derivatives of $5 million and $11 million as of September 30, 2018 and December 31, 2017 , respectively. At September 30, 2018 and December 31, 2017 , 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, 2018 Derivative Category and Balance Sheet Location Southern Company (*) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (*) (in millions) Energy-related derivatives: Other regulatory assets, current $ (9 ) $ (2 ) $ (4 ) $ (6 ) $ (2 ) $ (1 ) Other regulatory assets, deferred (20 ) (4 ) (11 ) (2 ) (5 ) — Assets held for sale, current (6 ) — — — — — Assets held for sale (2 ) — — — — — Other regulatory liabilities, current 8 3 — — — 5 Total energy-related derivative gains (losses) $ (29 ) $ (3 ) $ (15 ) $ (8 ) $ (7 ) $ 4 (*) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $3 million at September 30, 2018 . Regulatory Hedge Unrealized Gain (Loss) Recognized in the Balance Sheet at December 31, 2017 Derivative Category and Balance Sheet Location Southern Company (*) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (*) (in millions) Energy-related derivatives: Other regulatory assets, current $ (34 ) $ (4 ) $ (7 ) $ (14 ) $ (5 ) $ (4 ) Other regulatory assets, deferred (18 ) (3 ) (6 ) (7 ) (2 ) — Other regulatory liabilities, current 7 — — — — 7 Other regulatory liabilities, deferred 1 1 — — — — Total energy-related derivative gains (losses) $ (44 ) $ (6 ) $ (13 ) $ (21 ) $ (7 ) $ 3 (*) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $6 million at December 31, 2017 . For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of cash flow hedge accounting on accumulated OCI were as follows: Gain (Loss) Recognized in OCI on Derivative For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Southern Company Energy-related derivatives $ (5 ) $ (6 ) $ 7 $ (26 ) Interest rate derivatives — (1 ) (2 ) (2 ) Foreign currency derivatives (10 ) 46 (31 ) 114 Total $ (15 ) $ 39 $ (26 ) $ 86 Southern Power Energy-related derivatives $ (5 ) $ (6 ) $ 5 $ (21 ) Foreign currency derivatives (10 ) 46 (31 ) 114 Total $ (15 ) $ 40 $ (26 ) $ 93 Southern Company Gas Energy-related derivatives $ — $ — $ 2 $ (4 ) For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of energy-related derivatives and interest rate derivatives designated as cash flow hedging instruments on accumulated OCI were immaterial for the other registrants. For the three and nine months ended September 30, 2017 , there was no material ineffectiveness recorded in earnings for any registrant. Upon the adoption of ASU 2017-12, beginning in 2018, ineffectiveness was no longer separately measured and recorded in earnings. See Note (A) for additional information. For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of cash flow and fair value hedge accounting on income were as follows: Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Southern Company Cost of natural gas $ 104 $ 134 $ 1,053 $ 1,085 Gain (loss) on cash flow hedges (a) Energy-related derivatives — — (2 ) — Depreciation and amortization 787 767 2,338 2,236 Gain (loss) on cash flow hedges (a) Energy-related derivatives — (6 ) 2 (12 ) Interest expense, net of amounts capitalized (458 ) (407 ) (1,386 ) (1,248 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives (5 ) (5 ) (16 ) (15 ) Foreign currency derivatives (6 ) (5 ) (18 ) (17 ) Gain (loss) on fair value hedges (b) Interest rate derivatives (4 ) (5 ) (35 ) (6 ) Other income (expense), net 57 65 195 165 Gain (loss) on cash flow hedges (a)(c) Foreign currency derivatives (9 ) 43 (46 ) 139 Alabama Power Interest expense, net of amounts capitalized $ (82 ) $ (76 ) $ (240 ) $ (229 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives (1 ) (2 ) (4 ) (5 ) Georgia Power Interest expense, net of amounts capitalized $ (95 ) $ (105 ) $ (303 ) $ (310 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives (1 ) (1 ) (4 ) (3 ) Gain (loss) on fair value hedges (b) Interest rate derivatives — — (1 ) (1 ) Mississippi Power Interest expense, net of amounts capitalized $ (19 ) $ 13 $ (59 ) $ (23 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives — — (1 ) (1 ) Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Southern Power Depreciation and amortization $ 130 $ 131 $ 370 $ 379 Gain (loss) on cash flow hedges (a) Energy-related derivatives — (6 ) 2 (12 ) Interest expense, net of amounts capitalized (45 ) (47 ) (138 ) (144 ) Gain (loss) on cash flow hedges (a) Foreign currency derivatives (6 ) (5 ) (18 ) (17 ) Other income (expense), net 17 3 22 3 Gain (loss) on cash flow hedges (a)(c) Foreign currency derivatives (9 ) 43 (46 ) 139 Southern Company Gas Cost of natural gas $ 104 $ 134 $ 1,053 $ 1,085 Gain (loss) on cash flow hedges (a) Energy-related derivatives — — (2 ) — (a) Amounts reflect gains or losses on cash flow hedges that were reclassified from accumulated OCI into income. (b) For fair value hedges presented above, generally changes in the fair value of the derivative contracts are equal to changes in the fair value of the underlying debt and have no material impact on income. (c) 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 three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of cash flow hedge accounting on income for interest rate derivatives were immaterial for Gulf Power and Southern Company Gas. As of September 30, 2018 and December 31, 2017, the following amounts were recorded on the balance sheets related to cumulative basis adjustments for fair value hedges: Carrying Amount of the Hedged Item Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item Balance Sheet Location of Hedged Items As of September 30, 2018 As of December 31, 2017 As of September 30, 2018 As of December 31, 2017 (in millions) (in millions) Southern Company Securities due within one year $ (499 ) $ (746 ) $ 1 $ 3 Long-term debt (2,526 ) (2,553 ) 65 35 Georgia Power Securities due within one year $ (499 ) $ (746 ) $ 1 $ 3 Long-term debt (497 ) (498 ) 2 1 For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of energy-related derivatives not designated as hedging instruments on the statements of income of Southern Company and Southern Company Gas were as follows: Gain (Loss) Three Months Ended September 30, Nine Months Ended Derivatives in Non-Designated Hedging Relationships Statements of Income Location 2018 2017 2018 2017 (in millions) (in millions) Energy-related derivatives: Natural gas revenues (*) $ (36 ) $ (17 ) $ (79 ) $ 48 Cost of natural gas 2 2 5 (2 ) Total derivatives in non-designated hedging relationships $ (34 ) $ (15 ) $ (74 ) $ 46 (*) Excludes gains (losses) recorded in natural gas revenues associated with weather derivatives of $15 million for the nine months ended September 30, 2017 and immaterial amounts for all other periods presented. For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of energy-related derivatives and interest rate derivatives not designated as hedging instruments were immaterial f or the traditional electric operating companies and Southern Power. 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, 2018 , the registrants had no collateral posted with derivative counterparties to satisfy these arrangements. For the registrants with interest rate derivatives at September 30, 2018 , the fair value of interest rate derivative liabilities with contingent features and the maximum potential collateral requirements arising from the credit-risk-related contingent features, at a rating below BBB- and/or Baa3, was immaterial. At September 30, 2018 , the fair value of energy-related derivative liabilities with contingent features and 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, 2018 , 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, 2018 , cash collateral held on deposit in broker margin accounts was $189 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 the creditworthiness of counterparties in order to mitigate Southern Company's, the traditional electric operating companies', Southern Power's, and Southern Company Gas' exposure to counterparty credit risk. Southern Company Gas may require counterparties to pledge additional collateral when deemed necessary. In addition, Southern Company Gas conducts credit evaluations and obtains appropriate internal approvals for the counterparty's line of credit before any transaction with the counterparty is executed. In most cases, the counterparty must have an investment grade rating, which includes a minimum long-term debt rating of Baa3 from Moody's and BBB- from S&P. Generally, Southern Company Gas requires credit enhancements by way of a guaranty, cash deposit, or letter of credit for transaction counterparties that do not have investment grade ratings. Southern Company Gas also utilizes master netting agreements whenever possible to mitigate exposure to counterparty credit risk. When Southern Company Gas is engaged in more than one outstanding derivative transaction with the same counterparty and it also has a legally enforceable netting agreement with that counterparty, the "net" mark-to-market exposure represents the netting of the positive and negative exposures with that counterparty and a reasonable measure of Southern Company Gas' credit risk. Southern Company Gas also uses other netting agreements with certain counterparties with whom it conducts significant transactions. Master netting agreements enable Southern Company Gas to net certain assets and liabilities by counterparty. Southern Company Gas also nets across product lines and against cash collateral provided the master netting and cash collateral agreements include such provisions. Southern Company Gas may require counterparties to pledge additional collateral when deemed necessary. Southern Company, the traditional electric operating companies, Southern Power, and Southern Company Gas do not anticipate a ma

Acquisitions and Dispositions

Acquisitions and Dispositions	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
ACQUISITIONS AND DISPOSITIONS	ACQUISITIONS AND DISPOSITIONS Southern Company's Sale of Gulf Power On May 20, 2018, Southern Company entered into a stock purchase agreement (Gulf Power SPA) with NextEra Energy and its wholly-owned subsidiary 700 Universe, LLC, which provides for the sale of all of the capital stock of Gulf Power for an aggregate cash purchase price of $5.75 billion (less the amount of indebtedness assumed at closing, which is currently estimated at approximately $1.3 billion ), subject to (i) customary adjustments for indebtedness and working capital and (ii) reduction by the amount (if any) by which Gulf Power fails to meet a specified capital expenditure target. The Gulf Power SPA contains customary representations, warranties, and covenants of Southern Company, 700 Universe, LLC, and NextEra Energy. These covenants include, among others, an obligation of Southern Company to cause Gulf Power to operate its business in the ordinary course until the sale is consummated and an obligation for each of the parties to use reasonable best efforts to obtain the governmental and regulatory approvals described below. The completion of the sale is subject to the satisfaction or waiver of certain closing conditions, including, among others, (i) approval by the FERC and the Federal Communications Commission, (ii) the entry into certain ancillary agreements, including transmission-related agreements and a transition services agreement, among the parties and their affiliates, and (iii) other customary closing conditions. The Gulf Power SPA may be terminated by either Southern Company or 700 Universe, LLC under certain circumstances, including if the sale is not consummated by June 28, 2019 (subject to extension to December 31, 2019, if all of the conditions to closing, other than the conditions related to obtaining regulatory approvals, have been satisfied). The Gulf Power SPA further provides that, upon the termination thereof, (i) under certain specified circumstances, 700 Universe, LLC will be required to pay Southern Company a termination fee of $100 million or $200 million (such amount depending on the specific circumstances of such termination) and (ii) upon certain other specified circumstances Southern Company will be required to pay 700 Universe, LLC a termination fee of $100 million . The sale of Gulf Power is expected to occur in the first quarter 2019. The assets and liabilities of Gulf Power are classified as assets held for sale and liabilities held for sale on Southern Company's balance sheet as of September 30, 2018 . See " Assets Held for Sale " below for additional information. The ultimate outcome of this matter cannot be determined at this time. Southern Power See Note 11 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, 2018 During the nine months ended September 30, 2018, one of Southern Power's wholly-owned subsidiaries acquired and completed construction of the Gaskell West 1 solar facility . 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 Gaskell West 1 Solar Recurrent Energy Development Holdings, LLC January 26, 2018 20 Kern County, CA 100% of Class B (*) March 2018 20 years (*) Southern Power owns 100% of the class B membership interests under a tax equity partnership agreement. The Gaskell West 1 facility did not have operating revenues or activities prior to completion of construction and the assets being placed in service during March 2018. Construction Projects in Progress and/or Completed During the nine months ended September 30, 2018 , Southern Power started, continued, or completed construction of the projects set forth in the table below. Total aggregate construction costs, excluding the acquisition costs, are expected to be between $575 million and $640 million for the Mankato, Wild Horse Mountain, and Reading facilities. At September 30, 2018 , construction costs included in CWIP related to these projects totaled $246 million . 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 Cactus Flats (a) Wind 148 Concho County, TX July 2018 12-15 years Mankato Natural Gas 385 Mankato, MN First half 2019 20 years Wild Horse Mountain (b) Wind 100 Pushmataha County, OK Fourth quarter 2019 20 years Reading (c) Wind 200 Osage and Lyon Counties, KS Second quarter 2020 12 years (a) In July 2017, Southern Power purchased 100% of the Cactus Flats facility and commenced construction. In July 2018, the facility was placed in service and, in August 2018, Southern Power closed on a tax equity partnership agreement and owns 100% of the class B membership interests. (b) In May 2018, Southern Power purchased 100% of the Wild Horse Mountain facility and commenced construction. Southern Power may enter into a tax equity partnership agreement, in which case it would then own 100% of the class B membership interests. (c) In August 2018, Southern Power purchased 100% of the membership interests from the joint development arrangement with Renewable Energy Systems Americas, Inc. and commenced construction. Southern Power may enter into a tax equity partnership agreement, in which case it would then own 100% of the class B membership interests. Development Projects During 2017, as part of its renewable development strategy, Southern Power purchased wind turbine equipment from Siemens Gamesa Renewable Energy Inc. and Vestas-American Wind Technology, Inc. to be used for various development and construction projects. Any wind projects using this equipment and reaching commercial operation by the end of 2021 are expected to qualify for 80% PTCs. During 2016, Southern Power entered into a joint development agreement with Renewable Energy Systems Americas, Inc. to develop and construct wind projects. In addition, in 2016, Southern Power purchased wind turbine equipment from Siemens Wind Power, Inc. and Vestas-American Wind Technology, Inc. to be used for construction of the facilities. Any wind projects using this equipment and reaching commercial operation by the end of 2020 are expected to qualify for 100% PTCs. In response to the previously disclosed decrease of planned expenditures for plant acquisitions and placeholder growth, Southern Power continues to refine the deployment of the wind turbine equipment to potential joint development and construction projects as well as the amount of MW capacity to be constructed. During the third quarter 2018, as a result of a review of various options for probable dispositions of wind turbine equipment not already deployed to development or construction projects, Southern Power recorded a $36 million asset impairment charge on the equipment. The ultimate outcome of these matters cannot be determined at this time. Sale of Solar Facility Interests In May 2018, Southern Power sold a 33% equity interest in SPSH, a limited partnership indirectly owning substantially all of Southern Power's solar facilities, to Global Atlantic Financial Group Limited (Global Atlantic) for approximately $1.2 billion , subject to customary working capital adjustments. The proceeds were used to repay $770 million of existing indebtedness, to return capital of $250 million to Southern Company, and for other general corporate purposes, including working capital. Since Southern Power retains control of the limited partnership through its wholly-owned general partner, the sale was recorded as an equity transaction and Southern Power will continue to consolidate the results of SPSH. On the date of the transaction, the noncontrolling interest was increased by $511 million to reflect 33% of the carrying value of the partnership. This difference, partially offset by the tax impact and other related transaction charges, also resulted in a $410 million decrease to Southern Power's common stockholder's equity. Sale of Florida Plants In May 2018, Southern Power entered into an equity interest purchase agreement with NextEra Energy to sell all of its equity interests in the Florida Plants, for an aggregate purchase price of $195 million , subject to customary working capital and timing adjustments. The sale is subject to certain closing and timing conditions and approvals, including, but not limited to, approval by the FERC. The ultimate purchase price will decrease $110,000 per day for each day after December 31, 2018 through the closing of the transaction. Conversely, the ultimate purchase price will increase $110,000 per day for each day the closing occurs prior to December 31, 2018. The sale is expected to occur in the first quarter 2019. As a result of this pending transaction, Southern Power recorded an asset impairment charge of approximately $119 million ( $89 million after tax) in the second quarter 2018. The assets and liabilities of the Florida Plants are classified as assets held for sale and liabilities held for sale on Southern Company's and Southern Power's balance sheets as of September 30, 2018 . See " Assets Held for Sale " below for additional information. The ultimate outcome of this matter cannot be determined at this time. Sale of Wind Facility Interests On October 31, 2018, Southern Power entered into agreements with three financial investors for the sale of a noncontrolling interest for approximately $1.2 billion in tax equity in SP Wind, which owns a portfolio of eight operating wind facilities. The transaction is subject to Public Utility Commission of Texas approval and is expected to close by the end of 2018. Southern Power intends to use the proceeds to return capital of approximately $1.0 billion to Southern Company. The ultimate outcome of this matter cannot be determined at this time. Sale of Mankato Plant On November 5, 2018, Southern Power entered into an agreement with Northern States Power to sell all of its equity interests in Plant Mankato (including the 385 -MW expansion currently under construction) for an aggregate purchase price of $650 million , subject to customary working capital and timing adjustments. The ultimate purchase price will decrease $66,667 per day for each day after June 1, 2019, if the expansion has not achieved commercial operation, but such decrease will not exceed $15 million . This transaction is subject to the expiration or termination of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act and FERC and state commission approvals and is expected to close mid-2019. The ultimate outcome of this matter cannot be determined at this time. Assets Subject to Lien Under the terms of the PPA and the expansion PPA for the Mankato project, approximately $500 million of assets, primarily related to property, plant, and equipment, are subject to lien at September 30, 2018 . Southern Company Gas Sale of Pivotal Home Solutions On June 4, 2018, Southern Company Gas completed the stock sale of Pivotal Home Solutions to American Water Enterprises LLC for a total cash purchase price of $365 million , which includes the final working capital adjustment. This disposition resulted in an estimated net loss of $73 million , which includes $39 million of income tax expense, the calculation of which is expected to be finalized in the fourth quarter 2018. In contemplation of the transaction, a goodwill impairment charge of $42 million was recorded during the first quarter 2018. The after-tax loss included income tax expense on goodwill not deductible for tax purposes and for which a deferred tax liability had not been recorded previously. Southern Company Gas and American Water Enterprises LLC entered into a transition services agreement whereby Southern Company Gas provided certain administrative and operational services through November 4, 2018. Sale of Elizabethtown Gas and Elkton Gas On July 1, 2018, a Southern Company Gas subsidiary, Pivotal Utility Holdings, completed the sales 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 and an additional $40 million for working capital, subject to a final working capital adjustment expected in the fourth quarter 2018. This disposition resulted in an estimated pre-tax gain of approximately $230 million and an after-tax gain of approximately $18 million , the calculations of which are expected to be finalized in the fourth quarter 2018. The after-tax gain included income tax expense on goodwill not deductible for tax purposes and for which a deferred tax liability had not been recorded previously. Southern Company Gas and South Jersey Industries, Inc. entered into transition services agreements whereby Southern Company Gas will provide certain administrative and operational services through no later than January 31, 2020. Sale of Florida City Gas On July 29, 2018, Southern Company Gas and its wholly-owned direct subsidiary, NUI Corporation, completed the stock sale of Pivotal Utility Holdings, which primarily consisted of Florida City Gas, to NextEra Energy for a total cash purchase price of $530 million (less $3 million of indebtedness assumed at closing for customer deposits) and an additional $60 million for cash and other working capital, which includes the final working capital adjustment. This disposition resulted in an estimated pre-tax gain of approximately $121 million and an after-tax gain of approximately $20 million , the calculations of which are expected to be finalized in the fourth quarter 2018. The after-tax gain included income tax expense on goodwill not deductible for tax purposes and for which a deferred tax liability had not been recorded previously. Southern Company Gas and NextEra Energy entered into a transition services agreement whereby Southern Company Gas will provide certain administrative and operational services through no later than July 29, 2020. Assets Held for Sale As discussed above, Southern Company and Southern Power each have assets and liabilities held for sale on their balance sheets at September 30, 2018 . Assets and liabilities held for sale have been classified separately on each company's balance sheet at the lower of carrying value or fair value less costs to sell at the time the criteria for held-for-sale classification were met. For assets and liabilities held for sale recorded at fair value on a nonrecurring basis, the fair value of assets held for sale is based primarily on unobservable inputs (Level 3), which includes the agreed upon sales prices in executed sales agreements. Upon classification as held for sale in May 2018, Southern Power ceased recognizing depreciation on the Florida Plants' property, plant, and equipment to be sold. Since the depreciation of the assets to be sold in the Gulf Power transaction continues to be reflected in customer rates and will be reflected in the carryover basis of the assets when sold, Southern Company will continue to record depreciation on those assets through the date the transaction closes. Likewise, since the depreciation of the assets sold in the Elizabethtown Gas, Elkton Gas, and Florida City Gas transactions continued to be reflected in customer rates and was reflected in the carryover basis of the assets when sold, Southern Company Gas continued to record depreciation on those assets through the respective date that each transaction closed. The following table provides Southern Company's and Southern Power's major classes of assets and liabilities classified as held for sale at September 30, 2018 : Southern Company Southern Power (in millions) Assets Held for Sale: Current assets $ 407 $ 18 Total property, plant, and equipment 4,093 168 Other non-current assets 574 17 Total Assets Held for Sale $ 5,074 $ 203 Liabilities Held for Sale: Current liabilities $ 355 $ 4 Long-term debt 1,285 — Accumulated deferred income taxes 542 — Other non-current liabilities 1,008 — Total Liabilities Held for Sale $ 3,190 $ 4 Southern Company, Southern Power, and Southern Company Gas each concluded that the sale of their assets, both individually and combined, did not represent a strategic shift in operations that has, or is expected to have, a major effect on its operations and financial results; therefore, none of the assets related to the sales have been classified as discontinued operations for any of the periods presented. Gulf Power and the Florida Plants represent individually significant components of Southern Company and Southern Power, respectively; therefore, pre-tax profit for these components for the three and nine months ended September 30, 2018 and 2017 is presented below: For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Earnings before income taxes: Gulf Power $ 59 $ 103 $ 146 $ 199 Southern Power's Florida Plants $ 18 $ 11 $ 40 $ 28

Variable Interest Entity and Eq

Variable Interest Entity and Equity Method Investments	9 Months Ended
	Sep. 30, 2018
Regulated Operations [Abstract]
VARIABLE INTEREST ENTITY AND EQUITY METHOD INVESTMENTS	VARIABLE INTEREST ENTITY AND EQUITY METHOD INVESTMENTS Southern Power In May 2018, Southern Power sold a 33% limited partnership interest in SPSH to Global Atlantic. See Note (J) under "Southern Power" for additional information. A wholly-owned subsidiary of Southern Power is the general partner and holds a 1% ownership interest in SPSH and another wholly-owned subsidiary of Southern Power owns the remaining 66% ownership in SPSH. SPSH is a variable interest entity (VIE) because the arrangement is structured as a limited partnership and the 33% limited partner does not have substantive kick-out rights against the general partner. Southern Power previously consolidated SPSH and will continue to do so as the primary beneficiary of the VIE because it controls the most significant activities of the partnership, including operating and maintaining its assets. At September 30, 2018 , SPSH had total assets of $6.4 billion , total liabilities of $111 million , and noncontrolling interests related to other partners' interests of $1.2 billion . Cash distributions from SPSH are allocated 67% to Southern Power and 33% to Global Atlantic in accordance with their membership interests and the limited partnership agreement. Transfers and sales of the assets in the VIE are subject to limited partner consent and the liabilities do not have recourse to the general credit of Southern Power. Liabilities consist of customary working capital items and do not include any long-term debt. 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 on Southern Company Gas' equity method investments. Equity Method Investments The carrying amounts of Southern Company Gas' equity method investments as of September 30, 2018 and December 31, 2017 and related income from those investments for the three and nine -month periods ended September 30, 2018 and September 30, 2017 were as follows: Investment Balance September 30, 2018 December 31, 2017 (in millions) SNG $ 1,260 $ 1,262 Atlantic Coast Pipeline 73 41 PennEast Pipeline 70 57 Other 126 117 Total $ 1,529 $ 1,477 Earnings from Equity Method Investments Three Months Ended Three Months Ended Nine Months Ended Nine Months (in millions) SNG $ 29 $ 28 $ 95 $ 86 PennEast Pipeline 2 1 4 5 Atlantic Coast Pipeline 1 1 4 4 Other 2 2 5 5 Total $ 34 $ 32 $ 108 $ 100 Southern Natural Gas Selected financial information of SNG for the three and nine months ended September 30, 2018 and September 30, 2017 is as follows: Income Statement Information Three Months Ended September 30, 2018 Three Months Ended September 30, 2017 Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 (in millions) Revenues $ 145 $ 146 $ 451 $ 445 Operating income $ 71 $ 71 $ 230 $ 218 Net income $ 58 $ 57 $ 190 $ 172

Segment and Related Information

Segment and Related Information	9 Months Ended
	Sep. 30, 2018
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 develops, 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 its natural gas distribution utilities and is involved in several other complementary businesses including gas marketing services, wholesale gas services, and gas midstream operations. In July 2018, Southern Company Gas completed sales of three of its natural gas distribution utilities. See Note (J) under "Southern Company Gas" for additional information. 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 $134 million and $326 million for the three and nine months ended September 30, 2018 , respectively, and $105 million and $295 million for the three and nine months ended September 30, 2017 , respectively. Revenues from sales of natural gas from Southern Company Gas to the traditional electric operating companies were $14 million and $22 million for the three and nine months ended September 30, 2018 , respectively, and $9 million and $19 million for the three and nine months ended September 30, 2017 , respectively. Revenues from sales of natural gas from Southern Company Gas to Southern Power were $38 million and $96 million for the three and nine months ended September 30, 2018 , respectively, and $38 million and $94 million for the three and nine months ended September 30, 2017 , 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, 2018 and 2017 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 September 30, 2018: Operating revenues $ 5,014 $ 635 $ (140 ) $ 5,509 $ 492 $ 202 $ (44 ) $ 6,159 Segment net income (loss) (a)(b)(c)(d) 1,148 92 — 1,240 46 (119 ) (3 ) 1,164 Nine Months Ended September 30, 2018: Operating revenues $ 13,117 $ 1,699 $ (360 ) $ 14,456 $ 2,861 $ 984 $ (143 ) $ 18,158 Segment net income (loss) (a)(b)(c)(d) 1,711 235 — 1,946 294 (292 ) — 1,948 At September 30, 2018: Goodwill $ — $ 2 $ — $ 2 $ 5,015 $ 298 $ — $ 5,315 Total assets 75,069 15,355 (322 ) 90,102 20,398 3,086 (1,869 ) 111,717 Three Months Ended September 30, 2017: 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 September 30, 2017: Operating revenues $ 12,960 $ 1,597 $ (318 ) $ 14,239 $ 2,841 $ 442 $ (119 ) $ 17,403 Segment net income (loss) (a)(b)(e) — 276 — 276 303 (232 ) — 347 At December 31, 2017: Goodwill $ — $ 2 $ — $ 2 $ 5,967 $ 299 $ — $ 6,268 Total assets 72,204 15,206 (325 ) 87,085 22,987 2,552 (1,619 ) 111,005 (a) Attributable to Southern Company. (b) Segment net income (loss) for the traditional electric operating companies includes pre-tax charges for estimated losses on plants under construction of $1 million ( $1 million after tax) and $34 million ( $21 million after tax) for the three months ended September 30, 2018 and 2017 , respectively, and $1.1 billion ( $0.8 billion after tax) and $3.2 billion ( $2.2 billion after tax) for the nine months ended September 30, 2018 and 2017 , respectively. See Note 3 to the financial statements of Southern Company under "Kemper County Energy Facility" in Item 8 of the Form 10-K and Note (B) under " Nuclear Construction " and " Kemper County Energy Facility " for additional information. (c) Segment net income (loss) for Southern Power includes pre-tax impairment charges of $36 million ( $27 million after tax) and $155 million ( $116 million after tax) for the three and nine months ended September 30, 2018, respectively. See Note (J) under "Southern Power – Development Projects " and " – Sale of Florida Plants " for additional information. (d) Segment net income (loss) for Southern Company Gas includes a net gain on dispositions of $353 million ( $40 million gain after tax) and $317 million ( $35 million loss after tax) for the three and nine months ended September 30, 2018, respectively, related to the Southern Company Gas Dispositions and a goodwill impairment charge of $42 million for the nine months ended September 30, 2018 related to the sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas " for additional information. (e) Segment net income (loss) for the traditional electric operating companies 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 3 to the financial statements of Southern Company under "Regulatory Matters – Gulf Power – Retail Base Rate Cases" in Item 8 of the Form 10-K for additional information. Products and Services Electric Utilities' Revenues Period Retail Wholesale Other Total (in millions) Three Months Ended September 30, 2018 $ 4,605 $ 693 $ 211 $ 5,509 Three Months Ended September 30, 2017 4,615 718 190 5,523 Nine Months Ended September 30, 2018 $ 11,913 $ 1,923 $ 620 $ 14,456 Nine Months Ended September 30, 2017 11,786 1,867 586 14,239 Southern Company Gas' Revenues Period Gas Gas Other Total (in millions) Three Months Ended September 30, 2018 $ 438 $ 44 $ 10 $ 492 Three Months Ended September 30, 2017 430 143 (8 ) 565 Nine Months Ended September 30, 2018 $ 2,276 $ 403 $ 182 $ 2,861 Nine Months Ended September 30, 2017 2,119 597 125 2,841 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. In July 2018, Southern Company Gas sold three of its natural gas distribution utilities, Elizabethtown Gas, Elkton Gas, and Florida City Gas. See Note (J) under "Southern Company Gas" for additional information. Gas marketing services includes natural gas marketing to end-use customers primarily in Georgia and Illinois. On June 4, 2018, Southern Company Gas sold Pivotal Home Solutions. See Note (J) under "Southern Company Gas" for additional information. 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. Business segment financial data for the three and nine months ended September 30, 2018 and 2017 was as follows: Gas Distribution Operations (a)(c) Gas Marketing Services (b)(c) Wholesale Gas Services (d) Gas Midstream Operations Total All Other Eliminations Consolidated (in millions) Three Months Ended September 30, 2018: Operating revenues $ 441 $ 44 $ (8 ) $ 20 $ 497 $ 1 $ (6 ) $ 492 Segment net income (loss) 74 (8 ) (18 ) 16 64 (18 ) — 46 Nine Months Ended September 30, 2018: Operating revenues 2,297 403 142 60 2,902 3 (44 ) 2,861 Segment net income (loss) 290 (71 ) 65 54 338 (44 ) — 294 Total assets at September 30, 2018: 16,850 1,522 855 2,297 21,524 10,146 (11,272 ) 20,398 Three Months Ended September 30, 2017: Operating revenues $ 472 $ 143 $ (24 ) $ 16 $ 607 $ 2 $ (44 ) $ 565 Segment net income (loss) 52 1 (23 ) 14 44 (29 ) — 15 Nine Months Ended September 30, 2017: Operating revenues 2,255 597 95 53 3,000 7 (166 ) 2,841 Segment net income (loss) 223 36 28 38 325 (22 ) — 303 Total assets at December 31, 2017: 19,358 2,147 1,096 2,241 24,842 12,184 (14,039 ) 22,987 (a) Operating revenues for the three gas distribution operations dispositions were $8 million and $50 million for the three months ended September 30, 2018 and 2017 , respectively, and $245 million and $274 million for the nine months ended September 30, 2018 and 2017 , respectively. See Note (J) under " Southern Company Gas " for additional information. (b) Operating revenues for the gas marketing services disposition were $32 million for the three months ended September 30, 2017 and $55 million and $95 million for the nine months ended September 30, 2018 and 2017 , respectively. See Note (J) under " Southern Company Gas " for additional information. (c) Segment net income for gas distribution operations includes a gain on dispositions of $351 million ( $38 million after tax) for the three and nine months ended September 30, 2018 . Segment net income for gas marketing services includes a gain on disposition of $2 million ( $2 million after tax) for the three months ended September 30, 2018 and a loss on disposition of $34 million ( $73 million loss after tax) and a goodwill impairment charge of $42 million for the nine months ended September 30, 2018 recorded in contemplation of the sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas " for additional information. (d) 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) Three Months Ended September 30, 2018 $ 1,573 $ 82 $ 1,655 $ 1,663 $ (8 ) Three Months Ended September 30, 2017 1,411 103 1,514 1,538 (24 ) Nine Months Ended September 30, 2018 $ 4,847 $ 352 $ 5,199 $ 5,057 $ 142 Nine Months Ended September 30, 2017 4,781 362 5,143 5,048 95

Introduction (Policies)

Introduction (Policies)	9 Months Ended
	Sep. 30, 2018
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, 2017 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, 2018 and 2017 . 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.
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 Adopted Accounting Standards	Recently Adopted 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. Revenue 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. ASC 606 became effective on January 1, 2018 and the registrants adopted it using the modified retrospective method applied to open contracts and only to the version of the contracts in effect as of January 1, 2018. In accordance with the modified retrospective method, the registrants' previously issued financial statements have not been restated to comply with ASC 606 and the registrants did not have a cumulative-effect adjustment to retained earnings. The adoption of ASC 606 had no significant impact on the timing of revenue recognition compared to previously reported results; however, it requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenue and the related cash flows arising from contracts with customers, which are included in Note (C). ASC 606 provided additional clarity on financial statement presentation that resulted in reclassifications into other revenues and other operations and maintenance from other income/(expense), net at Alabama Power and Georgia Power related to certain unregulated sales of products and services. In addition, contract assets related to certain fixed retail revenues at Georgia Power and Southern Company's unregulated distributed generation business have been reclassified from unbilled revenue in accordance with the guidance in ASC 606. These reclassifications did not affect the timing or amount of revenues recognized or cash flows. ASC 606 also provided additional guidance on revenue recognized over time, resulting in a change in the timing of revenue recognized from guaranteed and fixed billing arrangements at Southern Company Gas. The changes in natural gas revenues recognized in the third quarter and year-to-date 2018 relate primarily to the seasonal nature of natural gas usage. The net impact of accounting for revenue under ASC 606 decreased Southern Company's and Southern Company Gas' consolidated net income by $4 million for the three months ended September 30, 2018 and increased Southern Company's and Southern Company Gas' consolidated net income by $1 million for the nine months ended September 30, 2018 . The specific impacts of applying ASC 606 to revenues from contracts with customers on the financial statements of Southern Company, Alabama Power, Georgia Power, and Southern Company Gas compared to previously recognized guidance is shown below. For the Three Months Ended For the Nine Months Ended Condensed Statements of Income As Reported Balances Without Adoption of Effect of Change As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) (in millions) Southern Company Natural gas revenues $ 492 $ 497 $ (5 ) $ 2,806 $ 2,805 $ 1 Other revenues 199 198 1 1,007 1,003 4 Other operations and maintenance 1,404 1,387 17 4,217 4,178 39 Operating income 2,174 2,195 (21 ) 3,613 3,647 (34 ) Other income (expense), net 57 41 16 195 160 35 Earnings (loss) before income taxes 1,845 1,850 (5 ) 2,629 2,628 1 Income taxes (benefit) 623 624 (1 ) 598 598 — Consolidated net income (loss) 1,222 1,226 (4 ) 2,031 2,030 1 Consolidated net income (loss) attributable to Southern Company 1,164 1,168 (4 ) 1,948 1,947 1 Alabama Power Other revenues $ 68 $ 59 $ 9 $ 199 $ 173 $ 26 Other operations and maintenance 401 390 11 1,191 1,159 32 Operating income 561 563 (2 ) 1,313 1,319 (6 ) Other income (expense), net 9 7 2 24 18 6 Georgia Power Other revenues $ 121 $ 97 $ 24 $ 349 $ 287 $ 62 Other operations and maintenance 460 437 23 1,325 1,268 57 Operating income (loss) 991 990 1 1,032 1,027 5 Other income (expense), net 30 31 (1 ) 104 109 (5 ) Southern Company Gas Natural gas revenues $ 487 $ 492 $ (5 ) $ 2,829 $ 2,828 $ 1 Operating income 374 379 (5 ) 810 809 1 Earnings before income taxes 362 367 (5 ) 769 768 1 Income taxes 316 317 (1 ) 475 475 — Net income (loss) 46 50 (4 ) 294 293 1 For the Nine Months Ended Condensed Statements of Cash Flows As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) Southern Company Consolidated net income $ 2,031 $ 2,030 $ 1 Changes in certain current assets and liabilities: Receivables 37 27 10 Other current assets (90 ) (80 ) (10 ) Other current liabilities (67 ) (68 ) 1 Georgia Power Changes in certain current assets and liabilities: Receivables $ (205 ) $ (242 ) $ 37 Other current assets (36 ) 1 (37 ) Southern Company Gas Net income $ 294 $ 293 $ 1 Changes in certain current assets and liabilities: Other current liabilities 35 34 1 At September 30, 2018 Condensed Balance Sheets As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) Southern Company Unbilled revenues $ 738 $ 776 $ (38 ) Other accounts and notes receivable 690 691 (1 ) Other current assets 232 193 39 Other current liabilities 763 764 (1 ) Retained earnings 9,048 9,047 1 Georgia Power Unbilled revenues $ 245 $ 310 $ (65 ) Other accounts and notes receivable 96 97 (1 ) Other current assets 91 25 66 Southern Company Gas Other current liabilities 122 123 (1 ) Accumulated deficit (273 ) (274 ) 1 Other In 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (ASU 2016-18). ASU 2016-18 eliminates the need to reflect transfers between cash and restricted cash in operating, investing, and financing activities in the statements of cash flows. In addition, the net change in cash and cash equivalents during the period includes amounts generally described as restricted cash or restricted cash equivalents. The registrants adopted ASU 2016-18 effective January 1, 2018 with no material impact on their financial statements. Southern Company, Southern Power, and Southern Company Gas retrospectively applied ASU 2016-18 effective January 1, 2018 and have restated prior periods in the statements of cash flows by immaterial amounts. The change in restricted cash in the statements of cash flows was previously disclosed in operating activities for Southern Company and Southern Company Gas and in investing activities for Southern Company and Southern Power. See " Restricted Cash " herein for additional information. In March 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 statements of income outside of income from operations. Additionally, only the service cost component is eligible for capitalization, when applicable. The registrants adopted ASU 2017-07 effective January 1, 2018 with no material impact on their financial statements. ASU 2017-07 has been applied retrospectively for the presentation of the service cost component and the other components of net periodic benefit costs in the statements of income for Southern Company, the traditional electric operating companies, and Southern Company Gas. Since Southern Power did not participate in the qualified pension and postretirement benefit plans until December 2017, no retrospective presentation of Southern Power's net periodic benefits costs is required. The requirement to limit capitalization to the service cost component of net periodic benefit costs has been applied on a prospective basis from the date of adoption for all registrants. The presentation changes resulted in a decrease in operating income and an increase in other income for the three and nine months ended September 30, 2018 and 2017 for Southern Company, the traditional electric operating companies, and Southern Company Gas. In August 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities (ASU 2017-12). 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, with early adoption permitted. The registrants adopted ASU 2017-12 effective January 1, 2018 with no material impact on their financial statements. See Note (I) for disclosures required by ASU 2017-12. On February 14, 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (ASU 2018-02) to address the application of ASC 740, Income Taxes (ASC 740) to certain provisions of the Tax Reform Legislation. ASU 2018-02 specifically addresses the ASC 740 requirement that the effect of a change in tax laws or rates on deferred tax assets and liabilities be included in income from continuing operations, even when the tax effects were initially recognized directly in OCI at the previous rate, which strands the income tax rate differential in accumulated OCI. The amendments in ASU 2018-02 allow a reclassification from accumulated OCI to retained earnings for stranded tax effects resulting from the Tax Reform Legislation. The registrants adopted ASU 2018-02 effective January 1, 2018 with no material impact on their financial statements.
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.
Restricted Cash	Restricted Cash The registrants adopted ASU 2016-18 as of January 1, 2018. See " Recently Adopted Accounting Standards – Other " herein for additional information. At December 31, 2017 , Southern Power had restricted cash primarily related to certain acquisitions and construction projects. At both September 30, 2018 and December 31, 2017 , Southern Company Gas had restricted cash held as collateral for worker's compensation, life insurance, and long-term disability insurance.
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, 2018 . 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.
Hypothetical Liquidation at Book Value	Hypothetical Liquidation at Book Value Southern Power has consolidated renewable generation projects that are partially funded by a third-party tax equity investor. The related contractual provisions represent profit-sharing arrangements because the allocations of cash distributions and tax benefits are not based on fixed ownership percentages. Therefore, the noncontrolling interest is accounted for under a balance sheet approach utilizing the hypothetical liquidation at book value (HLBV) method. The HLBV method calculates each partner's share of income based on the change in net equity the partner can legally claim in a hypothetical liquidation at the end of the period compared to the beginning of the period.
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 (I) for additional information on how these derivatives are used. For fair value measurements of the investments within the nuclear decommissioning trusts and the non-qualified deferred compensation 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. The NRC requires licensees of commissioned nuclear power reactors to establish a plan for providing reasonable assurance of funds for future decommissioning. 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 primarily obligated to make generation-based payments to the seller, which commenced at the commercial operation date of the respective facility and continue through 2026. 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. 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 stock-based compensation plans. See Note 8 to the financial statements of Southern Company in Item 8 of the Form 10-K for information on stock-based compensation plans. The effect of stock-based compensation plans was determined using the treasury stock method.
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 (D) 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. The registrants adopted ASU 2017-12 as of January 1, 2018. See Note (A) under " Recently Adopted Accounting Standards – Other " for additional information. 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 approved a moratorium on Gulf Power's fuel-hedging program until January 1, 2021. 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 operating revenues. 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 and in the same income statement line item as the earnings effect of the hedged transactions. • 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. 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 derivatives' fair value gains or losses are recorded in OCI and are reclassified into earnings at the same time and on the same income statement line as the earnings effect of the hedged transactions, including foreign currency gains or losses arising from changes in the U.S. currency exchange rates. The derivatives employed as hedging instruments are structured to minimize ineffectiveness. 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 derivatives' fair value gains or losses are recorded in OCI and are reclassified into earnings at the same time and presented on the same income statement line item as the earnings effect of the hedged transactions. 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 on the same income statement line item. 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.

Introduction (Tables)

Introduction (Tables)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The specific impacts of applying ASC 606 to revenues from contracts with customers on the financial statements of Southern Company, Alabama Power, Georgia Power, and Southern Company Gas compared to previously recognized guidance is shown below. For the Three Months Ended For the Nine Months Ended Condensed Statements of Income As Reported Balances Without Adoption of Effect of Change As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) (in millions) Southern Company Natural gas revenues $ 492 $ 497 $ (5 ) $ 2,806 $ 2,805 $ 1 Other revenues 199 198 1 1,007 1,003 4 Other operations and maintenance 1,404 1,387 17 4,217 4,178 39 Operating income 2,174 2,195 (21 ) 3,613 3,647 (34 ) Other income (expense), net 57 41 16 195 160 35 Earnings (loss) before income taxes 1,845 1,850 (5 ) 2,629 2,628 1 Income taxes (benefit) 623 624 (1 ) 598 598 — Consolidated net income (loss) 1,222 1,226 (4 ) 2,031 2,030 1 Consolidated net income (loss) attributable to Southern Company 1,164 1,168 (4 ) 1,948 1,947 1 Alabama Power Other revenues $ 68 $ 59 $ 9 $ 199 $ 173 $ 26 Other operations and maintenance 401 390 11 1,191 1,159 32 Operating income 561 563 (2 ) 1,313 1,319 (6 ) Other income (expense), net 9 7 2 24 18 6 Georgia Power Other revenues $ 121 $ 97 $ 24 $ 349 $ 287 $ 62 Other operations and maintenance 460 437 23 1,325 1,268 57 Operating income (loss) 991 990 1 1,032 1,027 5 Other income (expense), net 30 31 (1 ) 104 109 (5 ) Southern Company Gas Natural gas revenues $ 487 $ 492 $ (5 ) $ 2,829 $ 2,828 $ 1 Operating income 374 379 (5 ) 810 809 1 Earnings before income taxes 362 367 (5 ) 769 768 1 Income taxes 316 317 (1 ) 475 475 — Net income (loss) 46 50 (4 ) 294 293 1 For the Nine Months Ended Condensed Statements of Cash Flows As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) Southern Company Consolidated net income $ 2,031 $ 2,030 $ 1 Changes in certain current assets and liabilities: Receivables 37 27 10 Other current assets (90 ) (80 ) (10 ) Other current liabilities (67 ) (68 ) 1 Georgia Power Changes in certain current assets and liabilities: Receivables $ (205 ) $ (242 ) $ 37 Other current assets (36 ) 1 (37 ) Southern Company Gas Net income $ 294 $ 293 $ 1 Changes in certain current assets and liabilities: Other current liabilities 35 34 1 At September 30, 2018 Condensed Balance Sheets As Reported Balances Without Adoption of ASC 606 Effect of Change (in millions) Southern Company Unbilled revenues $ 738 $ 776 $ (38 ) Other accounts and notes receivable 690 691 (1 ) Other current assets 232 193 39 Other current liabilities 763 764 (1 ) Retained earnings 9,048 9,047 1 Georgia Power Unbilled revenues $ 245 $ 310 $ (65 ) Other accounts and notes receivable 96 97 (1 ) Other current assets 91 25 66 Southern Company Gas Other current liabilities 122 123 (1 ) Accumulated deficit (273 ) (274 ) 1
Schedule of Change in Asset Retirement Obligation	As of September 30, 2018 , details of the AROs, including those related to the CCR Rule, included in the condensed balance sheets of Southern Company, Alabama Power, Georgia Power, Gulf Power, and Mississippi Power were as follows: Southern Company Alabama Power Georgia Power Gulf Mississippi Power (in millions) Balance at December 31, 2017 $ 4,824 $ 1,709 $ 2,638 $ 142 $ 174 Liabilities incurred 2 — — — — Liabilities settled (160 ) (31 ) (82 ) (23 ) (22 ) Accretion 153 72 70 3 4 Cash flow revisions 1,510 1,451 (32 ) 42 21 Reclassification to held for sale (164 ) — — — — Balance at September 30, 2018 $ 6,165 $ 3,201 $ 2,594 $ 164 $ 177
Estimated Cost Of Decommissioning	In June 2018, Alabama Power completed an updated decommissioning cost site study for Plant Farley. The estimated costs of decommissioning based on the 2018 site study are as follows: Decommissioning periods: Beginning year 2037 Completion year 2076 (in millions) Site study costs: Radiated structures $ 1,621 Non-radiated structures 99 Total site study costs $ 1,720
Schedule of Goodwill	The following table presents year-to-date changes in goodwill balances for Southern Company and Southern Company Gas: Goodwill Southern Company Southern Company Gas Gas Distribution Operations Gas Marketing Services Total (in millions) Balance at December 31, 2017 $ 6,268 $ 4,702 $ 1,265 $ 5,967 Impairment (a) (42 ) — (42 ) (42 ) Dispositions (b) (910 ) (668 ) (242 ) (910 ) Balance at September 30, 2018 $ 5,315 (c) $ 4,034 $ 981 $ 5,015 (a) On April 11, 2018, Southern Company Gas entered into a stock purchase agreement for the sale of Pivotal Home Solutions. In contemplation of the transaction, a goodwill impairment charge of $42 million was recorded in the first quarter 2018. See Note (J) under " Southern Company Gas " for additional information. (b) Gas distribution operations reflects goodwill allocated to Elizabethtown Gas, Elkton Gas, and Florida City Gas, which were sold during the third quarter 2018. Gas marketing services reflects goodwill associated with Pivotal Home Solutions, which was sold on June 4, 2018. See Note (J) under " Southern Company Gas " for additional information. (c) Total does not add due to rounding.
Schedule of Other Intangible Assets, Finite-Lived	Other intangible assets were as follows: At September 30, 2018 At December 31, 2017 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 (*) $ 223 $ (87 ) $ 136 $ 288 $ (83 ) $ 205 Trade names (*) 70 (18 ) 52 159 (17 ) 142 Storage and transportation contracts 64 (49 ) 15 64 (34 ) 30 PPA fair value adjustments 456 (66 ) 390 456 (47 ) 409 Other 11 (5 ) 6 17 (5 ) 12 Total other intangible assets subject to amortization $ 824 $ (225 ) $ 599 $ 984 $ (186 ) $ 798 Other intangible assets not subject to amortization: Federal Communications Commission licenses 75 — 75 75 — 75 Total other intangible assets $ 899 $ (225 ) $ 674 $ 1,059 $ (186 ) $ 873 Southern Power Other intangible assets subject to amortization: PPA fair value adjustments $ 456 $ (66 ) $ 390 $ 456 $ (47 ) $ 409 Southern Company Gas Other intangible assets subject to amortization: Gas marketing services (*) Customer relationships $ 156 $ (78 ) $ 78 $ 221 $ (77 ) $ 144 Trade names 26 (6 ) 20 115 (9 ) 106 Wholesale gas services Storage and transportation contracts 64 (49 ) 15 64 (34 ) 30 Total other intangible assets subject to amortization $ 246 $ (133 ) $ 113 $ 400 $ (120 ) $ 280 (*) Balances as of September 30, 2018 reflect Southern Company Gas' sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas – Sale of Pivotal Home Solutions " for additional information.
Schedule of Other Intangible Assets, Indefinite-Lived	Other intangible assets were as follows: At September 30, 2018 At December 31, 2017 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 (*) $ 223 $ (87 ) $ 136 $ 288 $ (83 ) $ 205 Trade names (*) 70 (18 ) 52 159 (17 ) 142 Storage and transportation contracts 64 (49 ) 15 64 (34 ) 30 PPA fair value adjustments 456 (66 ) 390 456 (47 ) 409 Other 11 (5 ) 6 17 (5 ) 12 Total other intangible assets subject to amortization $ 824 $ (225 ) $ 599 $ 984 $ (186 ) $ 798 Other intangible assets not subject to amortization: Federal Communications Commission licenses 75 — 75 75 — 75 Total other intangible assets $ 899 $ (225 ) $ 674 $ 1,059 $ (186 ) $ 873 Southern Power Other intangible assets subject to amortization: PPA fair value adjustments $ 456 $ (66 ) $ 390 $ 456 $ (47 ) $ 409 Southern Company Gas Other intangible assets subject to amortization: Gas marketing services (*) Customer relationships $ 156 $ (78 ) $ 78 $ 221 $ (77 ) $ 144 Trade names 26 (6 ) 20 115 (9 ) 106 Wholesale gas services Storage and transportation contracts 64 (49 ) 15 64 (34 ) 30 Total other intangible assets subject to amortization $ 246 $ (133 ) $ 113 $ 400 $ (120 ) $ 280 (*) Balances as of September 30, 2018 reflect Southern Company Gas' sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas – Sale of Pivotal Home Solutions " for additional information.
Finite-lived Intangible Assets Amortization Expense	Amortization associated with other intangible assets was as follows: Three Months Ended Nine Months Ended September 30, 2018 (in millions) Southern Company $ 21 $ 70 Southern Power $ 6 $ 19 Southern Company Gas $ 12 $ 42
Schedule of Cash and Cash Equivalents	The following tables provide a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that total to the amounts shown in the condensed statements of cash flows for the registrants that had restricted cash at September 30, 2018 and/or December 31, 2017 : Southern Company Southern Company Gas (in millions) At September 30, 2018 Cash and cash equivalents $ 1,847 $ 56 Cash and cash equivalents classified as assets held for sale 37 — Restricted cash: Other accounts and notes receivable 6 6 Total cash, cash equivalents, and restricted cash $ 1,891 (*) $ 62 (*) Total does not add due to rounding. Southern Company Southern Power Southern Company Gas (in millions) At December 31, 2017 Cash and cash equivalents $ 2,130 $ 129 $ 73 Restricted cash: Other accounts and notes receivable 5 — 5 Deferred charges and other assets 12 11 — Total cash, cash equivalents, and restricted cash $ 2,147 $ 140 $ 78
Restrictions on Cash and Cash Equivalents	The following tables provide a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that total to the amounts shown in the condensed statements of cash flows for the registrants that had restricted cash at September 30, 2018 and/or December 31, 2017 : Southern Company Southern Company Gas (in millions) At September 30, 2018 Cash and cash equivalents $ 1,847 $ 56 Cash and cash equivalents classified as assets held for sale 37 — Restricted cash: Other accounts and notes receivable 6 6 Total cash, cash equivalents, and restricted cash $ 1,891 (*) $ 62 (*) Total does not add due to rounding. Southern Company Southern Power Southern Company Gas (in millions) At December 31, 2017 Cash and cash equivalents $ 2,130 $ 129 $ 73 Restricted cash: Other accounts and notes receivable 5 — 5 Deferred charges and other assets 12 11 — Total cash, cash equivalents, and restricted cash $ 2,147 $ 140 $ 78

Contingencies and Regulatory _2

Contingencies and Regulatory Matters (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Cost Recovery Clauses	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 under recovered regulatory clause revenues $ — $ 17 Under recovered regulatory clause revenues 7 — Rate CNP PPA Deferred under recovered regulatory clause revenues 30 12 Retail Energy Cost Recovery Deferred under recovered regulatory clause revenues 58 25 Under recovered regulatory clause revenues 41 — Natural Disaster Reserve Other regulatory liabilities, deferred 24 38 Regulatory clause recovery balances included in the balance sheets are as follows: Regulatory Clause Balance Sheet Line Item September 30, December 31, (in millions) Fuel Cost Recovery Under recovered regulatory clause revenues $ — $ 22 Fuel Cost Recovery Other regulatory liabilities, current 23 — Purchased Power Capacity Recovery Other regulatory liabilities, current 4 — Purchased Power Capacity Recovery Under recovered regulatory clause revenues — 2 Environmental Cost Recovery (*) Other regulatory liabilities, current 13 — Environmental Cost Recovery (*) Under recovered regulatory clause revenues — 2 Energy Conservation Cost Recovery Other regulatory liabilities, current 2 — (*) At September 30, 2018 and December 31, 2017 , the over and under recovered balances, respectively, included in the balance sheets represents the current portion of the regulatory assets associated with projected environmental expenditures of approximately $8 million and $13 million , respectively, net of the over recovered environmental cost recovery balance of approximately $21 million and $11 million , respectively.
Cost and Schedule	Georgia Power's approximate proportionate share of the remaining estimated capital cost to complete Plant Vogtle Units 3 and 4 by the expected in-service dates of November 2021 and November 2022, respectively, is as follows: (in billions) Base project capital cost forecast (a)(b) $ 8.0 Construction contingency estimate 0.4 Total project capital cost forecast (a)(b) 8.4 Net investment as of September 30, 2018 (b) (4.3 ) Remaining estimate to complete (a) $ 4.1 (a) Excludes financing costs expected to be capitalized through AFUDC of approximately $350 million . (b) Net of $1.7 billion received from Toshiba under the Guarantee Settlement Agreement and approximately $188 million in related Customer Refunds.

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	The following tables disaggregate revenue sources for the three and nine months ended September 30, 2018 : For the Three Months Ended September 30, 2018 For the Nine Months Ended September 30, 2018 (in millions) Southern Company Operating revenues Retail electric revenues (a) Residential $ 2,148 $ 5,266 Commercial 1,527 4,084 Industrial 901 2,471 Other 29 92 Natural gas distribution revenues 433 2,299 Alternative revenue programs (b) 5 (23 ) Total retail electric and gas distribution revenues $ 5,043 $ 14,189 Wholesale energy revenues (c)(d) 516 1,444 Wholesale capacity revenues (d) 177 479 Other natural gas revenues (e) 54 530 Other revenues (f) 369 1,516 Total operating revenues $ 6,159 $ 18,158 (a) Retail electric revenues include $17 million and $54 million of leases for the three and nine months ended September 30, 2018 , respectively, and a (net reduction) or net increase of $(98) million and $4 million for the three and nine months ended September 30, 2018 , respectively, from certain cost recovery mechanisms that are not accounted for as revenue under ASC 606. See Note 3 to the financial statements of Southern Company under "Regulatory Matters" in Item 8 of the Form 10-K for additional information on cost recovery mechanisms. (b) See Note 1 to the financial statements of Southern Company under "Revenues" in Item 8 of the Form 10-K for additional information on alternative revenue programs at the natural gas distribution utilities. Alternative revenue program revenues are presented net of any previously recognized program amounts billed to customers during the same accounting period. (c) Wholesale energy revenues include $63 million and $217 million for the three and nine months ended September 30, 2018 , respectively, of revenues accounted for as derivatives, primarily related to physical energy sales in the wholesale electricity market. See Note (I) for additional information on energy-related derivative contracts. (d) Wholesale energy and wholesale capacity revenues include $130 million and $31 million , respectively, for the three months ended September 30, 2018 and $318 million and $92 million , respectively, for the nine months ended September 30, 2018 of PPA contracts accounted for as leases. (e) Other natural gas revenues related to Southern Company Gas' energy and risk management activities are presented net of the related costs of those activities and include gross third-party revenues of $1.6 billion and $4.8 billion for the three and nine months ended September 30, 2018 , respectively, of which $0.9 billion and $2.7 billion , respectively, relates to contracts that are accounted for as derivatives. See Note (L) under " Southern Company Gas " for additional information on the components of wholesale gas services operating revenues. (f) Other revenues include $92 million and $274 million for the three and nine months ended September 30, 2018 , respectively, of revenues not accounted for under ASC 606. Alabama Power Georgia Power Gulf Power Mississippi Power (in millions) For the Three Months Ended September 30, 2018 Operating revenues Retail revenues (a)(b) Residential $ 721 $ 1,142 $ 200 $ 85 Commercial 464 877 103 82 Industrial 392 385 37 86 Other 7 21 1 1 Total retail electric revenues $ 1,584 $ 2,425 $ 341 $ 254 Wholesale energy revenues (c) 62 33 48 92 Wholesale capacity revenues 26 14 7 1 Other revenues (b)(d) 68 121 18 11 Total operating revenues $ 1,740 $ 2,593 $ 414 $ 358 For the Nine Months Ended September 30, 2018 Operating revenues Retail revenues (a)(b) Residential $ 1,848 $ 2,671 $ 537 $ 209 Commercial 1,238 2,343 291 212 Industrial 1,103 1,036 100 233 Other 19 62 4 6 Total retail electric revenues $ 4,208 $ 6,112 $ 932 $ 660 Wholesale energy revenues (c) 234 99 104 259 Wholesale capacity revenues 75 41 20 6 Other revenues (b)(d) 199 349 50 31 Total operating revenues $ 4,716 $ 6,601 $ 1,106 $ 956 (a) Retail revenues at Alabama Power, Georgia Power, Gulf Power, and Mississippi Power include a net increase or (net reduction) of $(12) million , $(47) million , $(36) million , and $(3) million , respectively, for the three months ended September 30, 2018 and $113 million , $(35) million , $(63) million , and $(11) million , respectively, for the nine months ended September 30, 2018 related to certain cost recovery mechanisms that are not accounted for as revenue under ASC 606. See Note 3 to the financial statements of Alabama Power, Georgia Power, Gulf Power, and Mississippi Power under "Retail Regulatory Matters" in Item 8 of the Form 10-K for additional information on cost recovery mechanisms. (b) Retail revenues and other revenues at Georgia Power include $17 million and $34 million , respectively, for the three months ended September 30, 2018 and $54 million and $100 million , respectively, for the nine months ended September 30, 2018 of revenues accounted for as leases. (c) Wholesale energy revenues at Alabama Power and Georgia Power include $6 million and $8 million , respectively, for the three months ended September 30, 2018 and $14 million and $21 million , respectively, for the nine months ended September 30, 2018 accounted for as derivatives primarily related to physical energy sales in the wholesale electricity market. See Note (I) for additional information on energy-related derivative contracts. (d) Other revenues at Alabama Power, Georgia Power, and Gulf Power include $27 million , $28 million , and $2 million , respectively, for the three months ended September 30, 2018 and $79 million , $80 million , and $5 million , respectively, for the nine months ended September 30, 2018 of revenues not accounted for under ASC 606. For the Three Months Ended September 30, 2018 For the Nine Months Ended September 30, 2018 (in millions) Southern Power PPA capacity revenues (a) $ 168 $ 450 PPA energy revenues (a) 336 892 Non-PPA revenues (b) 126 347 Other revenues 5 10 Total operating revenues $ 635 $ 1,699 (a) PPA capacity revenues and PPA energy revenues include $47 million and $139 million , respectively, for the three months ended September 30, 2018 and $141 million and $342 million , respectively, for the nine months ended September 30, 2018 related to PPAs accounted for as leases. See Note 1 to the financial statements of Southern Power under "Revenues" in Item 8 of the Form 10-K for additional information on capacity revenues accounted for as leases. (b) Non-PPA revenues include $47 million and $176 million for the three and nine months ended September 30, 2018 , respectively, of revenues from short-term sales related to physical energy sales from uncovered capacity in the wholesale electricity market. See Note 1 to the financial statements of Southern Power under "Revenues" in Item 8 of the Form 10-K and Note (I) for additional information on energy-related derivative contracts. For the Three Months Ended September 30, 2018 For the Nine Months Ended September 30, 2018 (in millions) Southern Company Gas Operating revenues Natural gas distribution revenues Residential $ 149 $ 1,082 Commercial 45 313 Transportation 203 708 Industrial 4 28 Other 32 168 Alternative revenue programs (a) 5 (23 ) Total natural gas distribution revenues $ 438 $ 2,276 Gas marketing services (b) 44 403 Wholesale gas services (c) (10 ) 121 Gas midstream operations 20 60 Other revenues — 1 Total operating revenues $ 492 $ 2,861 (a) See Note 1 to the financial statements of Southern Company Gas under "Revenues" in Item 8 of the Form 10-K for additional information on alternative revenue programs at the natural gas distribution utilities. Alternative revenue program revenues are presented net of any previously recognized program amounts billed to customers during the same accounting period. (b) Gas marketing services includes $4 million for the nine months ended September 30, 2018 of revenues not accounted for under ASC 606. (c) Wholesale gas services revenues are presented net of the related costs associated with its energy trading and risk management activities. Operating revenues, as presented, include gross third-party revenues of $1.6 billion and $4.8 billion for the three and nine months ended September 30, 2018 , respectively, of which $0.9 billion and $2.7 billion , respectively, relates to contracts that are accounted for as derivatives. See Note (L) under " Southern Company Gas " for additional information on the components of wholesale gas services operating revenues and Note (I) for additional information on energy-related derivative contracts.
Contract with Customer, Asset and Liability	The following table reflects the closing balances of receivables, contract assets, and contract liabilities related to revenues from contracts with customers as of September 30, 2018 : Receivables Contract Assets Contract Liabilities (in millions) Southern Company $ 2,778 $ 99 $ 34 Alabama Power 649 1 14 Georgia Power 924 70 3 Gulf Power 186 — — Mississippi Power 96 — — Southern Power 142 — 17 Southern Company Gas 523 — 1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction	Revenues from contracts with customers related to these performance obligations remaining at September 30, 2018 are expected to be recognized as follows: 2018 2019 2020 2021 2022 2023 and Thereafter (in millions) Southern Company (*) $ 168 $ 406 $ 322 $ 322 $ 310 $ 2,112 Alabama Power 6 22 22 26 23 161 Georgia Power 10 41 38 40 30 113 Gulf Power 5 22 — — — — Mississippi Power 1 3 3 1 — — Southern Power (*) 75 310 283 277 276 2,005 (*) Excludes amounts related to held for sale assets. See Note (J) under " Southern Company's Sale of Gulf Power " and " Southern Power – Sale of Florida Plants " for additional information.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Assets and Liabilities Measured at Fair Value on a Recurring Basis	As of September 30, 2018 , 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, 2018: 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) $ 271 $ 150 $ — $ — $ 421 Foreign currency derivatives — 122 — — 122 Nuclear decommissioning trusts (c) 828 1,007 — 37 1,872 Non-qualified deferred compensation trusts: Domestic equity — 11 — — 11 Foreign equity — 6 — — 6 Pooled funds – fixed income — 13 — — 13 Cash equivalents 15 — — — 15 Other 9 — — — 9 Cash equivalents 1,309 — — — 1,309 Total $ 2,432 $ 1,309 $ — $ 37 $ 3,778 Liabilities: Energy-related derivatives (a)(b) $ 416 $ 159 $ — $ — $ 575 Interest rate derivatives — 72 — — 72 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 22 — 22 Total $ 416 $ 254 $ 22 $ — $ 692 Fair Value Measurements Using: As of September 30, 2018: 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) Alabama Power Assets: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Nuclear decommissioning trusts: (d) Domestic equity 469 89 — — 558 Foreign equity 60 56 — — 116 U.S. Treasury and government agency securities — 18 — — 18 Corporate bonds 26 154 — — 180 Mortgage and asset backed securities — 22 — — 22 Private equity — — — 37 37 Other 7 — — — 7 Cash equivalents 513 — — — 513 Total $ 1,075 $ 346 $ — $ 37 $ 1,458 Liabilities: Energy-related derivatives $ — $ 10 $ — $ — $ 10 Georgia Power Assets: Energy-related derivatives $ — $ 8 $ — $ — $ 8 Nuclear decommissioning trusts: (d)(e) Domestic equity 250 1 — — 251 Foreign equity — 134 — — 134 U.S. Treasury and government agency securities — 236 — — 236 Municipal bonds — 82 — — 82 Corporate bonds — 163 — — 163 Mortgage and asset backed securities — 42 — — 42 Other 16 9 — — 25 Cash equivalents 350 — — — 350 Total $ 616 $ 675 $ — $ — $ 1,291 Liabilities: Energy-related derivatives $ — $ 22 $ — $ — $ 22 Interest rate derivatives — 6 — — 6 Total $ — $ 28 $ — $ — $ 28 Gulf Power Assets: Cash equivalents $ 27 $ — $ — $ — $ 27 Liabilities: Energy-related derivatives $ — $ 8 $ — $ — $ 8 Fair Value Measurements Using: As of September 30, 2018: 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) Mississippi Power Assets: Energy-related derivatives $ — $ 3 $ — $ — $ 3 Cash equivalents 346 — — — 346 Total $ 346 $ 3 $ — $ — $ 349 Liabilities: Energy-related derivatives $ — $ 9 $ — $ — $ 9 Southern Power Assets: Energy-related derivatives $ — $ 3 $ — $ — $ 3 Foreign currency derivatives — 122 — — 122 Total $ — $ 125 $ — $ — $ 125 Liabilities: Energy-related derivatives $ — $ 7 $ — $ — $ 7 Foreign currency derivatives — 23 — — 23 Contingent consideration — — 22 — 22 Total $ — $ 30 $ 22 $ — $ 52 Southern Company Gas Assets: Energy-related derivatives (a)(b) $ 271 $ 129 $ — $ — $ 400 Non-qualified deferred compensation trusts: Domestic equity — 11 — — 11 Foreign equity — 6 — — 6 Pooled funds – fixed income — 13 — — 13 Cash equivalents 4 — — — 4 Cash equivalents 26 — — — 26 Total $ 301 $ 159 $ — $ — $ 460 Liabilities: Energy-related derivatives (a)(b) $ 416 $ 101 $ — $ — $ 517 (a) Excludes $5 million associated with premiums and certain weather derivatives accounted for based on intrinsic value rather than fair value. (b) Excludes cash collateral of $189 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, 2018 , approximately $37 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.
Schedule of Increase (Decrease) In Fair Value Of Funds	The fair value of the funds, including reinvested interest and dividends and excluding the funds' expenses, increased for the three and nine months ended September 30, 2018 and 2017 by the amounts shown in the table below. The increases were recorded as a change to the regulatory assets and liabilities related to AROs for Georgia Power and Alabama Power, respectively. Three Months Ended September 30, 2018 Three Months Ended September 30, 2017 Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 (in millions) Southern Company $ 58 $ 50 $ 68 $ 168 Alabama Power 39 25 49 87 Georgia Power 19 25 19 81
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, 2018 , the fair value measurements of private equity investments held in the nuclear decommissioning trusts 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, 2018: Fair Value Unfunded Commitments (in millions) Southern Company $ 37 $ 47 Alabama Power $ 37 $ 47
Financial Instruments for Which Carrying Amount Did Not Equal Fair Value	As of September 30, 2018 , 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 $ 45,524 $ 45,500 Alabama Power 8,120 8,321 Georgia Power 10,227 10,159 Gulf Power 1,285 1,290 Mississippi Power 1,736 1,702 Southern Power 5,029 5,058 Southern Company Gas 5,908 5,935

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Earnings per Share	Shares used to compute diluted earnings per share were as follows: Three Months Ended Three Months Ended Nine Months Ended Nine Months Ended (in millions) As reported shares 1,023 1,003 1,016 998 Effect of stock-based compensation 6 7 5 7 Diluted shares 1,029 1,010 1,021 1,005
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 (a) (in thousands) (in millions) Balance at December 31, 2017 1,008,532 (929 ) $ 24,167 $ — $ 1,361 $ 25,528 Consolidated net income attributable to Southern Company — — 1,948 — — 1,948 Other comprehensive income — — 52 — — 52 Stock issued 21,342 — 878 — — 878 Stock-based compensation — — 74 — — 74 Cash dividends on common stock — — (1,805 ) — — (1,805 ) Contributions from noncontrolling interests — — — — 154 154 Distributions to noncontrolling interests — — — — (87 ) (87 ) Net income attributable to noncontrolling interests — — — — 71 71 Sale of noncontrolling interests (b) — — (410 ) — 1,690 1,280 Other — (57 ) (27 ) — (1 ) (28 ) Balance at September 30, 2018 1,029,874 (986 ) $ 24,877 $ — $ 3,188 $ 28,065 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 (a) Primarily related to Southern Power and excludes redeemable noncontrolling interests. See Note 10 to the financial statements of Southern Power in Item 8 of the Form 10-K for additional information. (b) See Note (J) under "Southern Power – Sale of Solar Facility Interests " for additional information.

Financing (Tables)

Financing (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Credit Arrangements by Company	The following table outlines the committed credit arrangements by company as of September 30, 2018 : Expires Executable Term Loans Expires Within One Year Company 2018 2019 2020 2022 Total Unused One Year Term Out No Term Out (in millions) Southern Company (a) $ — $ — $ — $ 2,000 $ 2,000 $ 1,999 $ — $ — $ — Alabama Power — 33 500 800 1,333 1,333 — — 33 Georgia Power — — — 1,750 1,750 1,736 — — — Gulf Power 20 25 235 — 280 280 45 45 — Mississippi Power — 100 — — 100 100 — — — Southern Power Company (b) — — — 750 750 728 — — — Southern Company Gas (c) — — — 1,900 1,900 1,895 — — — Other — 30 — — 30 30 — — 30 Southern Company Consolidated $ 20 $ 188 $ 735 $ 7,200 $ 8,143 $ 8,101 $ 45 $ 45 $ 63 (a) Represents the Southern Company parent entity. (b) Does not include Southern Power Company's $120 million continuing letter of credit facility for standby letters of credit expiring in 2019, of which $22 million remains unused at September 30, 2018 . (c) Southern Company Gas, as the parent entity, guarantees the obligations of Southern Company Gas Capital, which is the borrower of $1.4 billion of these arrangements. Southern Company Gas' committed credit arrangements also include $500 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 2018 : Company Senior Note Issuances Senior Note Maturities, Redemptions, and Repurchases Revenue Bond Other Long-Term Debt Issuances Other Long-Term Debt Redemptions and Maturities (a) (in millions) Southern Company (b) $ 750 $ 1,000 $ — $ — $ — Alabama Power 500 — — — — Georgia Power — 1,000 469 — 107 Mississippi Power 600 — 43 — 900 Southern Power — 350 — — 420 Southern Company Gas — — 200 100 — Other — — — — 10 Elimination (c) — — — — (1 ) Southern Company Consolidated $ 1,850 $ 2,350 $ 712 $ 100 $ 1,436 (a) Includes reductions in capital lease obligations resulting from cash payments under capital leases. (b) Represents the Southern Company parent entity. (c) Represents reductions in affiliate capital lease obligations at Georgia Power, which are eliminated in Southern Company's Consolidated Financial Statements.

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [Abstract]
Pension Plans and Postretirement Plans	Components of the net periodic benefit costs for the three and nine months ended September 30, 2018 and 2017 are presented in the following tables. Three Months Ended September 30, 2018 Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Power Southern Company Gas (in millions) Pension Plans Service cost $ 90 $ 19 $ 22 $ 4 $ 5 $ 3 $ 8 Interest cost 116 26 34 5 5 1 10 Expected return on plan assets (236 ) (51 ) (74 ) (10 ) (11 ) (3 ) (18 ) Amortization: Prior service costs 1 — 1 — — — (1 ) Regulatory asset — — — — — — 4 Net (gain)/loss 53 13 18 2 3 — 3 Net periodic pension cost (income) $ 24 $ 7 $ 1 $ 1 $ 2 $ 1 $ 6 Postretirement Benefits Service cost $ 6 $ 1 $ 2 $ — $ — $ 1 $ — Interest cost 19 5 7 1 — — 2 Expected return on plan assets (17 ) (7 ) (6 ) — — — (1 ) Amortization: Prior service costs 2 1 — — — — — Regulatory asset — — — — — — 2 Net (gain)/loss 3 — 2 — — — — Net periodic postretirement benefit cost $ 13 $ — $ 5 $ 1 $ — $ 1 $ 3 Nine Months Ended September 30, 2018 Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Power Southern Company Gas (in millions) Pension Plans Service cost $ 269 $ 58 $ 65 $ 12 $ 13 $ 7 $ 24 Interest cost 348 76 104 15 15 4 29 Expected return on plan assets (707 ) (155 ) (222 ) (30 ) (31 ) (8 ) (53 ) Amortization: Prior service costs 3 1 2 — — — (2 ) Regulatory asset — — — — — — 11 Net (gain)/loss 160 40 52 7 8 1 9 Net periodic pension cost (income) $ 73 $ 20 $ 1 $ 4 $ 5 $ 4 $ 18 Postretirement Benefits Service cost $ 18 $ 4 $ 5 $ 1 $ 1 $ 1 $ 1 Interest cost 56 13 21 2 2 — 7 Expected return on plan assets (51 ) (20 ) (19 ) (1 ) (1 ) — (5 ) Amortization: Prior service costs 5 3 1 — — — — Regulatory asset — — — — — — 5 Net (gain)/loss 10 1 6 — — — — Net periodic postretirement benefit cost $ 38 $ 1 $ 14 $ 2 $ 2 $ 1 $ 8 Three Months Ended September 30, 2017 (*) Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (in millions) Pension Plans Service cost $ 73 $ 15 $ 19 $ 3 $ 4 $ 6 Interest cost 114 25 34 5 5 10 Expected return on plan assets (224 ) (49 ) (71 ) (10 ) (9 ) (18 ) Amortization: Prior service costs 3 1 — — — — Net (gain)/loss 41 10 15 2 1 5 Net periodic pension cost (income) $ 7 $ 2 $ (3 ) $ — $ 1 $ 3 Postretirement Benefits Service cost $ 6 $ 1 $ 2 $ — $ — $ 1 Interest cost 19 4 6 1 1 3 Expected return on plan assets (16 ) (5 ) (6 ) — — (2 ) Amortization: Prior service costs 2 1 — — — (1 ) Net (gain)/loss 3 — 3 — — 1 Net periodic postretirement benefit cost $ 14 $ 1 $ 5 $ 1 $ 1 $ 2 (*) Excludes Southern Power since Southern Power did not participate in the qualified pension and postretirement benefit plans until December 2017. Nine Months Ended September 30, 2017 (*) Southern Company Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (in millions) Pension Plans Service cost $ 220 $ 47 $ 56 $ 10 $ 11 $ 17 Interest cost 341 73 103 15 15 30 Expected return on plan assets (673 ) (147 ) (212 ) (29 ) (29 ) (53 ) Amortization: Prior service costs 9 2 2 — 1 (1 ) Net (gain)/loss 122 31 43 5 5 15 Net periodic pension cost (income) $ 19 $ 6 $ (8 ) $ 1 $ 3 $ 8 Postretirement Benefits Service cost $ 18 $ 4 $ 5 $ 1 $ 1 $ 2 Interest cost 59 13 21 2 3 8 Expected return on plan assets (49 ) (19 ) (18 ) (1 ) (1 ) (5 ) Amortization: Prior service costs 5 3 1 — — (2 ) Net (gain)/loss 10 1 6 — — 3 Net periodic postretirement benefit cost $ 43 $ 2 $ 15 $ 2 $ 3 $ 6 (*) Excludes Southern Power since Southern Power did not participate in the qualified pension and postretirement benefit plans until December 2017.

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Summary of Valuation Allowances	Valuation Allowances Georgia Power Mississippi Power Southern Company Gas Southern Company (in millions) Federal $ 6 $ — $ 11 $ 19 State (net of federal benefit) 33 124 1 171 Balance at September 30, 2018 $ 39 $ 124 $ 12 $ 190

Derivatives (Tables)

Derivatives (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Energy-Related Derivatives	At September 30, 2018 , 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 (*) 595 2022 2029 Alabama Power 82 2022 — Georgia Power 165 2022 — Gulf Power 9 2020 — Mississippi Power 69 2022 — Southern Power 15 2020 — Southern Company Gas (*) 255 2021 2029 (*) 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 4.3 billion mmBtu and short natural gas positions of 4 billion mmBtu as of September 30, 2018 , which is also included in Southern Company's total volume.
Schedule of Interest Rate Derivatives	At September 30, 2018 , 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, 2018 (in millions) (in millions) Fair Value Hedges of Existing Debt Southern Company (*) $ 300 2.75% 3-month June 2020 $ (6 ) Southern Company (*) 1,500 2.35% 1-month July 2021 (60 ) Georgia Power 500 1.95% 3-month December 2018 (3 ) Georgia Power 200 4.25% 3-month December 2019 (3 ) Southern Company Consolidated $ 2,500 $ (72 ) (*) Represents the Southern Company parent entity.
Schedule of Foreign Currency Derivatives	At September 30, 2018 , the following foreign currency derivatives were outstanding: Pay Notional Pay Rate Receive Notional Receive Rate Hedge Fair Value Gain (Loss) at September 30, 2018 (in millions) (in millions) (in millions) Cash Flow Hedges of Existing Debt Southern Power $ 677 2.95% € 600 1.00% June 2022 $ 48 Southern Power 564 3.78% 500 1.85% June 2026 51 Total $ 1,241 € 1,100 $ 99
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, 2018 As of December 31, 2017 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 $ 15 $ 17 $ 10 $ 43 Other deferred charges and assets/Other deferred credits and liabilities 5 26 7 24 Assets held for sale, current/Liabilities held for sale, current — 6 — — Assets held for sale/Liabilities held for sale — 2 — — Total derivatives designated as hedging instruments for regulatory purposes $ 20 $ 51 $ 17 $ 67 Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 3 $ 6 $ 3 $ 14 Other deferred charges and assets/Other deferred credits and liabilities 1 1 — — Interest rate derivatives: Other current assets/Other current liabilities — 22 1 4 Other deferred charges and assets/Other deferred credits and liabilities — 50 — 34 Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 23 Other deferred charges and assets/Other deferred credits and liabilities 122 — 129 — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 126 $ 102 $ 133 $ 75 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ 263 $ 317 $ 380 $ 437 Other deferred charges and assets/Other deferred credits and liabilities 134 198 170 215 Total derivatives not designated as hedging instruments $ 397 $ 515 $ 550 $ 652 Gross amounts recognized $ 543 $ 668 $ 700 $ 794 Gross amounts offset (a) $ (303 ) $ (491 ) $ (405 ) $ (598 ) Net amounts recognized in the Balance Sheets (b) $ 240 $ 177 $ 295 $ 196 As of September 30, 2018 As of December 31, 2017 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 $ 5 $ 4 $ 2 $ 6 Other deferred charges and assets/Other deferred credits and liabilities 2 6 2 4 Total derivatives designated as hedging instruments for regulatory purposes $ 7 $ 10 $ 4 $ 10 Gross amounts recognized $ 7 $ 10 $ 4 $ 10 Gross amounts offset $ (4 ) $ (4 ) $ (4 ) $ (4 ) Net amounts recognized in the Balance Sheets $ 3 $ 6 $ — $ 6 Georgia Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ 5 $ 9 $ 2 $ 9 Other deferred charges and assets/Other deferred credits and liabilities 2 13 4 10 Total derivatives designated as hedging instruments for regulatory purposes $ 7 $ 22 $ 6 $ 19 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ — $ 5 $ — $ 4 Other deferred charges and assets/Other deferred credits and liabilities — 1 — 1 Total derivatives designated as hedging instruments in cash flow and fair value hedges $ — $ 6 $ — $ 5 Gross amounts recognized $ 7 $ 28 $ 6 $ 24 Gross amounts offset $ (7 ) $ (7 ) $ (6 ) $ (6 ) Net amounts recognized in the Balance Sheets $ — $ 21 $ — $ 18 Gulf Power Derivatives designated as hedging instruments for regulatory purposes Energy-related derivatives: Other current assets/Other current liabilities $ — $ 6 $ — $ 14 Other deferred charges and assets/Other deferred credits and liabilities — 2 — 7 Total derivatives designated as hedging instruments for regulatory purposes $ — $ 8 $ — $ 21 Gross amounts recognized $ — $ 8 $ — $ 21 Net amounts recognized in the Balance Sheets $ — $ 8 $ — $ 21 As of September 30, 2018 As of December 31, 2017 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 $ 2 $ 3 $ 1 $ 6 Other deferred charges and assets/Other deferred credits and liabilities 1 6 1 3 Total derivatives designated as hedging instruments for regulatory purposes $ 3 $ 9 $ 2 $ 9 Derivatives designated as hedging instruments in cash flow and fair value hedges Interest rate derivatives: Other current assets/Other current liabilities $ — $ — $ 1 $ — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ — $ — $ 1 $ — Gross amounts recognized $ 3 $ 9 $ 3 $ 9 Gross amounts offset $ (3 ) $ (3 ) $ (2 ) $ (2 ) Net amounts recognized in the Balance Sheets $ — $ 6 $ 1 $ 7 Southern Power Derivatives designated as hedging instruments in cash flow and fair value hedges Energy-related derivatives: Other current assets/Other current liabilities $ 2 $ 6 $ 3 $ 11 Other deferred charges and assets/Other deferred credits and liabilities 1 1 — — Foreign currency derivatives: Other current assets/Other current liabilities — 23 — 23 Other deferred charges and assets/Other deferred credits and liabilities 122 — 129 — Total derivatives designated as hedging instruments in cash flow and fair value hedges $ 125 $ 30 $ 132 $ 34 Derivatives not designated as hedging instruments Energy-related derivatives: Other current assets/Other current liabilities $ — $ — $ — $ 2 Gross amounts recognized $ 125 $ 30 $ 132 $ 36 Gross amounts offset $ (2 ) $ (2 ) $ (3 ) $ (3 ) Net amounts recognized in the Balance Sheets $ 123 $ 28 $ 129 $ 33 As of September 30, 2018 As of December 31, 2017 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 $ 3 $ 1 $ 5 $ 8 Other deferred charges and assets/Other deferred credits and liabilities — 1 — — Total derivatives designated as hedging instruments for regulatory purposes $ 3 $ 2 $ 5 $ 8 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 $ 1 $ — $ — $ 3 Derivatives not designated as hedging instruments Energy-related derivatives: Assets from risk management activities/Liabilities from risk management activities-current $ 262 $ 316 $ 379 $ 434 Other deferred charges and assets/Other deferred credits and liabilities 134 198 170 215 Total derivatives not designated as hedging instruments $ 396 $ 514 $ 549 $ 649 Gross amounts of recognized $ 400 $ 516 $ 554 $ 660 Gross amounts offset (a) $ (287 ) $ (475 ) $ (390 ) $ (583 ) Net amounts recognized in the Balance Sheets (b) $ 113 $ 41 $ 164 $ 77 (a) Gross amounts offset include cash collateral held on deposit in broker margin accounts of $189 million and $193 million as of September 30, 2018 and December 31, 2017 , respectively. (b) Net amounts of derivative instruments outstanding exclude premium and intrinsic value associated with weather derivatives of $5 million and $11 million as of September 30, 2018 and December 31, 2017 , respectively.
Pre-tax Effects of Unrealized Derivative Gains (Losses) Arising from Energy-Related Derivative Instruments	At September 30, 2018 and December 31, 2017 , 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, 2018 Derivative Category and Balance Sheet Location Southern Company (*) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (*) (in millions) Energy-related derivatives: Other regulatory assets, current $ (9 ) $ (2 ) $ (4 ) $ (6 ) $ (2 ) $ (1 ) Other regulatory assets, deferred (20 ) (4 ) (11 ) (2 ) (5 ) — Assets held for sale, current (6 ) — — — — — Assets held for sale (2 ) — — — — — Other regulatory liabilities, current 8 3 — — — 5 Total energy-related derivative gains (losses) $ (29 ) $ (3 ) $ (15 ) $ (8 ) $ (7 ) $ 4 (*) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $3 million at September 30, 2018 . Regulatory Hedge Unrealized Gain (Loss) Recognized in the Balance Sheet at December 31, 2017 Derivative Category and Balance Sheet Location Southern Company (*) Alabama Power Georgia Power Gulf Power Mississippi Power Southern Company Gas (*) (in millions) Energy-related derivatives: Other regulatory assets, current $ (34 ) $ (4 ) $ (7 ) $ (14 ) $ (5 ) $ (4 ) Other regulatory assets, deferred (18 ) (3 ) (6 ) (7 ) (2 ) — Other regulatory liabilities, current 7 — — — — 7 Other regulatory liabilities, deferred 1 1 — — — — Total energy-related derivative gains (losses) $ (44 ) $ (6 ) $ (13 ) $ (21 ) $ (7 ) $ 3 (*) Fair value gains and losses recorded in regulatory assets and liabilities include cash collateral held on deposit in broker margin accounts of $6 million at December 31, 2017 .
Pre-tax Effects of Interest Rate Derivatives, Designated as Cash Flow Hedging Instruments	For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of cash flow hedge accounting on accumulated OCI were as follows: Gain (Loss) Recognized in OCI on Derivative For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Southern Company Energy-related derivatives $ (5 ) $ (6 ) $ 7 $ (26 ) Interest rate derivatives — (1 ) (2 ) (2 ) Foreign currency derivatives (10 ) 46 (31 ) 114 Total $ (15 ) $ 39 $ (26 ) $ 86 Southern Power Energy-related derivatives $ (5 ) $ (6 ) $ 5 $ (21 ) Foreign currency derivatives (10 ) 46 (31 ) 114 Total $ (15 ) $ 40 $ (26 ) $ 93 Southern Company Gas Energy-related derivatives $ — $ — $ 2 $ (4 )
Schedule of Cash Flow Hedging Instruments, Statements of Financial Performance and Financial Position, Location	For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of cash flow and fair value hedge accounting on income were as follows: Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Southern Company Cost of natural gas $ 104 $ 134 $ 1,053 $ 1,085 Gain (loss) on cash flow hedges (a) Energy-related derivatives — — (2 ) — Depreciation and amortization 787 767 2,338 2,236 Gain (loss) on cash flow hedges (a) Energy-related derivatives — (6 ) 2 (12 ) Interest expense, net of amounts capitalized (458 ) (407 ) (1,386 ) (1,248 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives (5 ) (5 ) (16 ) (15 ) Foreign currency derivatives (6 ) (5 ) (18 ) (17 ) Gain (loss) on fair value hedges (b) Interest rate derivatives (4 ) (5 ) (35 ) (6 ) Other income (expense), net 57 65 195 165 Gain (loss) on cash flow hedges (a)(c) Foreign currency derivatives (9 ) 43 (46 ) 139 Alabama Power Interest expense, net of amounts capitalized $ (82 ) $ (76 ) $ (240 ) $ (229 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives (1 ) (2 ) (4 ) (5 ) Georgia Power Interest expense, net of amounts capitalized $ (95 ) $ (105 ) $ (303 ) $ (310 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives (1 ) (1 ) (4 ) (3 ) Gain (loss) on fair value hedges (b) Interest rate derivatives — — (1 ) (1 ) Mississippi Power Interest expense, net of amounts capitalized $ (19 ) $ 13 $ (59 ) $ (23 ) Gain (loss) on cash flow hedges (a) Interest rate derivatives — — (1 ) (1 ) Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Southern Power Depreciation and amortization $ 130 $ 131 $ 370 $ 379 Gain (loss) on cash flow hedges (a) Energy-related derivatives — (6 ) 2 (12 ) Interest expense, net of amounts capitalized (45 ) (47 ) (138 ) (144 ) Gain (loss) on cash flow hedges (a) Foreign currency derivatives (6 ) (5 ) (18 ) (17 ) Other income (expense), net 17 3 22 3 Gain (loss) on cash flow hedges (a)(c) Foreign currency derivatives (9 ) 43 (46 ) 139 Southern Company Gas Cost of natural gas $ 104 $ 134 $ 1,053 $ 1,085 Gain (loss) on cash flow hedges (a) Energy-related derivatives — — (2 ) — (a) Amounts reflect gains or losses on cash flow hedges that were reclassified from accumulated OCI into income. (b) For fair value hedges presented above, generally changes in the fair value of the derivative contracts are equal to changes in the fair value of the underlying debt and have no material impact on income. (c) 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.
Schedule of Fair Value Hedging Instruments, Statements of Financial Performance and Financial Position, Location	As of September 30, 2018 and December 31, 2017, the following amounts were recorded on the balance sheets related to cumulative basis adjustments for fair value hedges: Carrying Amount of the Hedged Item Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item Balance Sheet Location of Hedged Items As of September 30, 2018 As of December 31, 2017 As of September 30, 2018 As of December 31, 2017 (in millions) (in millions) Southern Company Securities due within one year $ (499 ) $ (746 ) $ 1 $ 3 Long-term debt (2,526 ) (2,553 ) 65 35 Georgia Power Securities due within one year $ (499 ) $ (746 ) $ 1 $ 3 Long-term debt (497 ) (498 ) 2 1
Pre-tax Effect of Interest Rate and Energy Related Derivatives	For the three and nine months ended September 30, 2018 and 2017 , the pre-tax effects of energy-related derivatives not designated as hedging instruments on the statements of income of Southern Company and Southern Company Gas were as follows: Gain (Loss) Three Months Ended September 30, Nine Months Ended Derivatives in Non-Designated Hedging Relationships Statements of Income Location 2018 2017 2018 2017 (in millions) (in millions) Energy-related derivatives: Natural gas revenues (*) $ (36 ) $ (17 ) $ (79 ) $ 48 Cost of natural gas 2 2 5 (2 ) Total derivatives in non-designated hedging relationships $ (34 ) $ (15 ) $ (74 ) $ 46 (*) Excludes gains (losses) recorded in natural gas revenues associated with weather derivatives of $15 million for the nine months ended September 30, 2017 and immaterial amounts for all other periods presented.

Acquisitions and Dispositions (

Acquisitions and Dispositions (Tables)	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Schedule of Business Acquisitions	During the nine months ended September 30, 2018, one of Southern Power's wholly-owned subsidiaries acquired and completed construction of the Gaskell West 1 solar facility . 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 Gaskell West 1 Solar Recurrent Energy Development Holdings, LLC January 26, 2018 20 Kern County, CA 100% of Class B (*) March 2018 20 years (*) Southern Power owns 100% of the class B membership interests under a tax equity partnership agreement.
Schedule of Construction Projects	Project Facility Resource Approximate Nameplate Capacity ( MW ) Location Actual/Expected COD PPA Contract Period Cactus Flats (a) Wind 148 Concho County, TX July 2018 12-15 years Mankato Natural Gas 385 Mankato, MN First half 2019 20 years Wild Horse Mountain (b) Wind 100 Pushmataha County, OK Fourth quarter 2019 20 years Reading (c) Wind 200 Osage and Lyon Counties, KS Second quarter 2020 12 years (a) In July 2017, Southern Power purchased 100% of the Cactus Flats facility and commenced construction. In July 2018, the facility was placed in service and, in August 2018, Southern Power closed on a tax equity partnership agreement and owns 100% of the class B membership interests. (b) In May 2018, Southern Power purchased 100% of the Wild Horse Mountain facility and commenced construction. Southern Power may enter into a tax equity partnership agreement, in which case it would then own 100% of the class B membership interests. (c) In August 2018, Southern Power purchased 100% of the membership interests from the joint development arrangement with Renewable Energy Systems Americas, Inc. and commenced construction. Southern Power may enter into a tax equity partnership agreement, in which case it would then own 100% of the class B membership interests.
Disposal Groups, Including Discontinued Operations	Gulf Power and the Florida Plants represent individually significant components of Southern Company and Southern Power, respectively; therefore, pre-tax profit for these components for the three and nine months ended September 30, 2018 and 2017 is presented below: For the Three Months For the Nine Months 2018 2017 2018 2017 (in millions) (in millions) Earnings before income taxes: Gulf Power $ 59 $ 103 $ 146 $ 199 Southern Power's Florida Plants $ 18 $ 11 $ 40 $ 28 The following table provides Southern Company's and Southern Power's major classes of assets and liabilities classified as held for sale at September 30, 2018 : Southern Company Southern Power (in millions) Assets Held for Sale: Current assets $ 407 $ 18 Total property, plant, and equipment 4,093 168 Other non-current assets 574 17 Total Assets Held for Sale $ 5,074 $ 203 Liabilities Held for Sale: Current liabilities $ 355 $ 4 Long-term debt 1,285 — Accumulated deferred income taxes 542 — Other non-current liabilities 1,008 — Total Liabilities Held for Sale $ 3,190 $ 4

Variable Interest Entity and _2

Variable Interest Entity and Equity Method Investments (Tables)	9 Months Ended
	Sep. 30, 2018
Regulated Operations [Abstract]
Equity Method Investments	The carrying amounts of Southern Company Gas' equity method investments as of September 30, 2018 and December 31, 2017 and related income from those investments for the three and nine -month periods ended September 30, 2018 and September 30, 2017 were as follows: Investment Balance September 30, 2018 December 31, 2017 (in millions) SNG $ 1,260 $ 1,262 Atlantic Coast Pipeline 73 41 PennEast Pipeline 70 57 Other 126 117 Total $ 1,529 $ 1,477 Selected financial information of SNG for the three and nine months ended September 30, 2018 and September 30, 2017 is as follows: Income Statement Information Three Months Ended September 30, 2018 Three Months Ended September 30, 2017 Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 (in millions) Revenues $ 145 $ 146 $ 451 $ 445 Operating income $ 71 $ 71 $ 230 $ 218 Net income $ 58 $ 57 $ 190 $ 172
Schedule of Other Nonoperating Income, by Component	Earnings from Equity Method Investments Three Months Ended Three Months Ended Nine Months Ended Nine Months (in millions) SNG $ 29 $ 28 $ 95 $ 86 PennEast Pipeline 2 1 4 5 Atlantic Coast Pipeline 1 1 4 4 Other 2 2 5 5 Total $ 34 $ 32 $ 108 $ 100

Segment and Related Informati_2

Segment and Related Information (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 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 September 30, 2018: Operating revenues $ 5,014 $ 635 $ (140 ) $ 5,509 $ 492 $ 202 $ (44 ) $ 6,159 Segment net income (loss) (a)(b)(c)(d) 1,148 92 — 1,240 46 (119 ) (3 ) 1,164 Nine Months Ended September 30, 2018: Operating revenues $ 13,117 $ 1,699 $ (360 ) $ 14,456 $ 2,861 $ 984 $ (143 ) $ 18,158 Segment net income (loss) (a)(b)(c)(d) 1,711 235 — 1,946 294 (292 ) — 1,948 At September 30, 2018: Goodwill $ — $ 2 $ — $ 2 $ 5,015 $ 298 $ — $ 5,315 Total assets 75,069 15,355 (322 ) 90,102 20,398 3,086 (1,869 ) 111,717 Three Months Ended September 30, 2017: 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 September 30, 2017: Operating revenues $ 12,960 $ 1,597 $ (318 ) $ 14,239 $ 2,841 $ 442 $ (119 ) $ 17,403 Segment net income (loss) (a)(b)(e) — 276 — 276 303 (232 ) — 347 At December 31, 2017: Goodwill $ — $ 2 $ — $ 2 $ 5,967 $ 299 $ — $ 6,268 Total assets 72,204 15,206 (325 ) 87,085 22,987 2,552 (1,619 ) 111,005 (a) Attributable to Southern Company. (b) Segment net income (loss) for the traditional electric operating companies includes pre-tax charges for estimated losses on plants under construction of $1 million ( $1 million after tax) and $34 million ( $21 million after tax) for the three months ended September 30, 2018 and 2017 , respectively, and $1.1 billion ( $0.8 billion after tax) and $3.2 billion ( $2.2 billion after tax) for the nine months ended September 30, 2018 and 2017 , respectively. See Note 3 to the financial statements of Southern Company under "Kemper County Energy Facility" in Item 8 of the Form 10-K and Note (B) under " Nuclear Construction " and " Kemper County Energy Facility " for additional information. (c) Segment net income (loss) for Southern Power includes pre-tax impairment charges of $36 million ( $27 million after tax) and $155 million ( $116 million after tax) for the three and nine months ended September 30, 2018, respectively. See Note (J) under "Southern Power – Development Projects " and " – Sale of Florida Plants " for additional information. (d) Segment net income (loss) for Southern Company Gas includes a net gain on dispositions of $353 million ( $40 million gain after tax) and $317 million ( $35 million loss after tax) for the three and nine months ended September 30, 2018, respectively, related to the Southern Company Gas Dispositions and a goodwill impairment charge of $42 million for the nine months ended September 30, 2018 related to the sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas " for additional information. (e) Segment net income (loss) for the traditional electric operating companies 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 3 to the financial statements of Southern Company under "Regulatory Matters – Gulf Power – Retail Base Rate Cases" in Item 8 of the Form 10-K for additional information. Business segment financial data for the three and nine months ended September 30, 2018 and 2017 was as follows: Gas Distribution Operations (a)(c) Gas Marketing Services (b)(c) Wholesale Gas Services (d) Gas Midstream Operations Total All Other Eliminations Consolidated (in millions) Three Months Ended September 30, 2018: Operating revenues $ 441 $ 44 $ (8 ) $ 20 $ 497 $ 1 $ (6 ) $ 492 Segment net income (loss) 74 (8 ) (18 ) 16 64 (18 ) — 46 Nine Months Ended September 30, 2018: Operating revenues 2,297 403 142 60 2,902 3 (44 ) 2,861 Segment net income (loss) 290 (71 ) 65 54 338 (44 ) — 294 Total assets at September 30, 2018: 16,850 1,522 855 2,297 21,524 10,146 (11,272 ) 20,398 Three Months Ended September 30, 2017: Operating revenues $ 472 $ 143 $ (24 ) $ 16 $ 607 $ 2 $ (44 ) $ 565 Segment net income (loss) 52 1 (23 ) 14 44 (29 ) — 15 Nine Months Ended September 30, 2017: Operating revenues 2,255 597 95 53 3,000 7 (166 ) 2,841 Segment net income (loss) 223 36 28 38 325 (22 ) — 303 Total assets at December 31, 2017: 19,358 2,147 1,096 2,241 24,842 12,184 (14,039 ) 22,987 (a) Operating revenues for the three gas distribution operations dispositions were $8 million and $50 million for the three months ended September 30, 2018 and 2017 , respectively, and $245 million and $274 million for the nine months ended September 30, 2018 and 2017 , respectively. See Note (J) under " Southern Company Gas " for additional information. (b) Operating revenues for the gas marketing services disposition were $32 million for the three months ended September 30, 2017 and $55 million and $95 million for the nine months ended September 30, 2018 and 2017 , respectively. See Note (J) under " Southern Company Gas " for additional information. (c) Segment net income for gas distribution operations includes a gain on dispositions of $351 million ( $38 million after tax) for the three and nine months ended September 30, 2018 . Segment net income for gas marketing services includes a gain on disposition of $2 million ( $2 million after tax) for the three months ended September 30, 2018 and a loss on disposition of $34 million ( $73 million loss after tax) and a goodwill impairment charge of $42 million for the nine months ended September 30, 2018 recorded in contemplation of the sale of Pivotal Home Solutions. See Note (J) under " Southern Company Gas " for additional information. (d) 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) Three Months Ended September 30, 2018 $ 1,573 $ 82 $ 1,655 $ 1,663 $ (8 ) Three Months Ended September 30, 2017 1,411 103 1,514 1,538 (24 ) Nine Months Ended September 30, 2018 $ 4,847 $ 352 $ 5,199 $ 5,057 $ 142 Nine Months Ended September 30, 2017 4,781 362 5,143 5,048 95
Financial Data for Products and Services	Products and Services Electric Utilities' Revenues Period Retail Wholesale Other Total (in millions) Three Months Ended September 30, 2018 $ 4,605 $ 693 $ 211 $ 5,509 Three Months Ended September 30, 2017 4,615 718 190 5,523 Nine Months Ended September 30, 2018 $ 11,913 $ 1,923 $ 620 $ 14,456 Nine Months Ended September 30, 2017 11,786 1,867 586 14,239 Southern Company Gas' Revenues Period Gas Gas Other Total (in millions) Three Months Ended September 30, 2018 $ 438 $ 44 $ 10 $ 492 Three Months Ended September 30, 2017 430 143 (8 ) 565 Nine Months Ended September 30, 2018 $ 2,276 $ 403 $ 182 $ 2,861 Nine Months Ended September 30, 2017 2,119 597 125 2,841

Introduction - Narrative (Detai

Introduction - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Net income (loss)		$ 1,222,000,000	$ 1,109,000,000	$ 2,031,000,000	$ 427,000,000
ALABAMA POWER CO
Related Party Transaction [Line Items]
Net income (loss)		377,000,000	330,000,000	$ 868,000,000	743,000,000
Increase in ARO due to new CCR rule	$ 1,200,000,000
Inflation rate assumption				4.50%
Trust earnings rate assumption				7.00%
MISSISSIPPI POWER CO
Related Party Transaction [Line Items]
Net income (loss)		47,000,000	40,000,000	$ 87,000,000	(2,033,000,000)
Increase in ARO due to new CCR rule	11,000,000
SOUTHERN Co GAS
Related Party Transaction [Line Items]
Net income (loss)		46,000,000	15,000,000	294,000,000	303,000,000
Inventory decrement		0		0
Wholesale Gas Services | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Inventory write-down		0	$ 0	0	$ 0
Plant Farley | ALABAMA POWER CO
Related Party Transaction [Line Items]
Increase in ARO	$ 300,000,000
Accounting Standards Update 2014-09 | Effect of Change
Related Party Transaction [Line Items]
Net income (loss)		(4,000,000)		1,000,000
Accounting Standards Update 2014-09 | Effect of Change | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Net income (loss)		$ (4,000,000)		$ 1,000,000

Introduction - ASC 606 (Details

Introduction - ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Condensed Statements of Income
Total operating revenues	$ 6,159	$ 6,201	$ 18,158	$ 17,403
Other operations and maintenance	1,404	1,341	4,217	4,100
Operating income (loss)	2,174	1,991	3,613	1,594
Other income (expense), net	57	65	195	165
Earnings (loss) before income taxes	1,845	1,699	2,629	744
Income taxes (benefit)	623	590	598	317
Consolidated net income (loss)	1,222	1,109	2,031	427
Consolidated net income (loss) attributable to Southern Company	1,164	1,069	1,948	347
Condensed Statements of Cash Flows
Net income (loss)	1,222	1,109	2,031	427
Changes in certain current assets and liabilities:
Receivables			37	423
Other current assets			(90)	(66)
Other current liabilities			(67)	(129)
Condensed Balance Sheets
Unbilled revenues	738		738		$ 810
Other accounts and notes receivable	690		690		698
Other current assets	232		232		199
Other current liabilities	763		763		874
Retained earnings (accumulated deficit)	9,048		9,048		8,885
Balances Without Adoption of ASC 606
Condensed Statements of Income
Other operations and maintenance	1,387		4,178
Operating income (loss)	2,195		3,647
Other income (expense), net	41		160
Earnings (loss) before income taxes	1,850		2,628
Income taxes (benefit)	624		598
Consolidated net income (loss)	1,226		2,030
Consolidated net income (loss) attributable to Southern Company	1,168		1,947
Condensed Statements of Cash Flows
Net income (loss)	1,226		2,030
Changes in certain current assets and liabilities:
Receivables			27
Other current assets			(80)
Other current liabilities			(68)
Condensed Balance Sheets
Unbilled revenues	776		776
Other accounts and notes receivable	691		691
Other current assets	193		193
Other current liabilities	764		764
Retained earnings (accumulated deficit)	9,047		9,047
ALABAMA POWER CO
Condensed Statements of Income
Total operating revenues	1,740	1,740	4,716	4,606
Other operations and maintenance	401	406	1,191	1,177
Operating income (loss)	561	601	1,313	1,403
Other income (expense), net	9	10	24	35
Earnings (loss) before income taxes	504	546	1,140	1,236
Income taxes (benefit)	127	216	272	493
Consolidated net income (loss)	377	330	868	743
Consolidated net income (loss) attributable to Southern Company	373	325	857	729
Condensed Statements of Cash Flows
Net income (loss)	377	330	868	743
Changes in certain current assets and liabilities:
Receivables			(207)	(163)
Other current assets			66	33
Other current liabilities			60	7
Condensed Balance Sheets
Unbilled revenues	159		159		162
Other accounts and notes receivable	54		54		55
Other current assets	8		8		5
Other current liabilities	46		46		52
Retained earnings (accumulated deficit)	2,902		2,902		2,647
ALABAMA POWER CO | Balances Without Adoption of ASC 606
Condensed Statements of Income
Other operations and maintenance	390		1,159
Operating income (loss)	563		1,319
Other income (expense), net	7		18
GEORGIA POWER CO
Condensed Statements of Income
Total operating revenues	2,593	2,546	6,601	6,426
Other operations and maintenance	460	430	1,325	1,248
Operating income (loss)	991	1,017	1,032	2,121
Other income (expense), net	30	22	104	95
Earnings (loss) before income taxes	926	934	833	1,906
Income taxes (benefit)	262	350	212	705
Consolidated net income (loss)	664	584	621	1,201
Consolidated net income (loss) attributable to Southern Company	664	580	621	1,188
Condensed Statements of Cash Flows
Net income (loss)	664	584	621	1,201
Changes in certain current assets and liabilities:
Receivables			(205)	(254)
Other current assets			(36)	(24)
Other current liabilities			(111)	(11)
Condensed Balance Sheets
Unbilled revenues	245		245		255
Other accounts and notes receivable	96		96		76
Other current assets	91		91		14
Other current liabilities	180		180		198
Retained earnings (accumulated deficit)	3,792		3,792		4,215
GEORGIA POWER CO | Balances Without Adoption of ASC 606
Condensed Statements of Income
Other operations and maintenance	437		1,268
Operating income (loss)	990		1,027
Other income (expense), net	31		109
Changes in certain current assets and liabilities:
Receivables			(242)
Other current assets			1
Condensed Balance Sheets
Unbilled revenues	310		310
Other accounts and notes receivable	97		97
Other current assets	25		25
SOUTHERN Co GAS
Condensed Statements of Income
Total operating revenues	492	565	2,861	2,841
Other operations and maintenance	216	206	730	675
Operating income (loss)	374	67	810	551
Other income (expense), net	6	19	21	30
Earnings (loss) before income taxes	362	67	769	536
Income taxes (benefit)	316	52	475	233
Consolidated net income (loss)	46	15	294	303
Consolidated net income (loss) attributable to Southern Company	46	15	294	303
Condensed Statements of Cash Flows
Net income (loss)	46	15	294	303
Changes in certain current assets and liabilities:
Receivables			445	531
Other current assets			21	(42)
Other current liabilities			35	8
Condensed Balance Sheets
Unbilled revenues	58		58		285
Other accounts and notes receivable	110		110		91
Other current assets	88		88		78
Other current liabilities	122		122		159
Retained earnings (accumulated deficit)	(273)		(273)		$ (212)
SOUTHERN Co GAS | Balances Without Adoption of ASC 606
Condensed Statements of Income
Operating income (loss)	379		809
Earnings (loss) before income taxes	367		768
Income taxes (benefit)	317		475
Consolidated net income (loss)	50		293
Condensed Statements of Cash Flows
Net income (loss)	50		293
Changes in certain current assets and liabilities:
Other current liabilities			34
Condensed Balance Sheets
Other current liabilities	123		123
Retained earnings (accumulated deficit)	(274)		(274)
SOUTHERN Co GAS
Condensed Statements of Income
Total operating revenues	492	565	2,861	2,841
Accounting Standards Update 2014-09 | Effect of Change
Condensed Statements of Income
Other operations and maintenance	17		39
Operating income (loss)	(21)		(34)
Other income (expense), net	16		35
Earnings (loss) before income taxes	(5)		1
Income taxes (benefit)	(1)		0
Consolidated net income (loss)	(4)		1
Consolidated net income (loss) attributable to Southern Company	(4)		1
Condensed Statements of Cash Flows
Net income (loss)	(4)		1
Changes in certain current assets and liabilities:
Receivables			10
Other current assets			(10)
Other current liabilities			1
Condensed Balance Sheets
Unbilled revenues	(38)		(38)
Other accounts and notes receivable	(1)		(1)
Other current assets	39		39
Other current liabilities	(1)		(1)
Retained earnings (accumulated deficit)	1		1
Accounting Standards Update 2014-09 | ALABAMA POWER CO | Effect of Change
Condensed Statements of Income
Other operations and maintenance	11		32
Operating income (loss)	(2)		(6)
Other income (expense), net	2		6
Accounting Standards Update 2014-09 | GEORGIA POWER CO | Effect of Change
Condensed Statements of Income
Other operations and maintenance	23		57
Operating income (loss)	1		5
Other income (expense), net	(1)		(5)
Changes in certain current assets and liabilities:
Receivables			37
Other current assets			(37)
Condensed Balance Sheets
Unbilled revenues	(65)		(65)
Other accounts and notes receivable	(1)		(1)
Other current assets	66		66
Accounting Standards Update 2014-09 | SOUTHERN Co GAS | Effect of Change
Condensed Statements of Income
Operating income (loss)	(5)		1
Earnings (loss) before income taxes	(5)		1
Income taxes (benefit)	(1)		0
Consolidated net income (loss)	(4)		1
Condensed Statements of Cash Flows
Net income (loss)	(4)		1
Changes in certain current assets and liabilities:
Other current liabilities			1
Condensed Balance Sheets
Other current liabilities	(1)		(1)
Retained earnings (accumulated deficit)	1		1
Natural gas
Condensed Statements of Income
Total operating revenues	492	532	2,806	2,746
Natural gas | Balances Without Adoption of ASC 606
Condensed Statements of Income
Total operating revenues	497		2,805
Natural gas | SOUTHERN Co GAS
Condensed Statements of Income
Total operating revenues	487	532	2,829	2,737
Natural gas | SOUTHERN Co GAS | Balances Without Adoption of ASC 606
Condensed Statements of Income
Total operating revenues	492		2,828
Natural gas | Accounting Standards Update 2014-09 | Effect of Change
Condensed Statements of Income
Total operating revenues	(5)		1
Natural gas | Accounting Standards Update 2014-09 | SOUTHERN Co GAS | Effect of Change
Condensed Statements of Income
Total operating revenues	(5)		1
Other revenues
Condensed Statements of Income
Total operating revenues	199	168	1,007	494
Other revenues | Balances Without Adoption of ASC 606
Condensed Statements of Income
Total operating revenues	198		1,003
Other revenues | ALABAMA POWER CO
Condensed Statements of Income
Total operating revenues	68	50	199	158
Other revenues | ALABAMA POWER CO | Balances Without Adoption of ASC 606
Condensed Statements of Income
Total operating revenues	59		173
Other revenues | GEORGIA POWER CO
Condensed Statements of Income
Total operating revenues	121	93	349	284
Other revenues | GEORGIA POWER CO | Balances Without Adoption of ASC 606
Condensed Statements of Income
Total operating revenues	97		287
Other revenues | SOUTHERN Co GAS
Condensed Statements of Income
Total operating revenues	0	$ 33	55	$ 95
Other revenues | Accounting Standards Update 2014-09 | Effect of Change
Condensed Statements of Income
Total operating revenues	1		4
Other revenues | Accounting Standards Update 2014-09 | ALABAMA POWER CO | Effect of Change
Condensed Statements of Income
Total operating revenues	9		26
Other revenues | Accounting Standards Update 2014-09 | GEORGIA POWER CO | Effect of Change
Condensed Statements of Income
Total operating revenues	$ 24		$ 62

Introduction - Asset Retirement

Introduction - Asset Retirement Obligation (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2017	$ 4,824
Liabilities incurred	2
Liabilities settled	(160)
Accretion	153
Cash flow revisions	1,510
Reclassification to held for sale	(164)
Balance at September 30, 2018	6,165
ALABAMA POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2017	1,709
Liabilities incurred	0
Liabilities settled	(31)
Accretion	72
Cash flow revisions	1,451
Reclassification to held for sale	0
Balance at September 30, 2018	3,201
GEORGIA POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2017	2,638
Liabilities incurred	0
Liabilities settled	(82)
Accretion	70
Cash flow revisions	(32)
Reclassification to held for sale	0
Balance at September 30, 2018	2,594
GULF POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2017	142
Liabilities incurred	0
Liabilities settled	(23)
Accretion	3
Cash flow revisions	42
Reclassification to held for sale	0
Balance at September 30, 2018	164
MISSISSIPPI POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2017	174
Liabilities incurred	0
Liabilities settled	(22)
Accretion	4
Cash flow revisions	21
Reclassification to held for sale	0
Balance at September 30, 2018	$ 177

Introduction - Nuclear Decommis

Introduction - Nuclear Decommissioning (Details) - ALABAMA POWER CO - Plant Farley $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Decommissioning periods:
Beginning year	2,037
Completion year	2,076
Site study costs	$ 1,720
Radiated structures
Decommissioning periods:
Site study costs	1,621
Non-radiated structures
Decommissioning periods:
Site study costs	$ 99

Introduction - Goodwill (Detail

Introduction - Goodwill (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Roll Forward]
Balance at December 31, 2017		$ 6,268		$ 6,268
Impairment				(42)
Dispositions				(910)
Balance at September 30, 2018	$ 5,315			5,315
SOUTHERN Co GAS
Goodwill [Roll Forward]
Balance at December 31, 2017		5,967		5,967
Impairment	0	(42)	$ 0	(42)	$ 0
Dispositions				(910)
Balance at September 30, 2018	5,015			5,015
SOUTHERN Co GAS | Gas Distribution Operations
Goodwill [Roll Forward]
Balance at December 31, 2017		4,702		4,702
Impairment				0
Dispositions				(668)
Balance at September 30, 2018	4,034			4,034
SOUTHERN Co GAS | Gas Marketing Services
Goodwill [Roll Forward]
Balance at December 31, 2017		$ 1,265		1,265
Impairment				(42)
Dispositions				(242)
Balance at September 30, 2018	$ 981			$ 981

Introduction - Other Intangible

Introduction - Other Intangible Assets Subject to Amortization (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ (225)	$ (186)
Total other intangible assets - net	674	873
SOUTHERN POWER CO
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	(66)	(47)
SOUTHERN POWER CO | PPA fair value adjustments
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(66)	(47)
Other Intangible Assets, Net	390	409
SOUTHERN Co GAS
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	246	400
Accumulated Amortization	(133)	(120)
Other Intangible Assets, Net	113	280
Total other intangible assets - net	113	280
SOUTHERN Co GAS | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	156	221
Accumulated Amortization	(78)	(77)
Other Intangible Assets, Net	78	144
SOUTHERN Co GAS | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	26	115
Accumulated Amortization	(6)	(9)
Other Intangible Assets, Net	20	106
SOUTHERN Co GAS | Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(49)	(34)
Other Intangible Assets, Net	15	30
Southern Company
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	824	984
Accumulated Amortization	(225)	(186)
Other Intangible Assets, Net	599	798
Total other intangible assets - gross	899	1,059
Total other intangible assets - net	674	873
Southern Company | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	223	288
Accumulated Amortization	(87)	(83)
Other Intangible Assets, Net	136	205
Southern Company | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	70	159
Accumulated Amortization	(18)	(17)
Other Intangible Assets, Net	52	142
Southern Company | Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(49)	(34)
Other Intangible Assets, Net	15	30
Southern Company | PPA fair value adjustments
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(66)	(47)
Other Intangible Assets, Net	390	409
Southern Company | Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	11	17
Accumulated Amortization	(5)	(5)
Other Intangible Assets, Net	$ 6	$ 12

Introduction - Other Intangib_2

Introduction - Other Intangible Assets Not Subject to Amortization (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (225)	$ (186)
Total other intangible assets - net	674	873
Southern Company
Indefinite-lived Intangible Assets [Line Items]
Total other intangible assets - gross	899	1,059
Accumulated Amortization	(225)	(186)
Total other intangible assets - net	674	873
Federal Communications Commission licenses | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	75	75
SOUTHERN POWER CO
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	(66)	(47)
SOUTHERN Co GAS
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	(133)	(120)
Total other intangible assets - net	$ 113	$ 280

Introduction - Intangibles, Amo

Introduction - Intangibles, Amortization (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 21	$ 70
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	$ 12	$ 42

Introduction - Restricted Cash

Introduction - Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 1,847	$ 2,130
Restricted cash:
Total cash, cash equivalents, and restricted cash	1,891	2,147	$ 1,858	$ 1,992
SOUTHERN POWER CO
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	192	129
Restricted cash:
Other accounts and notes receivable		0
Deferred charges and other assets		11
Total cash, cash equivalents, and restricted cash	192	140	189	1,112
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	56	73
Cash and cash equivalents classified as assets held for sale	0
Restricted cash:
Other accounts and notes receivable	6	5
Deferred charges and other assets		0
Total cash, cash equivalents, and restricted cash	62	78	$ 26	$ 24
Southern Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	1,847	2,130
Cash and cash equivalents classified as assets held for sale	37
Restricted cash:
Other accounts and notes receivable	6	5
Deferred charges and other assets		12
Total cash, cash equivalents, and restricted cash	$ 1,891	$ 2,147

Contingencies and Regulatory _3

Contingencies and Regulatory Matters - General Litigation Matters (Details) $ in Thousands	May 18, 2018USD ($)	May 04, 2018	Apr. 25, 2018	Mar. 02, 2018USD ($)generating_plant	Feb. 07, 2018	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
ALABAMA POWER CO
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage						47.00%
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage					9.33%
MISSISSIPPI POWER CO | MRA Fuel Costs
Loss Contingencies [Line Items]
Over recovered fuel cost						$ 7,000	$ 0
MISSISSIPPI POWER CO | MB Fuel Costs
Loss Contingencies [Line Items]
Over recovered fuel cost						0	0
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability						294,000	388,000
GEORGIA POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability						25,000	22,000
GULF POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability						48,000	52,000
Location One | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability						$ 2,000
Location One | SOUTHERN Co GAS | Elizabethtown Gas
Loss Contingencies [Line Items]
Environmental remediation liability							$ 85,000
Alabama Department of Environmental Management (ADEM) | ALABAMA POWER CO
Loss Contingencies [Line Items]
Litigation settlement, amount awarded to other party				$ 1,250
Number of electric generating plants | generating_plant				5
Open Access Transmission Tariff
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage						11.25%
RE Roserock, LLC | RE Roserock, LLC | SOUTHERN POWER CO
Loss Contingencies [Line Items]
Ownership percentage by parent						51.00%
Maximum | Open Access Transmission Tariff
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage						8.65%
Martin Product Sales, LLC Litigation | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Damages sought by litigation	$ 143,000
Deepwater Horizon Oil Spill Litigation Case | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Litigation settlement awarded from other party						$ 18,000
Nicor Energy Services Company Litigation | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Litigation amount awarded to other party						$ 11,000
Pending Litigation | Shareholder Derivative Lawsuits
Loss Contingencies [Line Items]
Stay period		30 days	30 days

Contingencies and Regulatory _4

Contingencies and Regulatory Matters - Cost Recovery Clauses (Details) - ALABAMA POWER CO - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 0	$ 17
Rate CNP PPA	30	12
Retail Energy Cost Recovery	58	25
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	7	0
Retail Energy Cost Recovery	41	0
Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 24	$ 38

Contingencies and Regulatory _5

Contingencies and Regulatory Matters - Alabama Power (Details)	Aug. 06, 2018	May 01, 2018USD ($)$ / KWH_Kilowatt_hour	Apr. 30, 2018$ / KWH_Kilowatt_hour	Jan. 31, 2019$ / KWH_Kilowatt_hour	Jun. 30, 2018	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2025	Dec. 31, 2018USD ($)	Dec. 31, 2017
ALABAMA POWER CO
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage							47.00%
Public utilities, allowed weighted common equity return, percentage					6.15%					6.21%
Customer refundable fees, refund payments, tranche one, percentage							25.00%
Customer refundable fees, refund payments, tranche two, percentage							40.00%
Customer refundable fees, refund payments, tranche three, percentage							75.00%
Customer refund							$ 50,000,000
Customer refundable fees, refund payments							151,000,000
Current billing rates under rate ecr in terms of per units | $ / KWH_Kilowatt_hour		2.353	2.015
Approved increase (decrease) in fuel rates amount		$ 100,000,000
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit, maximum amount used to offset under recovered amounts		$ 30,000,000
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit							$ 30,000,000
ALABAMA POWER CO | Minimum
Loss Contingencies [Line Items]
Public utilities, actual weighted common equity return, threshold, percentage							6.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche one							6.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche two							6.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche three							7.15%
ALABAMA POWER CO | Maximum
Loss Contingencies [Line Items]
Public utilities, actual weighted common equity return, threshold, percentage							7.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche one							6.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche two							7.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche three							7.65%
Customer refundable fees, refund payments, percentage							7.65%
ALABAMA POWER CO | Scenario, Forecast
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage								55.00%
Customer refundable fees, refund payments						$ 257,000,000
Current billing rates under rate ecr in terms of per units | $ / KWH_Kilowatt_hour				5.910
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit									$ 50,000,000
Reserve Margin Plan | Plant Greene County Units 1 and 2 | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Accelerated retirement period	4 years

Contingencies and Regulatory _6

Contingencies and Regulatory Matters - Georgia Power (Details) $ in Millions	Apr. 03, 2018USD ($)	Feb. 29, 2020USD ($)	Jun. 30, 2019USD ($)	Oct. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2019	Dec. 31, 2017USD ($)
Loss Contingencies [Line Items]
Effective tax rate					22.70%	42.60%
Deferred income tax benefit					$ 286	$ (15)
GEORGIA POWER CO
Loss Contingencies [Line Items]
Customer refund					$ 330
Effective tax rate					25.50%	37.00%
Deferred income tax benefit	$ 700				$ 185	$ (328)
Customer refund liability in absence of base rate case	$ 185
Retail regulatory equity ratio					0.53
Under recovered fuel balance, threshold for interim fuel rider					$ 200
Annual accrual for incremental operating and maintenance costs of damage from major storms					30
GEORGIA POWER CO | Storm Damage
Loss Contingencies [Line Items]
Regulatory asset					311
GEORGIA POWER CO | Subsequent Event
Loss Contingencies [Line Items]
Customer refundable fees, refund payments				$ 130
GEORGIA POWER CO | Other current liabilities
Loss Contingencies [Line Items]
Under recovered fuel balance					105			$ 165
GEORGIA POWER CO | Maximum
Loss Contingencies [Line Items]
Retail regulatory equity ratio	0.55
GEORGIA POWER CO | Maximum | Subsequent Event | Loss from Catastrophes
Loss Contingencies [Line Items]
Estimated restoration costs				150
GEORGIA POWER CO | Minimum | Subsequent Event | Loss from Catastrophes
Loss Contingencies [Line Items]
Estimated restoration costs				$ 125
GEORGIA POWER CO | State (net of federal benefit)
Loss Contingencies [Line Items]
Effective tax rate	6.00%
GEORGIA POWER CO | Scenario, Forecast
Loss Contingencies [Line Items]
Customer refundable fees, refund payments		$ 105	$ 95
GEORGIA POWER CO | Scenario, Forecast | State (net of federal benefit)
Loss Contingencies [Line Items]
Effective tax rate							5.75%
Impact of Tax Reform Legislation | GEORGIA POWER CO
Loss Contingencies [Line Items]
Regulatory liabilities					$ 655

Contingencies and Regulatory _7

Contingencies and Regulatory Matters - Gulf Power (Details) - GULF POWER CO $ in Millions	Nov. 05, 2018USD ($)	Oct. 30, 2018USD ($)	Oct. 10, 2018USD ($)	Mar. 26, 2018USD ($)intervenor	Mar. 25, 2018	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Loss Contingencies [Line Items]
Property damage reserve						$ 48
Property damage reserve replenishment						40
Number of intervenors | intervenor				3
Tax reform settlement agreement, annual reduction of revenue from base rates				$ 18.2
Tax reform settlement agreement, annual reduction of revenue from environmental cost recovery rates				15.6
Tax reform settlement agreement, refund				$ 69.4
Customer refundable fees, refund payments						53
Retail regulatory equity ratio				0.535	0.525
Environmental Cost Recovery						21	$ 11
Projected environmental expenditures						8	13
Property Basis Differences
Loss Contingencies [Line Items]
Deferred tax asset, regulatory liability						7
Subsequent Event
Loss Contingencies [Line Items]
Recovery from customers, period following the filing of cost recovery petition			60 days
Tax reform settlement agreement, annual reduction of revenue from base rates		$ 9.6
Decrease in annual revenues	$ 38
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Fuel Cost Recovery						0	22
Purchased Power Capacity Recovery						0	2
Environmental Cost Recovery						0	2
Other regulatory liabilities, current
Loss Contingencies [Line Items]
Fuel Cost Recovery						23	0
Purchased Power Capacity Recovery						4	0
Environmental Cost Recovery						13	0
Energy Conservation Cost Recovery						$ 2	$ 0
Loss from Catastrophes | Subsequent Event | Minimum
Loss Contingencies [Line Items]
Estimated restoration costs			$ 350
Loss from Catastrophes | Subsequent Event | Maximum
Loss Contingencies [Line Items]
Estimated restoration costs			$ 400

Contingencies and Regulatory _8

Contingencies and Regulatory Matters - Mississippi Power (Details) - MISSISSIPPI POWER CO - USD ($)	Jul. 27, 2018	May 08, 2018	Mar. 22, 2018	Feb. 07, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 03, 2018
Loss Contingencies [Line Items]
Refund due to customers											$ 5,000,000
Performance evaluation plan, prior year lookback, surcharge (refund)			$ 0					$ 5,000,000	$ 5,000,000	$ 0	$ 0
Public utilities, interim rate increase (decrease), percentage						4.00%	0.00%	0.00%
Public utilities, interim rate						$ 38,000,000
Public utilities, requested revenue requirement increase (decrease), amount				$ 26,000,000
Public utilities, requested return on equity, percentage				9.33%
Performance evaluation plan, equity ratio				55.00%
Increase retail base rate, annual base revenue		$ 3,000,000
Ad valorem tax adjustment, annual rate increase, percentage		0.80%
Ad valorem tax adjustment, annual rate increase, amount		$ 7,000,000
Environmental compliance overview plan, maximum annual increase in revenue, amount												$ 17,000,000
Over (under) recovered retail fuel costs					$ 13,000,000	$ 13,000,000	$ (6,000,000)
PEP Settlement Agreement
Loss Contingencies [Line Items]
Public utilities, approved revenue requirement increase (decrease), amount	$ 21,600,000
Amount subsequently approved for recovery	$ 2,000,000
Public utilities, performance-adjusted allowed return on equity percentage	9.31%
Public utilities, approved return on equity percentage						45.00%
Deferred income taxes	$ 44,000,000
Equity ratio, variance threshold	1.00%
Retail electric revenues					5,000,000
PEP Settlement Agreement | Maximum
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage	50.00%
Other Regulatory Assets | PEP Settlement Agreement
Loss Contingencies [Line Items]
Regulatory asset					3,000,000	$ 3,000,000
Other Regulatory Assets
Loss Contingencies [Line Items]
Regulatory liabilities					2,000,000	2,000,000
Other Regulatory Assets | PEP Settlement Agreement
Loss Contingencies [Line Items]
Regulatory liabilities					$ 5,000,000	$ 5,000,000

Contingencies and Regulatory _9

Contingencies and Regulatory Matters - Southern Company Gas (Details) - USD ($) $ in Millions	Oct. 25, 2018	Oct. 15, 2018	May 15, 2018	May 02, 2018	Feb. 23, 2018	Oct. 31, 2018	Jul. 31, 2018	Mar. 31, 2018	Sep. 30, 2018
Florida City Gas
Loss Contingencies [Line Items]
Public utilities, interim rate increase (decrease), percentage			4.00%
Public utilities, approved return on equity percentage			55.00%
Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount					$ (16)
Customer credit							$ 8
Atlanta Gas Light | Pipeline Replacement Program
Loss Contingencies [Line Items]
Litigation settlement awarded from other party								$ 7
Atlanta Gas Light | Subsequent Event
Loss Contingencies [Line Items]
Customer credit						$ 8
Nicor Gas
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage				9.80%
Approved decrease in fuel rates amount				$ 44
Chattanooga Gas | Subsequent Event
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 1
Public utilities, requested return on equity, percentage		9.80%
Virginia Natural Gas | Impact of Tax Reform Legislation
Loss Contingencies [Line Items]
Regulatory liabilities									$ 9
Virginia Natural Gas | Subsequent Event
Loss Contingencies [Line Items]
Customer refund	$ 14
Minimum | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage					10.55%
Maximum | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			10.95%

Contingencies and Regulatory_10

Contingencies and Regulatory Matters - Nuclear Construction (Details) - GEORGIA POWER CO

Jan. 01, 2021

Jan. 01, 2020

Nov. 09, 2018USD ($)

Aug. 31, 2018USD ($)

Jan. 11, 2018USD ($)

Jan. 01, 2016

Sep. 30, 2018USD ($)subcontractor

Jun. 30, 2018USD ($)

Sep. 30, 2018USD ($)subcontractorMW

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)report

Dec. 31, 2009USD ($)utilityMW

Dec. 31, 2022USD ($)

Dec. 31, 2017USD ($)

Jun. 01, 2021

Mar. 08, 2018appeal

Sep. 30, 2017USD ($)

Dec. 31, 2013

Loss Contingencies [Line Items]

Number of construction units approved | utility

2

Electric generating capacity in mega watts under consortium agreement | MW

1,100

Additional construction capital costs

$ 3,300,000,000

Estimated in-service capital cost

$ 4,418,000,000

Financing costs collected, net of tax

$ 1,800,000,000

Amendment to estimated in-service capital cost

$ 5,680,000,000

Retail rate of return on common equity

10.95%

Eligible project costs to be reimbursed

3,460,000,000

Scenario, Forecast

Loss Contingencies [Line Items]

Increase in NCCR tariff

$ 90,000,000

FFB Loan

Loss Contingencies [Line Items]

Long-term line of credit

$ 2,600,000,000

2,600,000,000

Conditional borrowing commitment

$ 1,670,000,000

Plant Vogtle Units 3 And 4

Loss Contingencies [Line Items]

Construction financing costs

1,800,000,000

Increase in base project capital cost forecast

$ 700,000,000

Estimated project costs

1,100,000,000

Increase in expected costs

$ 800,000,000

Base project capital cost forecast, monthly

50,000,000

Monthly AFUDC

12,000,000

Percentage of ownership interest required for voting for continuing construction

90.00%

Project capital cost forecast

$ 8,400,000,000

8,400,000,000

Additional construction capital costs

$ 4,900,000,000

Construction costs, threshold

$ 2,100,000,000

Percentage of remaining share of construction costs to pay

100.00%

Percentage of cost savings

60.70%

Increase costs amount

$ 100,000

Percentage of approval required to change primary construction contractor

90.00%

90.00%

Percent disallowed for recovery

6.00%

Disallowed for recovery reporting period

6 months

Extension in project schedule

1 year

Percentage of approval required for material amendments

67.00%

67.00%

Voted in favor period

30 days

Ownership interests voted in favor

50.00%

Share of plants (mw) | MW

206

Aggregate purchase price not to exceed

$ 300,000,000

Funding for project

250,000,000

Cost settlement agreement revised forecast, net of payments

$ 7,300,000,000

7,300,000,000

Public utilities, approved return on equity percentage

10.00%

Return on equity reduction, negative impact on earnings

$ 25,000,000

Number Of Appeals | appeal

2

Number of VCM reports approved | report

18

Guarantor obligations

$ 1,700,000,000

1,700,000,000

Customer refund

$ 188,000,000

Requested capital construction costs

$ 578,000,000

Plant Vogtle Units 3 And 4 | Scenario, Forecast

Loss Contingencies [Line Items]

Construction financing costs

$ 3,200,000,000

Public utilities, approved return on equity percentage

5.30%

8.30%

Approved return on equity, monthly percentage decrease

0.10%

Return on equity reduction, negative impact on earnings

$ 100,000,000

$ 680,000,000

Plant Vogtle Units 3 And 4 | Vogtle Owners

Loss Contingencies [Line Items]

Period of notice required in the event letters of credit are not renewed

30 days

Percentage of cost savings

39.30%

Plant Vogtle Units 3 And 4 | Minimum

Loss Contingencies [Line Items]

Number of subcontractors | subcontractor

60

60

Category I | Plant Vogtle Units 3 And 4

Loss Contingencies [Line Items]

Additional construction capital costs

$ 800,000,000

Category II | Plant Vogtle Units 3 And 4

Loss Contingencies [Line Items]

Percentage of construction costs responsible to pay

55.70%

Category II | Plant Vogtle Units 3 And 4 | Vogtle Owners

Loss Contingencies [Line Items]

Percentage of construction costs responsible to pay

44.30%

Category II | Plant Vogtle Units 3 And 4 | Minimum

Loss Contingencies [Line Items]

Additional construction capital costs

$ 80,000,000

Construction costs, threshold

800,000,000

Category II | Plant Vogtle Units 3 And 4 | Maximum

Loss Contingencies [Line Items]

Construction costs, threshold

$ 1,600,000,000

Category III | Plant Vogtle Units 3 And 4

Loss Contingencies [Line Items]

Percentage of construction costs responsible to pay

65.70%

Category III | Plant Vogtle Units 3 And 4 | Vogtle Owners

Loss Contingencies [Line Items]

Percentage of construction costs responsible to pay

34.30%

Category III | Plant Vogtle Units 3 And 4 | Minimum

Loss Contingencies [Line Items]

Additional construction capital costs

$ 100,000,000

Construction costs, threshold

1,600,000,000

Category III | Plant Vogtle Units 3 And 4 | Maximum

Loss Contingencies [Line Items]

Construction costs, threshold

$ 2,100,000,000

Contingencies and Regulatory_11

Contingencies and Regulatory Matters - Nuclear Construction Cost and Schedule (Details) - GEORGIA POWER CO - Plant Vogtle Units 3 And 4 $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Loss Contingencies [Line Items]
Base project capital cost forecast	$ 8,000
Construction contingency estimate	400
Project capital cost forecast	8,400
Net investment as of June 30, 2018	(4,300)
Remaining estimate to complete	4,100
Expected capitalized costs	350
Guarantor obligations	1,700
Customer refund	$ 188

Contingencies and Regulatory_12

Contingencies and Regulatory Matters - Kemper County Energy Facility (Details) - MISSISSIPPI POWER CO - USD ($) $ in Millions	Aug. 06, 2018	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Loss Contingencies [Line Items]
Accrued plant closure costs			$ 30	$ 30			$ 35
Kemper County Energy Facility
Loss Contingencies [Line Items]
Public utilities, charges to income			0	45
Public utilities, charges to income, after tax				34
Accrued plant closure costs			$ 20	$ 20
Reserve Margin Plan | Plant Watson, Unit 4
Loss Contingencies [Line Items]
Accelerated retirement period	2 years
Reserve Margin Plan | Plant Watson, Unit 5
Loss Contingencies [Line Items]
Accelerated retirement period	7 years
Reserve Margin Plan | Plant Greene County Units 1 and 2
Loss Contingencies [Line Items]
Accelerated retirement period	4 years
Scenario, Forecast | Kemper County Energy Facility
Loss Contingencies [Line Items]
Production costs, period cost		$ 2			$ 4	$ 8

Contingencies and Regulatory_13

Contingencies and Regulatory Matters - Other Matters (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018USD ($)cavern	Dec. 31, 2017USD ($)
Southern Company Holdings Subsidiary
Loss Contingencies [Line Items]
Lease receivable	$ 86,000,000
Number of salt dome caverns | cavern	2
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Impairment of long-lived assets		$ 0
Natural Gas, Storage
Loss Contingencies [Line Items]
Property, plant and equipment, net book value	$ 110,000,000
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Percentage of total property, plant and equipment	20.00%

Revenue from Contracts with C_3

Revenue from Contracts with Customers - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 6,159		$ 18,158
Total operating revenues	6,159	$ 6,201	18,158	$ 17,403
Other Income	92		274
Retail electric revenues
Disaggregation of Revenue [Line Items]
Leases revenues	17		54
Increase (decrease) in revenue from cost recovery mechanisms	(98)		4
Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	2,148		5,266
Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	1,527		4,084
Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	901		2,471
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	29		92
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	433		2,299
Alternative revenue programs
Disaggregation of Revenue [Line Items]
Total operating revenues	5		(23)
Total retail electric and gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	5,043		14,189
Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	516		1,444
Revenue from transactions accounted for as derivatives	63		217
Lease income from PPA contracts	130		318
Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	177		479
Lease income from PPA contracts	31		92
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	54		530
Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	369		1,516
Total operating revenues	199	168	1,007	494
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	492		2,861
Total operating revenues	492	565	2,861	2,841
SOUTHERN Co GAS | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	438		2,276
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	149		1,082
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	45		313
SOUTHERN Co GAS | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	203		708
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	4		28
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	32		168
SOUTHERN Co GAS | Alternative revenue programs
Disaggregation of Revenue [Line Items]
Total operating revenues	5		(23)
SOUTHERN Co GAS | Gas marketing Services
Disaggregation of Revenue [Line Items]
Total operating revenues	44		403
Other Income			4
SOUTHERN Co GAS | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	(10)		121
SOUTHERN Co GAS | Gas midstream operations
Disaggregation of Revenue [Line Items]
Total operating revenues	20		60
SOUTHERN Co GAS | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0		1
Total operating revenues	0	33	55	95
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	1,740		4,716
Total operating revenues	1,740	1,740	4,716	4,606
Other Income	27		79
ALABAMA POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,584		4,208
Increase (decrease) in revenue from cost recovery mechanisms	(12)		113
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	721		1,848
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	464		1,238
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	392		1,103
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	7		19
ALABAMA POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	62		234
Revenue from transactions accounted for as derivatives	6		14
ALABAMA POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	26		75
ALABAMA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	68		199
Total operating revenues	68	50	199	158
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	2,593		6,601
Total operating revenues	2,593	2,546	6,601	6,426
Other Income	28		80
GEORGIA POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2,425		6,112
Leases revenues	17		54
Increase (decrease) in revenue from cost recovery mechanisms	(47)		(35)
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,142		2,671
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	877		2,343
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	385		1,036
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	21		62
GEORGIA POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	33		99
Revenue from transactions accounted for as derivatives	8		21
GEORGIA POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	14		41
GEORGIA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	121		349
Leases revenues	34		100
Total operating revenues	121	93	349	284
GULF POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	414		1,106
Total operating revenues	414	437	1,106	1,144
Other Income	2		5
GULF POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	341		932
Increase (decrease) in revenue from cost recovery mechanisms	(36)		(63)
GULF POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	200		537
GULF POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	103		291
GULF POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	37		100
GULF POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	1		4
GULF POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	48		104
GULF POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	7		20
GULF POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	18		50
Total operating revenues	18	20	50	53
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	358		956
Total operating revenues	358	341	956	915
MISSISSIPPI POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	254		660
Increase (decrease) in revenue from cost recovery mechanisms	(3)		(11)
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	85		209
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	82		212
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	86		233
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	1		6
MISSISSIPPI POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	92		259
MISSISSIPPI POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1		6
MISSISSIPPI POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	11		31
Total operating revenues	11	5	31	14
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	635		1,699
Total operating revenues	635	618	1,699	1,597
SOUTHERN POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	168		450
Lease income from PPA contracts	47		141
SOUTHERN POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	336		892
Revenue from transactions accounted for as derivatives	47		176
Lease income from PPA contracts	139		342
SOUTHERN POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	126		347
SOUTHERN POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	5		10
Total operating revenues	5	3	10	9
Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	497	607	2,902	3,000
Wholesale gas services | Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	(8)	(24)	142	95
Wholesale gas services | Third Party Gross Revenues | Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Revenue from transactions accounted for as derivatives	900		2,700
Total operating revenues	$ 1,573	$ 1,411	$ 4,847	$ 4,781

Revenue from Contracts with C_4

Revenue from Contracts with Customers - Contract Balances (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	$ 2,778
Contract Assets	99
Contract Liabilities	34
Unregulated Distributed Generation
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract Assets	27
Contract Liabilities	17
ALABAMA POWER CO
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	649
Contract Assets	1
Contract Liabilities	14
GEORGIA POWER CO
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	924
Contract Assets	70
Contract Liabilities	$ 3
Performance obligation, expected timing of satisfaction	Georgia Power had contract assets primarily related to fixed retail customer bill programs where the payment is contingent upon Georgia Power's continued performance and the customer's continued participation in the program over the one-year contract term.
GULF POWER CO
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	$ 186
Contract Assets	0
Contract Liabilities	0
MISSISSIPPI POWER CO
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	96
Contract Assets	0
Contract Liabilities	0
SOUTHERN POWER CO
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	142
Contract Assets	0
Contract Liabilities	17
SOUTHERN Co GAS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Receivables	523
Contract Assets	0
Contract Liabilities	$ 1

Revenue from Contracts with C_5

Revenue from Contracts with Customers - Performance Obligations (Details) $ in Millions	Sep. 30, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 168
Performance obligation, expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 406
Performance obligation, expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 322
Performance obligation, expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 322
Performance obligation, expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 310
Performance obligation, expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 2,112
Performance obligation, expected timing of satisfaction
ALABAMA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 6
Performance obligation, expected timing of satisfaction	1 year
ALABAMA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 22
Performance obligation, expected timing of satisfaction	1 year
ALABAMA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 22
Performance obligation, expected timing of satisfaction	1 year
ALABAMA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 26
Performance obligation, expected timing of satisfaction	1 year
ALABAMA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 23
Performance obligation, expected timing of satisfaction	1 year
ALABAMA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 161
Performance obligation, expected timing of satisfaction
GEORGIA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 10
Performance obligation, expected timing of satisfaction	1 year
GEORGIA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 41
Performance obligation, expected timing of satisfaction	1 year
GEORGIA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 38
Performance obligation, expected timing of satisfaction	1 year
GEORGIA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 40
Performance obligation, expected timing of satisfaction	1 year
GEORGIA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 30
Performance obligation, expected timing of satisfaction	1 year
GEORGIA POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 113
Performance obligation, expected timing of satisfaction
GULF POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 5
Performance obligation, expected timing of satisfaction	1 year
GULF POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 22
Performance obligation, expected timing of satisfaction	1 year
GULF POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0
Performance obligation, expected timing of satisfaction	1 year
GULF POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0
Performance obligation, expected timing of satisfaction	1 year
GULF POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0
Performance obligation, expected timing of satisfaction	1 year
GULF POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0
Performance obligation, expected timing of satisfaction
MISSISSIPPI POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 1
Performance obligation, expected timing of satisfaction	1 year
MISSISSIPPI POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 3
Performance obligation, expected timing of satisfaction	1 year
MISSISSIPPI POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 3
Performance obligation, expected timing of satisfaction	1 year
MISSISSIPPI POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 1
Performance obligation, expected timing of satisfaction	1 year
MISSISSIPPI POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0
Performance obligation, expected timing of satisfaction	1 year
MISSISSIPPI POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0
Performance obligation, expected timing of satisfaction
SOUTHERN POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 75
Performance obligation, expected timing of satisfaction	1 year
SOUTHERN POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 310
Performance obligation, expected timing of satisfaction	1 year
SOUTHERN POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 283
Performance obligation, expected timing of satisfaction	1 year
SOUTHERN POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 277
Performance obligation, expected timing of satisfaction	1 year
SOUTHERN POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 276
Performance obligation, expected timing of satisfaction	1 year
SOUTHERN POWER CO | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 2,005
Performance obligation, expected timing of satisfaction

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Collateral already posted, aggregate fair value	$ 189
Net Asset Value as a Practical Expedient (NAV)
Assets:
Net Asset Value as a Practical Expedient (NAV)	37
Net Asset Value as a Practical Expedient (NAV) | Nuclear Decommissioning Trusts
Assets:
Net Asset Value as a Practical Expedient (NAV)	37
ALABAMA POWER CO
Assets:
Energy-related derivatives	7	$ 4
Liabilities:
Energy-related derivatives	10	10
ALABAMA POWER CO | Net Asset Value as a Practical Expedient (NAV)
Assets:
Net Asset Value as a Practical Expedient (NAV)	37
ALABAMA POWER CO | Net Asset Value as a Practical Expedient (NAV) | Nuclear Decommissioning Trusts
Assets:
Net Asset Value as a Practical Expedient (NAV)	37
GEORGIA POWER CO
Assets:
Energy-related derivatives	7	6
Nuclear decommissioning trusts	37
Liabilities:
Energy-related derivatives	28	24
GEORGIA POWER CO | Net Asset Value as a Practical Expedient (NAV)
Assets:
Net Asset Value as a Practical Expedient (NAV)	0
GULF POWER CO
Assets:
Energy-related derivatives	0	0
Liabilities:
Energy-related derivatives	8	21
MISSISSIPPI POWER CO
Assets:
Energy-related derivatives	3	3
Liabilities:
Energy-related derivatives	9	9
SOUTHERN POWER CO
Assets:
Energy-related derivatives	125	132
Liabilities:
Energy-related derivatives	30	36
SOUTHERN Co GAS
Assets:
Energy-related derivatives	400	554
Liabilities:
Energy-related derivatives	516	660
Collateral already posted, aggregate fair value	189	$ 193
Fair Value, Measurements, Recurring
Assets:
Energy-related derivatives	421
Foreign currency derivatives	122
Nuclear decommissioning trusts	1,872
Cash equivalents	1,309
Total	3,778
Liabilities:
Energy-related derivatives	575
Interest rate derivatives	72
Foreign currency derivatives	23
Contingent consideration	22
Total	692
Fair Value, Measurements, Recurring | Weather Derivative
Assets:
Energy-related derivatives	5
Fair Value, Measurements, Recurring | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	11
Fair Value, Measurements, Recurring | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	6
Fair Value, Measurements, Recurring | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	13
Fair Value, Measurements, Recurring | Cash equivalents
Assets:
Non-qualified deferred compensation trusts:	15
Fair Value, Measurements, Recurring | Other
Assets:
Non-qualified deferred compensation trusts:	9
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	271
Foreign currency derivatives	0
Nuclear decommissioning trusts	828
Cash equivalents	1,309
Total	2,432
Liabilities:
Energy-related derivatives	416
Interest rate derivatives	0
Foreign currency derivatives	0
Contingent consideration	0
Total	416
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents
Assets:
Non-qualified deferred compensation trusts:	15
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Non-qualified deferred compensation trusts:	9
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	150
Foreign currency derivatives	122
Nuclear decommissioning trusts	1,007
Cash equivalents	0
Total	1,309
Liabilities:
Energy-related derivatives	159
Interest rate derivatives	72
Foreign currency derivatives	23
Contingent consideration	0
Total	254
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	11
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	6
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	13
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Cash equivalents
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Foreign currency derivatives	0
Nuclear decommissioning trusts	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Interest rate derivatives	0
Foreign currency derivatives	0
Contingent consideration	22
Total	22
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Cash equivalents
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO
Assets:
Energy-related derivatives	7
Cash equivalents	513
Total	1,458
Liabilities:
Energy-related derivatives	10
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Domestic equity
Assets:
Nuclear decommissioning trusts	558
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Foreign equity
Assets:
Nuclear decommissioning trusts	116
Fair Value, Measurements, Recurring | ALABAMA POWER CO | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	18
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Corporate bonds
Assets:
Nuclear decommissioning trusts	180
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	22
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Private equity
Assets:
Nuclear decommissioning trusts	37
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Other
Assets:
Nuclear decommissioning trusts	7
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Cash equivalents	513
Total	1,075
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Assets:
Nuclear decommissioning trusts	469
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity
Assets:
Nuclear decommissioning trusts	60
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Assets:
Nuclear decommissioning trusts	26
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Private equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Nuclear decommissioning trusts	7
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	7
Cash equivalents	0
Total	346
Liabilities:
Energy-related derivatives	10
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Domestic equity
Assets:
Nuclear decommissioning trusts	89
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Foreign equity
Assets:
Nuclear decommissioning trusts	56
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	18
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Corporate bonds
Assets:
Nuclear decommissioning trusts	154
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	22
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Private equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Domestic equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Foreign equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Corporate bonds
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Private equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO
Assets:
Energy-related derivatives	8
Cash equivalents	350
Total	1,291
Liabilities:
Energy-related derivatives	22
Interest rate derivatives	6
Total	28
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Domestic equity
Assets:
Nuclear decommissioning trusts	251
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Foreign equity
Assets:
Nuclear decommissioning trusts	134
Fair Value, Measurements, Recurring | GEORGIA POWER CO | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	236
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Municipal bonds
Assets:
Nuclear decommissioning trusts	82
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Corporate bonds
Assets:
Nuclear decommissioning trusts	163
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	42
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Other
Assets:
Non-qualified deferred compensation trusts:	25
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Cash equivalents	350
Total	616
Liabilities:
Energy-related derivatives	0
Interest rate derivatives	0
Total	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Assets:
Nuclear decommissioning trusts	250
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Non-qualified deferred compensation trusts:	16
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	8
Cash equivalents	0
Total	675
Liabilities:
Energy-related derivatives	22
Interest rate derivatives	6
Total	28
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Domestic equity
Assets:
Nuclear decommissioning trusts	1
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Foreign equity
Assets:
Nuclear decommissioning trusts	134
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	236
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets:
Nuclear decommissioning trusts	82
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Corporate bonds
Assets:
Nuclear decommissioning trusts	163
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	42
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other
Assets:
Non-qualified deferred compensation trusts:	9
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Interest rate derivatives	0
Total	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Domestic equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Foreign equity
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | U.S. Treasury and government agency securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Municipal bonds
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Corporate bonds
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed securities
Assets:
Nuclear decommissioning trusts	0
Fair Value, Measurements, Recurring | GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | GULF POWER CO
Assets:
Cash equivalents	27
Liabilities:
Energy-related derivatives	8
Fair Value, Measurements, Recurring | GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	27
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | GULF POWER CO | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0
Liabilities:
Energy-related derivatives	8
Fair Value, Measurements, Recurring | GULF POWER CO | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | MISSISSIPPI POWER CO
Assets:
Energy-related derivatives	3
Cash equivalents	346
Total	349
Liabilities:
Energy-related derivatives	9
Fair Value, Measurements, Recurring | MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Cash equivalents	346
Total	346
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	3
Cash equivalents	0
Total	3
Liabilities:
Energy-related derivatives	9
Fair Value, Measurements, Recurring | MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | SOUTHERN POWER CO
Assets:
Energy-related derivatives	3
Foreign currency derivatives	122
Total	125
Liabilities:
Energy-related derivatives	7
Foreign currency derivatives	23
Contingent consideration	22
Total	52
Fair Value, Measurements, Recurring | SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	0
Foreign currency derivatives	0
Total	0
Liabilities:
Energy-related derivatives	0
Foreign currency derivatives	0
Contingent consideration	0
Total	0
Fair Value, Measurements, Recurring | SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	3
Foreign currency derivatives	122
Total	125
Liabilities:
Energy-related derivatives	7
Foreign currency derivatives	23
Contingent consideration	0
Total	30
Fair Value, Measurements, Recurring | SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Foreign currency derivatives	0
Total	0
Liabilities:
Energy-related derivatives	0
Foreign currency derivatives	0
Contingent consideration	22
Total	22
Fair Value, Measurements, Recurring | SOUTHERN Co GAS
Assets:
Energy-related derivatives	400
Cash equivalents	26
Total	460
Liabilities:
Energy-related derivatives	517
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	11
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	6
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	13
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Other
Assets:
Non-qualified deferred compensation trusts:	4
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Energy-related derivatives	271
Cash equivalents	26
Total	301
Liabilities:
Energy-related derivatives	416
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Non-qualified deferred compensation trusts:	4
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2)
Assets:
Energy-related derivatives	129
Cash equivalents	0
Total	159
Liabilities:
Energy-related derivatives	101
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	11
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	6
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	13
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Other
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3)
Assets:
Energy-related derivatives	0
Cash equivalents	0
Total	0
Liabilities:
Energy-related derivatives	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Domestic equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Foreign equity
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pooled funds – fixed income
Assets:
Non-qualified deferred compensation trusts:	0
Fair Value, Measurements, Recurring | SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Other
Assets:
Non-qualified deferred compensation trusts:	$ 0

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Increase (Decrease) In Fair Value Of Funds (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 58	$ 50	$ 68	$ 168
ALABAMA POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	39	25	49	87
GEORGIA POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 19	$ 25	$ 19	$ 81

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Measurements Of Investments Calculated At Net Asset Value Per Share (Details) $ in Millions	Sep. 30, 2018USD ($)
Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 47
ALABAMA POWER CO | Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	47
Net Asset Value as a Practical Expedient (NAV)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	37
Net Asset Value as a Practical Expedient (NAV) | Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	37
Net Asset Value as a Practical Expedient (NAV) | ALABAMA POWER CO
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	37
Net Asset Value as a Practical Expedient (NAV) | ALABAMA POWER CO | Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 37

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	9 Months Ended
	Sep. 30, 2018
Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Liquidations term	10 years

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2018USD ($)
Southern Company
Long-term debt, including securities due within one year:
Carrying Amount	$ 45,524
Fair Value	45,500
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	8,120
Fair Value	8,321
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	10,227
Fair Value	10,159
GULF POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	1,285
Fair Value	1,290
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	1,736
Fair Value	1,702
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	5,029
Fair Value	5,058
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying Amount	5,908
Fair Value	$ 5,935

Stockholders' Equity - Earnings

Stockholders' Equity - Earnings per Share (Details) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
As reported shares (in shares)	1,023	1,003	1,016	998
Effect of stock-based compensation (in shares)	6	7	5	7
Diluted shares (in shares)	1,029	1,010	1,021	1,005

Stockholders' Equity - Changes

Stockholders' Equity - Changes in Stockholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)			(900)
Beginning balance			$ 25,528	$ 26,612
Consolidated net income attributable to Southern Company	$ 1,164	$ 1,069	1,948	347
Other comprehensive income (loss)			52	(2)
Stock issued			878	613
Stock-based compensation			74	97
Cash dividends on common stock			(1,805)	(1,716)
Preference stock redemption				(150)
Contributions from noncontrolling interests			154	77
Distributions to noncontrolling interests			(87)	(87)
Net income attributable to noncontrolling interests			71	45
Sale of noncontrolling interests			1,280
Reclassification from redeemable noncontrolling interests				114
Other			$ (28)	(11)
Ending balance, treasury shares (in shares)	(1,000)		(1,000)
Ending balance	$ 28,065	$ 25,939	$ 28,065	$ 25,939
Number of Common Shares Issued
Changes in Stockholders' Equity
Beginning balance (in shares)			1,008,532	991,213
Stock issued (in shares)			21,342	13,308
Other (in shares)			0
Ending balance (in shares)	1,029,874	1,004,521	1,029,874	1,004,521
Number of Common Shares Treasury Issued
Changes in Stockholders' Equity
Beginning balance, treasury shares (in shares)			(929)	(819)
Other (in shares)			(57)	(75)
Ending balance, treasury shares (in shares)	(986)	(894)	(986)	(894)
Common Stockholders' Equity
Changes in Stockholders' Equity
Beginning balance			$ 24,167	$ 24,758
Consolidated net income attributable to Southern Company			1,948	347
Other comprehensive income (loss)			52	(2)
Stock issued			878	613
Stock-based compensation			74	97
Cash dividends on common stock			(1,805)	(1,716)
Sale of noncontrolling interests			(410)
Other			(27)	(15)
Ending balance	$ 24,877	$ 24,082	24,877	24,082
Preferred and Preference Stock of Subsidiaries
Changes in Stockholders' Equity
Beginning balance			0	609
Preference stock redemption				(150)
Other				3
Ending balance	0	462	0	462
Noncontrolling Interests
Changes in Stockholders' Equity
Beginning balance			1,361	1,245
Contributions from noncontrolling interests			154	77
Distributions to noncontrolling interests			(87)	(87)
Net income attributable to noncontrolling interests			71	45
Sale of noncontrolling interests			1,690
Reclassification from redeemable noncontrolling interests				114
Other			(1)	1
Ending balance	$ 3,188	$ 1,395	$ 3,188	$ 1,395

Financing - Narrative (Details)

Financing - Narrative (Details)

1 Months Ended

3 Months Ended

9 Months Ended

Nov. 06, 2018USD ($)shares

Aug. 31, 2018USD ($)

Jun. 30, 2018USD ($)agreement

May 31, 2018USD ($)agreement

Apr. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Sep. 30, 2018USD ($)

Sep. 30, 2017USD ($)

Jul. 31, 2018USD ($)

Jan. 31, 2018USD ($)

Jan. 04, 2018USD ($)

Debt Instrument [Line Items]

Variable rate pollution control revenue bonds outstanding

$ 1,500,000,000

$ 1,500,000,000

Maximum borrowing capacity

8,143,000,000

8,143,000,000

Repayments of short-term debt

1,800,000,000

$ 409,000,000

Stock redeemed during period, par value

150,000,000

Floating rate Promissory Note Due July 2018

Debt Instrument [Line Items]

Debt face amount

$ 100,000,000

Southern Company

Debt Instrument [Line Items]

Maximum borrowing capacity

2,000,000,000

2,000,000,000

Southern Company | Floating Rate Bank Term Loan Agreement 1 | Notes Payable to Banks

Debt Instrument [Line Items]

Debt face amount

$ 900,000,000

Southern Company | Uncommitted Bank Credit Arrangement | Notes Payable to Banks

Debt Instrument [Line Items]

Debt face amount

$ 250,000,000

Debt instrument, payable by demand from bank, term

30 days

Repayments of short-term debt

$ 250,000,000

Southern Company | Floating Rate Bank Term Loan Agreement 6 | Notes Payable to Banks

Debt Instrument [Line Items]

Debt face amount

1,500,000,000

Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 1

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

Southern Company | Senior Notes | Series 2018A Floating Rate Senior Notes Due February 14, 2020

Debt Instrument [Line Items]

Debt face amount

750,000,000

ALABAMA POWER CO

Debt Instrument [Line Items]

Variable rate pollution control revenue bonds outstanding

854,000,000

854,000,000

Fixed rate pollution control revenue bonds outstanding

120,000,000

120,000,000

Maximum borrowing capacity

1,333,000,000

1,333,000,000

ALABAMA POWER CO | Senior Notes | Series 2018A Floating Rate Senior Notes

Debt Instrument [Line Items]

Debt face amount

$ 500,000,000

$ 500,000,000

Debt stated interest rate

4.30%

4.30%

ALABAMA POWER CO | Senior Notes | Series 2008B 5.40% Senior Notes due June 1, 2018

Debt Instrument [Line Items]

Debt stated interest rate

5.40%

Repurchased face amount

$ 250,000,000

ALABAMA POWER CO | Subsequent Event | Pollution Control Revenue Bonds

Debt Instrument [Line Items]

Debt instrument, repurchase amount

$ 120,000,000

GEORGIA POWER CO

Debt Instrument [Line Items]

Variable rate pollution control revenue bonds outstanding

550,000,000

550,000,000

Fixed rate pollution control revenue bonds outstanding

345,000,000

345,000,000

Maximum borrowing capacity

1,750,000,000

$ 1,750,000,000

Amortization period for line of credit facility

5 years

Percent of eligible project costs to be reimbursed

70.00%

Repayments of short-term debt

$ 150,000,000

300,000,000

GEORGIA POWER CO | Loan Guarantee Agreement

Debt Instrument [Line Items]

Conditional borrowing commitment

1,670,000,000

GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 2

Debt Instrument [Line Items]

Debt face amount

$ 150,000,000

GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 3

Debt Instrument [Line Items]

Debt face amount

$ 100,000,000

GEORGIA POWER CO | Senior Notes

Debt Instrument [Line Items]

Repayments of debt

$ 902,000,000

GEORGIA POWER CO | Senior Notes | Series 2007A 5.65% Senior Notes

Debt Instrument [Line Items]

Debt instrument, repurchase amount

89,000,000

Debt face amount

$ 250,000,000

Debt stated interest rate

5.65%

GEORGIA POWER CO | Senior Notes | Series 2009A 5.95% Senior Notes

Debt Instrument [Line Items]

Debt instrument, repurchase amount

$ 326,000,000

Debt face amount

$ 500,000,000

Debt stated interest rate

5.95%

GEORGIA POWER CO | Senior Notes | Series 2010B 5.40% Senior Notes

Debt Instrument [Line Items]

Debt instrument, repurchase amount

$ 335,000,000

Debt face amount

$ 600,000,000

Debt stated interest rate

5.40%

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

104,600,000

104,600,000

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

173,000,000

173,000,000

GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fifth Series 1994

Debt Instrument [Line Items]

Debt face amount

55,000,000

55,000,000

GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Second Series 2008

Debt Instrument [Line Items]

Debt face amount

65,000,000

65,000,000

GEORGIA POWER CO | Municipal bonds | Development Authority Of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), First Series 2013

Debt Instrument [Line Items]

Debt face amount

71,735,000

71,735,000

GEORGIA POWER CO | FFB Loan

Debt Instrument [Line Items]

Conditional borrowing commitment

1,670,000,000

Maximum borrowing capacity

2,600,000,000

2,600,000,000

GULF POWER CO

Debt Instrument [Line Items]

Variable rate pollution control revenue bonds outstanding

82,000,000

82,000,000

Fixed rate pollution control revenue bonds outstanding

58,000,000

58,000,000

Maximum borrowing capacity

280,000,000

280,000,000

MISSISSIPPI POWER CO

Debt Instrument [Line Items]

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

Repayments of short-term debt

300,000,000

$ 109,000,000

MISSISSIPPI POWER CO | Notes Payable to Banks

Debt Instrument [Line Items]

Debt face amount

300,000,000

MISSISSIPPI POWER CO | Senior Notes | Series 2018A Floating Rate Senior Notes

Debt Instrument [Line Items]

Debt face amount

300,000,000

MISSISSIPPI POWER CO | Senior Notes | Series 2018B 3.95% Senior Notes

Debt Instrument [Line Items]

Debt face amount

$ 300,000,000

Debt stated interest rate

3.95%

MISSISSIPPI POWER CO | Municipal bonds | Pollution Control Revenue Bonds

Debt Instrument [Line Items]

Debt instrument, repurchase amount

$ 43,000,000

MISSISSIPPI POWER CO | Promissory Notes | Promissory Notes

Debt Instrument [Line Items]

Repayments of debt

100,000,000

$ 200,000,000

MISSISSIPPI POWER CO | Unsecured Debt | Unsecured Term Loan

Debt Instrument [Line Items]

Repayments of debt

$ 900,000,000

MISSISSIPPI POWER CO | Subsequent Event | Senior Notes | Series G 5.40% Senior Notes Due July 1, 2035

Debt Instrument [Line Items]

Debt stated interest rate

5.40%

Aggregate principal amount outstanding

$ 30,000,000

MISSISSIPPI POWER CO | Subsequent Event | Senior Notes | Series 2009A 5.55% Senior Notes Due March 1, 2019

Debt Instrument [Line Items]

Debt stated interest rate

5.55%

Aggregate principal amount outstanding

$ 125,000,000

MISSISSIPPI POWER CO | 4.40% Series Preferred Stock | Subsequent Event

Debt Instrument [Line Items]

Stock redeemed during period (in shares) | shares

8,867

Stock redeemed during period, par value

$ 886,700

Preferred stock, dividend rate

4.40%

MISSISSIPPI POWER CO | 4.60% Series Preferred Stock | Subsequent Event

Debt Instrument [Line Items]

Stock redeemed during period (in shares) | shares

8,643

Stock redeemed during period, par value

$ 864,300

Preferred stock, dividend rate

4.60%

MISSISSIPPI POWER CO | 4.72% Series Preferred Stock | Subsequent Event

Debt Instrument [Line Items]

Stock redeemed during period (in shares) | shares

16,700

Stock redeemed during period, par value

$ 1,670,000

Preferred stock, dividend rate

4.72%

MISSISSIPPI POWER CO | Depository Shares | Subsequent Event

Debt Instrument [Line Items]

Stock redeemed during period (in shares) | shares

1,200,000

Stock redeemed during period, par value

$ 30,000,000

Interest in shares

25.00%

MISSISSIPPI POWER CO | 5.25% Series Preferred Stock | Subsequent Event

Debt Instrument [Line Items]

Preferred stock, dividend rate

5.25%

Traditional Electric Operating Companies

Debt Instrument [Line Items]

Fixed rate pollution control revenue bonds outstanding

573,000,000

573,000,000

SOUTHERN POWER CO

Debt Instrument [Line Items]

Maximum borrowing capacity

750,000,000

750,000,000

Repayments of debt

$ 770,000,000

Proceeds from third-party tax equity

148,000,000

SOUTHERN POWER CO | Floating Rate Bank Term Loan Agreement 1 | Notes Payable to Banks

Debt Instrument [Line Items]

Debt face amount

$ 100,000,000

Number of loan agreements | agreement

2

Repurchased face amount

$ 420,000,000

$ 420,000,000

SOUTHERN POWER CO | Senior Notes | Series 2015A 1.50% Senior Notes

Debt Instrument [Line Items]

Debt stated interest rate

1.50%

1.50%

Repurchased face amount

$ 350,000,000

$ 350,000,000

SOUTHERN Co GAS

Debt Instrument [Line Items]

Maximum borrowing capacity

1,900,000,000

1,900,000,000

SOUTHERN Co GAS | Uncommitted Bank Credit Arrangement | Notes Payable to Banks

Debt Instrument [Line Items]

Debt face amount

$ 95,000,000

Debt instrument, payable by demand from bank, term

30 days

SOUTHERN Co GAS | Senior Notes | Series 2017A

Debt Instrument [Line Items]

Debt face amount

$ 200,000,000

$ 200,000,000

Nicor Gas

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 500,000,000

$ 500,000,000

Nicor Gas | First Mortgage Bonds

Debt Instrument [Line Items]

Debt face amount

$ 300,000,000

Nicor Gas | First Mortgage Bonds | First Mortgage Bonds Issued August 2018

Debt Instrument [Line Items]

Debt face amount

$ 100,000,000

Nicor Gas | Subsequent Event | First Mortgage Bonds | First Mortgage Bonds Issued November 2018

Debt Instrument [Line Items]

Debt face amount

$ 200,000,000

Financing - Schedule of Credit

Financing - Schedule of Credit Arrangements (Details)	Sep. 30, 2018USD ($)
Line of Credit Facility [Line Items]
Expires, 2018	$ 20,000,000
Expires, 2019	188,000,000
Expires, 2020	735,000,000
Expires, 2022	7,200,000,000
Total	8,143,000,000
Unused	8,101,000,000
Executable Term Loans, One Year	45,000,000
Expires Within One Year, Term Out	45,000,000
Expires Within One Year, No Term Out	63,000,000
Southern Company
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	2,000,000,000
Total	2,000,000,000
Unused	1,999,000,000
Executable Term Loans, One Year	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, 2018	0
Expires, 2019	33,000,000
Expires, 2020	500,000,000
Expires, 2022	800,000,000
Total	1,333,000,000
Unused	1,333,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	33,000,000
GEORGIA POWER CO
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,750,000,000
Total	1,750,000,000
Unused	1,736,000,000
Executable Term Loans, One Year	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, 2018	20,000,000
Expires, 2019	25,000,000
Expires, 2020	235,000,000
Expires, 2022	0
Total	280,000,000
Unused	280,000,000
Executable Term Loans, One Year	45,000,000
Expires Within One Year, Term Out	45,000,000
Expires Within One Year, No Term Out	0
MISSISSIPPI POWER CO
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	100,000,000
Expires, 2020	0
Expires, 2022	0
Total	100,000,000
Unused	100,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	750,000,000
Total	750,000,000
Unused	728,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
SOUTHERN POWER CO | Standby Letters of Credit
Line of Credit Facility [Line Items]
Total	120,000,000
Unused	22,000,000
SOUTHERN Co GAS
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,900,000,000
Total	1,900,000,000
Unused	1,895,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
Other
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	30,000,000
Expires, 2020	0
Expires, 2022	0
Total	30,000,000
Unused	30,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	30,000,000
Southern Company Gas Capital
Line of Credit Facility [Line Items]
Total	1,400,000,000
Nicor Gas
Line of Credit Facility [Line Items]
Total	$ 500,000,000

Financing - Schedule of Long-Te

Financing - Schedule of Long-Term Debt Financing Activities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Debt Instrument [Line Items]
Senior Note Issuances	$ 1,850
Senior Note Maturities, Redemptions, and Repurchases	2,350
Revenue Bond Maturities, Redemptions, and Repurchases	712
Other Long-Term Debt Issuances	100
Other Long-Term Debt Redemptions and Maturities	1,436
Elimination
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	(1)
ALABAMA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	500
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	0
GEORGIA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	1,000
Revenue Bond Maturities, Redemptions, and Repurchases	469
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	107
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	600
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	43
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	900
SOUTHERN POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	350
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	420
SOUTHERN Co GAS
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	200
Other Long-Term Debt Issuances	100
Other Long-Term Debt Redemptions and Maturities	0
Other
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	10
Southern Company
Debt Instrument [Line Items]
Senior Note Issuances	750
Senior Note Maturities, Redemptions, and Repurchases	1,000
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	$ 0

Retirement Benefits (Details)

Retirement Benefits (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Amortization:
Net (gain)/loss
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future employer contributions, remainder of fiscal year	$ 0		$ 0
Pension Plans | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	19,000,000	15,000,000	58,000,000	$ 47,000,000
Interest cost	26,000,000	25,000,000	76,000,000	73,000,000
Expected return on plan assets	(51,000,000)	(49,000,000)	(155,000,000)	(147,000,000)
Amortization:
Prior service costs	0	1,000,000	1,000,000	2,000,000
Regulatory asset	0		0
Net (gain)/loss	13,000,000	10,000,000	40,000,000	31,000,000
Net periodic pension cost (income)	7,000,000	2,000,000	20,000,000	6,000,000
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	22,000,000	19,000,000	65,000,000	56,000,000
Interest cost	34,000,000	34,000,000	104,000,000	103,000,000
Expected return on plan assets	(74,000,000)	(71,000,000)	(222,000,000)	(212,000,000)
Amortization:
Prior service costs	1,000,000	0	2,000,000	2,000,000
Regulatory asset	0		0
Net (gain)/loss	18,000,000	15,000,000	52,000,000	43,000,000
Net periodic pension cost (income)	1,000,000	(3,000,000)	1,000,000	(8,000,000)
Pension Plans | GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	3,000,000	12,000,000	10,000,000
Interest cost	5,000,000	5,000,000	15,000,000	15,000,000
Expected return on plan assets	(10,000,000)	(10,000,000)	(30,000,000)	(29,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0		0
Net (gain)/loss	2,000,000	2,000,000	7,000,000	5,000,000
Net periodic pension cost (income)	1,000,000	0	4,000,000	1,000,000
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	5,000,000	4,000,000	13,000,000	11,000,000
Interest cost	5,000,000	5,000,000	15,000,000	15,000,000
Expected return on plan assets	(11,000,000)	(9,000,000)	(31,000,000)	(29,000,000)
Amortization:
Prior service costs	0	0	0	1,000,000
Regulatory asset	0		0
Net (gain)/loss	3,000,000	1,000,000	8,000,000	5,000,000
Net periodic pension cost (income)	2,000,000	1,000,000	5,000,000	3,000,000
Pension Plans | SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	3,000,000		7,000,000
Interest cost	1,000,000		4,000,000
Expected return on plan assets	(3,000,000)		(8,000,000)
Amortization:
Prior service costs	0		0
Regulatory asset	0		0
Net (gain)/loss	0		1,000,000
Net periodic pension cost (income)	1,000,000		4,000,000
Pension Plans | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	8,000,000	6,000,000	24,000,000	17,000,000
Interest cost	10,000,000	10,000,000	29,000,000	30,000,000
Expected return on plan assets	(18,000,000)	(18,000,000)	(53,000,000)	(53,000,000)
Amortization:
Prior service costs	(1,000,000)	0	(2,000,000)	(1,000,000)
Regulatory asset	4,000,000		11,000,000
Net (gain)/loss	3,000,000	5,000,000	9,000,000	15,000,000
Net periodic pension cost (income)	6,000,000	3,000,000	18,000,000	8,000,000
Pension Plans | Southern Company
Pension Plans and Postretirement Plans
Service cost	90,000,000	73,000,000	269,000,000	220,000,000
Interest cost	116,000,000	114,000,000	348,000,000	341,000,000
Expected return on plan assets	(236,000,000)	(224,000,000)	(707,000,000)	(673,000,000)
Amortization:
Prior service costs	1,000,000	3,000,000	3,000,000	9,000,000
Regulatory asset	0		0
Net (gain)/loss	53,000,000	41,000,000	160,000,000	122,000,000
Net periodic pension cost (income)	24,000,000	7,000,000	73,000,000	19,000,000
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	5,000,000	4,000,000	13,000,000	13,000,000
Expected return on plan assets	(7,000,000)	(5,000,000)	(20,000,000)	(19,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000	3,000,000	3,000,000
Regulatory asset	0		0
Net (gain)/loss	0	0	1,000,000	1,000,000
Net periodic pension cost (income)	0	1,000,000	1,000,000	2,000,000
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	7,000,000	6,000,000	21,000,000	21,000,000
Expected return on plan assets	(6,000,000)	(6,000,000)	(19,000,000)	(18,000,000)
Amortization:
Prior service costs	0	0	1,000,000	1,000,000
Regulatory asset	0		0
Net (gain)/loss	2,000,000	3,000,000	6,000,000	6,000,000
Net periodic pension cost (income)	5,000,000	5,000,000	14,000,000	15,000,000
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
Regulatory asset	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	0	1,000,000	2,000,000	3,000,000
Expected return on plan assets	0	0	(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0		0
Net (gain)/loss	0	0	0	0
Net periodic pension cost (income)	0	1,000,000	2,000,000	3,000,000
Postretirement Benefits | SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000		1,000,000
Interest cost	0		0
Expected return on plan assets	0		0
Amortization:
Prior service costs	0		0
Regulatory asset	0		0
Net (gain)/loss	0		0
Net periodic pension cost (income)	1,000,000		1,000,000
Postretirement Benefits | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	0	1,000,000	1,000,000	2,000,000
Interest cost	2,000,000	3,000,000	7,000,000	8,000,000
Expected return on plan assets	(1,000,000)	(2,000,000)	(5,000,000)	(5,000,000)
Amortization:
Prior service costs	0	(1,000,000)	0	(2,000,000)
Regulatory asset	2,000,000		5,000,000
Net (gain)/loss	0	1,000,000	0	3,000,000
Net periodic pension cost (income)	3,000,000	2,000,000	8,000,000	6,000,000
Postretirement Benefits | Southern Company
Pension Plans and Postretirement Plans
Service cost	6,000,000	6,000,000	18,000,000	18,000,000
Interest cost	19,000,000	19,000,000	56,000,000	59,000,000
Expected return on plan assets	(17,000,000)	(16,000,000)	(51,000,000)	(49,000,000)
Amortization:
Prior service costs	2,000,000	2,000,000	5,000,000	5,000,000
Regulatory asset	0		0
Net (gain)/loss	3,000,000	3,000,000	10,000,000	10,000,000
Net periodic pension cost (income)	$ 13,000,000	$ 14,000,000	$ 38,000,000	$ 43,000,000

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details)	Apr. 03, 2018	Sep. 30, 2018USD ($)	Dec. 31, 2018operating_facility	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2019	Dec. 31, 2017USD ($)
Income Tax Contingency [Line Items]
Valuation allowances		$ 190,000,000		$ 190,000,000				$ 190,000,000			$ 148,000,000
Effective tax rate decrease								14.00%
Effective tax (benefit) rate								22.70%	42.60%
Increase in effective tax rate due to loss on Kemper IGCC									$ 3,100,000,000
Impairment charges				36,000,000		$ 0		$ 197,000,000	0
Income taxes benefit				(623,000,000)		(590,000,000)		(598,000,000)	$ (317,000,000)
Unrecognized tax benefits		0		0				$ 0
Significantly increase (decrease) in amount of unrecognized tax benefits								12 months
State
Income Tax Contingency [Line Items]
Valuation allowances		171,000,000		171,000,000				$ 171,000,000
ALABAMA POWER CO
Income Tax Contingency [Line Items]
Effective tax (benefit) rate								23.90%	39.90%
Income taxes benefit				(127,000,000)		(216,000,000)		$ (272,000,000)	$ (493,000,000)
GEORGIA POWER CO
Income Tax Contingency [Line Items]
Valuation allowances		39,000,000		39,000,000				$ 39,000,000
Effective tax (benefit) rate								25.50%	37.00%
Income taxes benefit				(262,000,000)		(350,000,000)		$ (212,000,000)	$ (705,000,000)
GEORGIA POWER CO | Plant Vogtle Units 3 And 4
Income Tax Contingency [Line Items]
Estimated project costs					$ 1,100,000,000
GEORGIA POWER CO | State
Income Tax Contingency [Line Items]
Valuation allowances		33,000,000		33,000,000				$ 33,000,000
Effective tax (benefit) rate	6.00%
GEORGIA POWER CO | Scenario, Forecast | State
Income Tax Contingency [Line Items]
Effective tax (benefit) rate										5.75%
GULF POWER CO
Income Tax Contingency [Line Items]
Effective tax (benefit) rate								(0.50%)	39.40%
Income taxes benefit				4,000,000		(40,000,000)		$ 1,000,000	$ (78,000,000)
MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Valuation allowances		124,000,000		124,000,000				$ 124,000,000
Effective tax (benefit) rate								20.80%	(30.30%)
Increase in effective tax rate due to loss on Kemper IGCC								$ 3,100,000,000
Income taxes benefit				(14,000,000)		(24,000,000)		(23,000,000)	$ 885,000,000
MISSISSIPPI POWER CO | State
Income Tax Contingency [Line Items]
Valuation allowances		124,000,000		124,000,000				$ 124,000,000
SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Effective tax (benefit) rate								(220.30%)	(66.50%)
Impairment charges				36,000,000		0		$ 155,000,000	$ 0
Income taxes benefit				38,000,000		39,000,000	$ 54,000,000	210,000,000	$ 129,000,000
SOUTHERN POWER CO | State
Income Tax Contingency [Line Items]
Income taxes benefit		11,000,000
SOUTHERN POWER CO | Scenario, Forecast
Income Tax Contingency [Line Items]
Number of operating wind facilities | operating_facility			8
SOUTHERN POWER CO | Florida Plants | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Tax Contingency [Line Items]
Pre-tax impairment charge								119,000,000
Impairment charges								36,000,000
SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Valuation allowances		12,000,000		12,000,000				$ 12,000,000
Effective tax (benefit) rate								61.80%	43.40%
Income taxes benefit				(316,000,000)		$ (52,000,000)		$ (475,000,000)	$ (233,000,000)
SOUTHERN Co GAS | State
Income Tax Contingency [Line Items]
Valuation allowances		1,000,000		1,000,000				1,000,000
Investment Tax Credit Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward, amount		$ 2,400,000,000		$ 2,400,000,000				$ 2,400,000,000			$ 2,100,000,000

Income Taxes - Summary of Valua

Income Taxes - Summary of Valuation Allowances (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Valuation Allowance [Line Items]
Valuation allowances	$ 190	$ 148
Federal
Valuation Allowance [Line Items]
Valuation allowances	19
State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	171
GEORGIA POWER CO
Valuation Allowance [Line Items]
Valuation allowances	39
GEORGIA POWER CO | Federal
Valuation Allowance [Line Items]
Valuation allowances	6
GEORGIA POWER CO | State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	33
MISSISSIPPI POWER CO
Valuation Allowance [Line Items]
Valuation allowances	124
MISSISSIPPI POWER CO | Federal
Valuation Allowance [Line Items]
Valuation allowances	0
MISSISSIPPI POWER CO | State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	124
SOUTHERN Co GAS
Valuation Allowance [Line Items]
Valuation allowances	12
SOUTHERN Co GAS | Federal
Valuation Allowance [Line Items]
Valuation allowances	11
SOUTHERN Co GAS | State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	$ 1

Derivatives - Energy-Related an

Derivatives - Energy-Related and Interest Rate Derivatives (Details) MMBTU in Millions	9 Months Ended
	Sep. 30, 2018USD ($)MMBTU
Energy-related, Natural Gas | ALABAMA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	82
Longest Hedge Date	2,022
Energy-related, Natural Gas | GEORGIA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	165
Longest Hedge Date	2,022
Energy-related, Natural Gas | GULF POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	9
Longest Hedge Date	2,020
Energy-related, Natural Gas | MISSISSIPPI POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	69
Longest Hedge Date	2,022
Energy-related, Natural Gas | SOUTHERN POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	15
Longest Hedge Date	2,020
Energy-related, Natural Gas | SOUTHERN Co GAS
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	255
Longest Hedge Date	2,021
Longest Non-Hedge Date	2,029
Energy-related, Natural Gas | SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Long
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	4,300
Energy-related, Natural Gas | SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Short
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	4,000
Energy-related, Natural Gas | Southern Company
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	595
Longest Hedge Date	2,022
Longest Non-Hedge Date	2,029
Interest rate derivatives
Interest rate derivatives
Notional Amount	$ 2,500,000,000
Fair Value Gain (Loss) at September 30, 2018	(72,000,000)
Interest rate derivatives | GEORGIA POWER CO | December 2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 500,000,000
Interest Rate Received	1.95%
Fair Value Gain (Loss) at September 30, 2018	$ (3,000,000)
Interest rate derivatives | GEORGIA POWER CO | December 2019 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 200,000,000
Interest Rate Received	4.25%
Fair Value Gain (Loss) at September 30, 2018	$ (3,000,000)
Interest rate derivatives | Southern Company | June 2020 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 300,000,000
Interest Rate Received	2.75%
Fair Value Gain (Loss) at September 30, 2018	$ (6,000,000)
Interest rate derivatives | Southern Company | July 2021 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 1,500,000,000
Interest Rate Received	2.35%
Fair Value Gain (Loss) at September 30, 2018	$ (60,000,000)
LIBOR | Interest rate derivatives | GEORGIA POWER CO | December 2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.76%
LIBOR | Interest rate derivatives | GEORGIA POWER CO | December 2019 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	2.46%
LIBOR | Interest rate derivatives | Southern Company | June 2020 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.92%
LIBOR | Interest rate derivatives | Southern Company | July 2021 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.87%

Derivatives - Narrative (Detail

Derivatives - Narrative (Details) MMBTU in Millions	9 Months Ended
	Sep. 30, 2018USD ($)MMBTU	Dec. 31, 2017USD ($)
Derivative [Line Items]
Collateral already posted, aggregate fair value | $	$ 189,000,000
ALABAMA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	2
GEORGIA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	4
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	1
MISSISSIPPI POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	2
SOUTHERN POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	4
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)
Registrants | Derivative Counterparties
Derivative [Line Items]
Collateral already posted, aggregate fair value | $	0
SOUTHERN Co GAS
Derivative [Line Items]
Collateral already posted, aggregate fair value | $	$ 189,000,000	$ 193,000,000
Southern Company
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	13
Cash flow hedge gain (loss) to be reclassified within twelve months | $	$ (19,000,000)

Derivatives - Foreign Currency

Derivatives - Foreign Currency Derivatives (Details) - 9 months ended Sep. 30, 2018 - Cash Flow Hedges of Existing Debt - Foreign currency derivatives € in Millions, $ in Millions	USD ($)	EUR (€)
Derivative [Line Items]
Pay Notional	$ 1,241
Receive Notional | €		€ 1,100
Fair Value Gain (Loss) at September 30, 2018	99
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, 2018	$ 48
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, 2018	$ 51

Derivatives - Derivative Financ

Derivatives - Derivative Financial Statement Presentation and Amounts With Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	$ 189
Southern Company
Derivatives, Fair Value [Line Items]
Derivatives, assets	543	$ 700
Derivatives, liabilities	668	794
Derivative asset, Gross amounts offset	(303)	(405)
Derivative liability, Gross amounts offset	(491)	(598)
Derivative asset	240	295
Derivative liability	177	196
Southern Company | Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivatives, assets	397	550
Derivatives, liabilities	515	652
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	263	380
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	317	437
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	134	170
Southern Company | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	198	215
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	20	17
Derivatives, liabilities	51	67
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	15	10
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	17	43
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	5	7
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	26	24
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Liabilities held for sale, current
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	6	0
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets held for sale
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
Southern Company | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Liabilities held for sale
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	2	0
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	126	133
Derivatives, liabilities	102	75
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	3	3
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	14
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 deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	0
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 deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	0
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	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 current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	22	4
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	0
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	50	34
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 current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	23	23
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	122	129
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	0
ALABAMA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	7	4
Derivatives, liabilities	10	10
Derivative asset, Gross amounts offset	(4)	(4)
Derivative liability, Gross amounts offset	(4)	(4)
Derivative asset	3	0
Derivative liability	6	6
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	7	4
Derivatives, liabilities	10	10
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	5	2
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	6
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	2	2
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	6	4
GEORGIA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	7	6
Derivatives, liabilities	28	24
Derivative asset, Gross amounts offset	(7)	(6)
Derivative liability, Gross amounts offset	(7)	(6)
Derivative asset	0	0
Derivative liability	21	18
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	7	6
Derivatives, liabilities	22	19
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	5	2
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	9	9
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	2	4
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	13	10
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	0	0
Derivatives, liabilities	6	5
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	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 current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	5	4
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	1	1
GULF POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
Derivatives, liabilities	8	21
Derivative asset	0	0
Derivative liability	8	21
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	0
Derivatives, liabilities	8	21
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	0
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	6	14
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	0
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	2	7
MISSISSIPPI POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	3
Derivatives, liabilities	9	9
Derivative asset, Gross amounts offset	(3)	(2)
Derivative liability, Gross amounts offset	(3)	(2)
Derivative asset	0	1
Derivative liability	6	7
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	2
Derivatives, liabilities	9	9
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	2	1
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	3	6
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	1	1
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	6	3
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	0	1
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	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 current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	0
SOUTHERN POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	125	132
Derivatives, liabilities	30	36
Derivative asset, Gross amounts offset	(2)	(3)
Derivative liability, Gross amounts offset	(2)	(3)
Derivative asset	123	129
Derivative liability	28	33
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	2
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	125	132
Derivatives, liabilities	30	34
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	2	3
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	6	11
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 deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	0
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 deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	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 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 current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	23	23
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	122	129
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	0
SOUTHERN Co GAS
Derivatives, Fair Value [Line Items]
Derivatives, assets	400	554
Derivatives, liabilities	516	660
Derivative asset, Gross amounts offset	(287)	(390)
Derivative liability, Gross amounts offset	(475)	(583)
Derivative asset	113	164
Derivative liability	41	77
Collateral already posted, aggregate fair value	189	193
Weather derivative premium	5	11
SOUTHERN Co GAS | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	3	6
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Derivatives, assets	396	549
Derivatives, liabilities	514	649
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	134	170
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	198	215
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	262	379
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	316	434
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	3	5
Derivatives, liabilities	2	8
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	0
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	1	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	3	5
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	8
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	1	0
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	$ 0	$ 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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	$ 189		$ 189
Gain (Loss) Recognized in OCI on Derivative	(15)	$ 39	(26)	$ 86
SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(15)	40	(26)	93
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	189		189		$ 193
Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(29)		(29)		(44)
Gain (Loss) Recognized in OCI on Derivative	(5)	(6)	7	(26)
Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(9)		(9)		(34)
Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(20)		(20)		(18)
Energy-related derivatives | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(6)		(6)
Energy-related derivatives | Assets held for sale
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(2)
Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	8		8		7
Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)					1
Energy-related derivatives | ALABAMA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(3)		(3)		(6)
Energy-related derivatives | ALABAMA POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(2)		(4)
Energy-related derivatives | ALABAMA POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(4)		(4)		(3)
Energy-related derivatives | ALABAMA POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | ALABAMA POWER CO | Assets held for sale
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | ALABAMA POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	3		3		0
Energy-related derivatives | ALABAMA POWER CO | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)					1
Energy-related derivatives | GEORGIA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(15)		(15)		(13)
Energy-related derivatives | GEORGIA POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(4)		(4)		(7)
Energy-related derivatives | GEORGIA POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(11)		(11)		(6)
Energy-related derivatives | GEORGIA POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | GEORGIA POWER CO | Assets held for sale
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | GEORGIA POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0		0
Energy-related derivatives | GEORGIA POWER CO | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)					0
Energy-related derivatives | GULF POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(8)		(8)		(21)
Energy-related derivatives | GULF POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(6)		(6)		(14)
Energy-related derivatives | GULF POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(2)		(7)
Energy-related derivatives | GULF POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | GULF POWER CO | Assets held for sale
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | GULF POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0		0
Energy-related derivatives | GULF POWER CO | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)					0
Energy-related derivatives | MISSISSIPPI POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(7)		(7)		(7)
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(2)		(5)
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(5)		(5)		(2)
Energy-related derivatives | MISSISSIPPI POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | MISSISSIPPI POWER CO | Assets held for sale
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0		0
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)					0
Energy-related derivatives | SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(5)	(6)	5	(21)
Energy-related derivatives | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	4		4		3
Collateral already posted, aggregate fair value	3		3		6
Gain (Loss) Recognized in OCI on Derivative	0	0	2	(4)
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(1)		(1)		(4)
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0		0
Energy-related derivatives | SOUTHERN Co GAS | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | SOUTHERN Co GAS | Assets held for sale
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	5		5		7
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)					$ 0
Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	0	(1)	(2)	(2)
Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(10)	46	(31)	114
Foreign currency derivatives | SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	$ (10)	$ 46	$ (31)	$ 114

Derivatives - Location and Amou

Derivatives - Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Cost of natural gas	$ 104	$ 134	$ 1,053	$ 1,085
Gain (loss) recognized in income	(15)	39	(26)	86
Depreciation and amortization	787	767	2,338	2,236
Interest expense, net of amounts capitalized	(458)	(407)	(1,386)	(1,248)
Other income (expense), net	57	65	195	165
Energy-related derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(5)	(6)	7	(26)
Interest rate derivatives
Derivative [Line Items]
Gain (loss) recognized in income	0	(1)	(2)	(2)
Foreign currency derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(10)	46	(31)	114
Cash Flow Hedging | Energy-related derivatives | Cost of natural gas
Derivative [Line Items]
Gain (loss) recognized in income	0	0	(2)	0
Cash Flow Hedging | Energy-related derivatives | Depreciation and amortization
Derivative [Line Items]
Gain (loss) recognized in income	0	(6)	2	(12)
Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(5)	(5)	(16)	(15)
Cash Flow Hedging | Foreign currency derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(6)	(5)	(18)	(17)
Cash Flow Hedging | Foreign currency derivatives | Other income (expense), net
Derivative [Line Items]
Gain (loss) recognized in income	(9)	43	(46)	139
Fair Value Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(4)	(5)	(35)	(6)
ALABAMA POWER CO
Derivative [Line Items]
Depreciation and amortization	192	185	570	549
Interest expense, net of amounts capitalized	(82)	(76)	(240)	(229)
Other income (expense), net	9	10	24	35
ALABAMA POWER CO | Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(1)	(2)	(4)	(5)
GEORGIA POWER CO
Derivative [Line Items]
Depreciation and amortization	232	225	690	669
Interest expense, net of amounts capitalized	(95)	(105)	(303)	(310)
Other income (expense), net	30	22	104	95
GEORGIA POWER CO | Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(1)	(1)	(4)	(3)
GEORGIA POWER CO | Fair Value Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	0	0	(1)	(1)
MISSISSIPPI POWER CO
Derivative [Line Items]
Depreciation and amortization	42	39	126	120
Interest expense, net of amounts capitalized	(19)	13	(59)	(23)
Other income (expense), net	0	1	28	4
MISSISSIPPI POWER CO | Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	0	0	(1)	(1)
SOUTHERN POWER CO
Derivative [Line Items]
Gain (loss) recognized in income	(15)	40	(26)	93
Depreciation and amortization	130	131	370	379
Interest expense, net of amounts capitalized	(45)	(47)	(138)	(144)
Other income (expense), net	17	3	22	3
SOUTHERN POWER CO | Energy-related derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(5)	(6)	5	(21)
SOUTHERN POWER CO | Foreign currency derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(10)	46	(31)	114
SOUTHERN POWER CO | Cash Flow Hedging | Energy-related derivatives | Depreciation and amortization
Derivative [Line Items]
Gain (loss) recognized in income	0	(6)	2	(12)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(6)	(5)	(18)	(17)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Other income (expense), net
Derivative [Line Items]
Gain (loss) recognized in income	(9)	43	(46)	139
SOUTHERN Co GAS
Derivative [Line Items]
Cost of natural gas	104	134	1,053	1,085
Depreciation and amortization	119	125	374	370
Interest expense, net of amounts capitalized	(52)	(51)	(170)	(145)
Other income (expense), net	6	19	21	30
SOUTHERN Co GAS | Energy-related derivatives
Derivative [Line Items]
Gain (loss) recognized in income	0	0	2	(4)
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Cost of natural gas
Derivative [Line Items]
Gain (loss) recognized in income	$ 0	$ 0	$ (2)	$ 0

Derivatives - Cumulative Basis

Derivatives - Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Securities due within one year
Derivative [Line Items]
Carrying Amount of the Hedged Item	$ (499)	$ (746)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	1	3
Securities due within one year | GEORGIA POWER CO
Derivative [Line Items]
Carrying Amount of the Hedged Item	(499)	(746)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	1	3
Long-term debt
Derivative [Line Items]
Carrying Amount of the Hedged Item	(2,526)	(2,553)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	65	35
Long-term debt | GEORGIA POWER CO
Derivative [Line Items]
Carrying Amount of the Hedged Item	(497)	(498)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	$ 2	$ 1

Derivatives - Pre-tax Effects_2

Derivatives - Pre-tax Effects of Derivatives Not Designated as Hedging (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net			$ (23)	$ 32
Weather Derivatives | Southern Company
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	$ 0	$ 0	0	15
Derivatives not designated as hedging instruments | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(34)	(15)	(74)	46
Derivatives not designated as hedging instruments | Energy-related derivatives | Natural gas revenues
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(36)	(17)	(79)	48
Derivatives not designated as hedging instruments | Energy-related derivatives | Cost of natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	2	2	5	(2)
Derivatives not designated as hedging instruments | Energy-related derivatives | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(34)	(15)	(74)	46
Derivatives not designated as hedging instruments | Energy-related derivatives | SOUTHERN Co GAS | Natural gas revenues
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(36)	(17)	(79)	48
Derivatives not designated as hedging instruments | Energy-related derivatives | SOUTHERN Co GAS | Cost of natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	$ 2	$ 2	$ 5	$ (2)

Acquisitions and Dispositions -

Acquisitions and Dispositions - Southern Company (Details) - Scenario, Forecast - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Jun. 28, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock purchase agreement, contract termination fees		$ 100
700 Universe, LLC | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock purchase agreement, contract termination fees		100
700 Universe, LLC | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock purchase agreement, contract termination fees		$ 200
Disposal Group, Disposed of by Sale, Not Discontinued Operations | GULF POWER CO
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from divestiture of businesses, net of cash divested	$ 5,750
Amount of indebtedness assumed at closing	$ 1,300

Acquisitions and Dispositions_2

Acquisitions and Dispositions - Schedule of Business Acquisitions and Construction Projects (Details) - SOUTHERN POWER CO - MW	1 Months Ended		3 Months Ended		6 Months Ended
	Jul. 31, 2018	Mar. 31, 2018	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Sep. 30, 2018	Aug. 31, 2018	May 31, 2018	Jul. 31, 2017
Gaskell West 1
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)						20
PPA Contract Period		20 years
Gaskell West 1 | Gaskell West 1 | Common Class B
Business Acquisition [Line Items]
Southern Power Percentage Ownership						100.00%
Gaskell West 1 | Gaskell West 1 | Class B Membership Interest
Business Acquisition [Line Items]
Southern Power Percentage Ownership						100.00%
Cactus Flats
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	148
Cactus Flats | Minimum
Business Acquisition [Line Items]
PPA Contract Period	12 years
Cactus Flats | Maximum
Business Acquisition [Line Items]
PPA Contract Period	15 years
Cactus Flats | Cactus Flats
Business Acquisition [Line Items]
Southern Power Percentage Ownership									100.00%
Cactus Flats | Cactus Flats | Class B Membership Interest
Business Acquisition [Line Items]
Southern Power Percentage Ownership							100.00%
Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)					385
PPA Contract Period					20 years
Wild Horse Mountain | Scenario, Forecast
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)				100
PPA Contract Period				20 years
Wild Horse Mountain | Wild Horse Mountain
Business Acquisition [Line Items]
Southern Power Percentage Ownership								100.00%
Wild Horse Mountain | Wild Horse Mountain | Class B Membership Interest
Business Acquisition [Line Items]
Southern Power Percentage Ownership								100.00%
Reading | Scenario, Forecast
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)			200
PPA Contract Period			12 years
Reading | Reading
Business Acquisition [Line Items]
Southern Power Percentage Ownership							100.00%
Reading | Reading | Class B Membership Interest
Business Acquisition [Line Items]
Southern Power Percentage Ownership							100.00%

Acquisitions and Dispositions_3

Acquisitions and Dispositions - Southern Power Narrative (Details)	1 Months Ended	2 Months Ended	3 Months Ended			8 Months Ended	9 Months Ended		12 Months Ended	36 Months Ended
	May 31, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)operating_facility	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2019USD ($)MW	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Construction work in progress				$ 7,655,000,000			$ 7,655,000,000				$ 6,904,000,000
Impairment charges				36,000,000	$ 0		197,000,000	$ 0
Increase (decrease) due to sale of noncontrolling interests							1,280,000,000
Assets subject to lien				111,717,000,000			111,717,000,000				111,005,000,000
Common Stockholders' Equity
Business Acquisition [Line Items]
Increase (decrease) due to sale of noncontrolling interests							(410,000,000)
SOUTHERN POWER CO
Business Acquisition [Line Items]
Construction work in progress				586,000,000			586,000,000				511,000,000
Impairment charges				36,000,000	$ 0		155,000,000	0
Repayments of debt	$ 770,000,000
Repayments of capital	250,000,000						650,000,000	$ 0
Increase (decrease) due to sale of noncontrolling interests	511,000,000
Assets subject to lien				15,355,000,000			15,355,000,000				$ 15,206,000,000
SOUTHERN POWER CO | Scenario, Forecast
Business Acquisition [Line Items]
Number of operating wind facilities | operating_facility			8
SOUTHERN POWER CO | Common Stockholders' Equity
Business Acquisition [Line Items]
Increase (decrease) due to sale of noncontrolling interests							(410,000,000)
SOUTHERN POWER CO | Disposal Group, Held-for-sale, Not Discontinued Operations | Florida Plants
Business Acquisition [Line Items]
Impairment charges							36,000,000
Proceeds from sale of equity method investments	195,000,000
Decrease in consideration per day between end of fiscal year and closing	110,000
Increase in consideration per day if closing is prior to end of fiscal year	$ 110,000
Pre-tax impairment charge							119,000,000
After tax impairment charge							89,000,000
SOUTHERN POWER CO | Wind Generating Facility
Business Acquisition [Line Items]
Impairment charges				36,000,000			155,000,000
SOUTHERN POWER CO | Wind Generating Facility | Scenario, Forecast
Business Acquisition [Line Items]
Project qualification for production tax credits, percentage									80.00%	100.00%
SOUTHERN POWER CO | SP Solar Holdings I, LP
Business Acquisition [Line Items]
Limited liability company or limited partnership, members or limited partners, ownership interest	33.00%
Proceeds from sale of equity method investments	$ 1,200,000,000
SOUTHERN POWER CO | Series of Construction Projects
Business Acquisition [Line Items]
Construction work in progress				246,000,000			246,000,000
SOUTHERN POWER CO | Series of Construction Projects | Minimum
Business Acquisition [Line Items]
Estimated future construction payments				575,000,000			575,000,000
SOUTHERN POWER CO | Series of Construction Projects | Maximum
Business Acquisition [Line Items]
Estimated future construction payments				640,000,000			640,000,000
SOUTHERN POWER CO | Plant Mankato | Senior Lien
Business Acquisition [Line Items]
Assets subject to lien				$ 500,000,000			$ 500,000,000
SOUTHERN POWER CO | Plant Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Expansion currently under construction (MW) | MW						385
Tax Equity Funding | SOUTHERN POWER CO | Scenario, Forecast
Business Acquisition [Line Items]
Repayments of capital		$ 1,000,000,000
Tax equity funding		$ 1,200,000,000	$ 1,200,000,000
Subsequent Event | SOUTHERN POWER CO | Disposal Group, Held-for-sale, Not Discontinued Operations | Plant Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Proceeds from sale of equity method investments						$ 650,000,000
Decrease per day after June 1, 2019						66,667
Maximum price decrease						$ 15,000,000
Subsequent Event | SOUTHERN POWER CO | Plant Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Expansion currently under construction (MW) | MW						385

Acquisitions and Dispositions_4

Acquisitions and Dispositions - Southern Company Gas (Details) $ in Millions	Jul. 29, 2018USD ($)	Jul. 01, 2018USD ($)utility	Jun. 04, 2018USD ($)	Sep. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Jul. 31, 2018utility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment							$ 42
Gain on disposition of business							324	$ 22
SOUTHERN Co GAS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment				$ 0	$ 42	$ 0	42	$ 0
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of natural gas distribution utilities disposed | utility									3
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Pivotal Home Solutions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business			$ 365
Gain (loss) on disposition of business, after tax			(73)
Tax on gain (loss) on disposition of business			$ 39
Goodwill impairment					$ 42		$ 42
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Elizabethtown Gas and Elkton Gas
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business		$ 1,700
Gain (loss) on disposition of business, after tax		$ 18
Number of natural gas distribution utilities disposed | utility		2
Working capital adjustment		$ 40
Gain on disposition of business		$ 230
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Florida City Gas
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business	$ 530
Gain (loss) on disposition of business, after tax	20
Gain on disposition of business	121
Long-term debt	3
Additional cash amount	$ 60

Acquisitions and Dispositions_5

Acquisitions and Dispositions - Held for Sale (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets Held for Sale:
Current assets	$ 407	$ 12
Liabilities Held for Sale:
Current liabilities	355	0
Other non-current liabilities	2,835	0
SOUTHERN POWER CO
Assets Held for Sale:
Current assets	18	1
Liabilities Held for Sale:
Current liabilities	4	$ 0
Disposal Group, Held-for-sale, Not Discontinued Operations
Assets Held for Sale:
Current assets	407
Total property, plant, and equipment	4,093
Other non-current assets	574
Total Assets Held for Sale	5,074
Liabilities Held for Sale:
Current liabilities	355
Long-term debt	1,285
Accumulated deferred income taxes	542
Other non-current liabilities	1,008
Total Liabilities Held for Sale	3,190
Disposal Group, Held-for-sale, Not Discontinued Operations | SOUTHERN POWER CO
Assets Held for Sale:
Current assets	18
Total property, plant, and equipment	168
Other non-current assets	17
Total Assets Held for Sale	203
Liabilities Held for Sale:
Current liabilities	4
Long-term debt	0
Accumulated deferred income taxes	0
Other non-current liabilities	0
Total Liabilities Held for Sale	$ 4

Acquisitions and Dispositions_6

Acquisitions and Dispositions - Pre-tax Profit (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
GULF POWER CO
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Earnings before income taxes	$ 59	$ 103	$ 146	$ 199
SOUTHERN POWER CO | Florida Plants
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Earnings before income taxes	$ 18	$ 11	$ 40	$ 28

Variable Interest Entity and _3

Variable Interest Entity and Equity Method Investments - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	May 31, 2018	Sep. 30, 2018
SOUTHERN POWER CO
Schedule of Equity Method Investments [Line Items]
Distribution made to limited partner, cash distributions paid, percentage		67.00%
Global Atlantic
Schedule of Equity Method Investments [Line Items]
Distribution made to limited partner, cash distributions paid, percentage		33.00%
SP Solar Holdings I, LP | SOUTHERN POWER CO
Schedule of Equity Method Investments [Line Items]
Limited liability company or limited partnership, members or limited partners, ownership interest	33.00%
SP Solar Holdings I, LP | Wholly Owned Subsidiary Of Southern Power | SOUTHERN POWER CO
Schedule of Equity Method Investments [Line Items]
Ownership interest	66.00%
SP Solar Holdings I, LP | Variable Interest Entity, Primary Beneficiary | SOUTHERN POWER CO
Schedule of Equity Method Investments [Line Items]
Assets		$ 6,400
Liabilities		111
Noncontrolling interests related to other partners' interests		$ 1,200
SP Solar Holdings I, LP | General Partner | Wholly Owned Subsidiary Of Southern Power | SOUTHERN POWER CO
Schedule of Equity Method Investments [Line Items]
Ownership interest	1.00%

Variable Interest Entity and _4

Variable Interest Entity and Equity Method Investments - Balance Sheet Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment Balance	$ 1,569	$ 1,513
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Investment Balance	1,529	1,477
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Investment Balance	1,260	1,262
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Investment Balance	73	41
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Investment Balance	70	57
SOUTHERN Co GAS | Other
Schedule of Equity Method Investments [Line Items]
Investment Balance	$ 126	$ 117

Variable Interest Entity and _5

Variable Interest Entity and Equity Method Investments - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	$ 36	$ 32	$ 108	$ 100
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	34	32	108	100
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	29	28	95	86
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	2	1	4	5
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	1	1	4	4
SOUTHERN Co GAS | Other
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	$ 2	$ 2	$ 5	$ 5

Variable Interest Entity and _6

Variable Interest Entity and Equity Method Investments - Investment in SNG (Details) - SNG - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement Information
Revenues	$ 145	$ 146	$ 451	$ 445
Operating income	71	71	230	218
Net income	$ 58	$ 57	$ 190	$ 172

Segment and Related Informati_3

Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)state	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)statesegment	Sep. 30, 2017USD ($)	Jul. 31, 2018utility
Segment Reporting Information [Line Items]
Number of reportable segments | segment			4
Number of states in which entity operates | state	4		4
Total operating revenues	$ 6,159	$ 6,201	$ 18,158	$ 17,403
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of reportable segments | segment			4
Number of states in which entity operates | state	7		7
Total operating revenues	$ 492	565	$ 2,861	2,841
SOUTHERN Co GAS | Traditional Electric Operating Companies
Segment Reporting Information [Line Items]
Natural gas revenues	14	9	22	19
SOUTHERN Co GAS | Southern Company
Segment Reporting Information [Line Items]
Natural gas revenues	38	38	96	94
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities disposed | utility					3
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Total operating revenues	635	618	1,699	1,597
SOUTHERN POWER CO | Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Total operating revenues	$ 134	$ 105	$ 326	$ 295

Segment and Related Informati_4

Segment and Related Information - Financial Data for Business Segments (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018USD ($)utility	Mar. 31, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)utility	Sep. 30, 2017USD ($)	Jul. 31, 2018utility	Dec. 31, 2017USD ($)
Segment Reporting Information [Line Items]
Operating revenues	$ 6,159		$ 6,201	$ 18,158	$ 17,403
Segment net income (loss)	1,164		1,069	1,948	347
Goodwill	5,315			5,315			$ 6,268
Total assets	111,717			111,717			111,005
Pre-tax impairment charges	36		0	197	0
Net gain (loss) on dispositions	353		0	317	19
Goodwill impairment				42
Plant Scherer Unit 3
Segment Reporting Information [Line Items]
Loss on Plant Scherer Unit 3					33
After tax charge to income					20
GEORGIA POWER AND MISSISSIPPI POWER CO | Kemper County Energy Facility
Segment Reporting Information [Line Items]
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment	1		34	1,100	3,200
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment after tax	1		21	800	2,200
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	635		618	1,699	1,597
Segment net income (loss)	92		124	235	276
Total assets	15,355			15,355			15,206
Pre-tax impairment charges	36		0	155	0
SOUTHERN POWER CO | Wind Generating Facility
Segment Reporting Information [Line Items]
Pre-tax impairment charges	36			155
After tax impairment charges	27			116
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	492		565	2,861	2,841
Segment net income (loss)	46		15	294	303
Goodwill	5,015			5,015			5,967
Total assets	20,398			20,398			22,987
Net gain (loss) on dispositions	353		0	317	0
Goodwill impairment	0	$ 42	0	42	0
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities disposed | utility						3
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Pivotal Home Solutions
Segment Reporting Information [Line Items]
Net gain (loss) on dispositions	353			317
Net gain (loss) on dispositions, net of tax	40			(35)
Goodwill impairment		$ 42		42
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,509		5,523	14,456	14,239
Segment net income (loss)	1,240		1,132	1,946	276
Goodwill	2			2			2
Total assets	90,102			90,102			87,085
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	492		565	2,861	2,841
Gas Distribution Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	4,034			4,034			4,702
Net gain (loss) on dispositions	351			351
Net gain (loss) on dispositions, net of tax	38			38
Goodwill impairment				0
Gas Marketing Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	981			981			1,265
Net gain (loss) on dispositions	2			(34)
Net gain (loss) on dispositions, net of tax	2			(73)
Goodwill impairment				42
Operating Segments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	497		607	2,902	3,000
Segment net income (loss)	64		44	338	325
Total assets	21,524			21,524			24,842
Operating Segments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	492		565	2,861	2,841
Segment net income (loss)	46		15	294	303
Goodwill	5,015			5,015			5,967
Total assets	20,398			20,398			22,987
Operating Segments | Gas Distribution Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	441		472	2,297	2,255
Segment net income (loss)	74		52	290	223
Total assets	16,850			16,850			19,358
Revenue on disposition	$ 8		50
Net income on disposition				$ 245	274
Operating Segments | Gas Distribution Operations | SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities disposed | utility	3			3
Operating Segments | Gas Marketing Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	$ 44		143	$ 403	597
Segment net income (loss)	(8)		1	(71)	36
Total assets	1,522			1,522			2,147
Revenue on disposition			32
Net income on disposition				55	95
Operating Segments | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	(8)		(24)	142	95
Segment net income (loss)	(18)		(23)	65	28
Total assets	855			855			1,096
Less Gross Gas Costs	1,663		1,538	5,057	5,048
Operating Segments | Gas Midstream Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	20		16	60	53
Segment net income (loss)	16		14	54	38
Total assets	2,297			2,297			2,241
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	202		153	984	442
Segment net income (loss)	(119)		(80)	(292)	(232)
Goodwill	298			298			299
Total assets	3,086			3,086			2,552
Operating Segments | All Other | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1		2	3	7
Segment net income (loss)	(18)		(29)	(44)	(22)
Total assets	10,146			10,146			12,184
Operating Segments | Third Party Gross Revenues | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,573		1,411	4,847	4,781
Operating Segments | Intercompany Revenues | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	82		103	352	362
Operating Segments | Total Gross Revenues | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,655		1,514	5,199	5,143
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(44)		(40)	(143)	(119)
Segment net income (loss)	(3)		2	0	0
Goodwill	0			0			0
Total assets	(1,869)			(1,869)			(1,619)
Eliminations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	(6)		(44)	(44)	(166)
Segment net income (loss)	0		0	0	0
Total assets	(11,272)			(11,272)			(14,039)
Reportable Subsegments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,014		5,017	13,117	12,960
Segment net income (loss)	1,148		1,008	1,711	0
Goodwill	0			0			0
Total assets	75,069			75,069			72,204
Reportable Subsegments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	635		618	1,699	1,597
Segment net income (loss)	92		124	235	276
Goodwill	2			2			2
Total assets	15,355			15,355			15,206
Intersubsegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(140)		(112)	(360)	(318)
Segment net income (loss)	0		$ 0	0	$ 0
Goodwill	0			0			0
Total assets	$ (322)			$ (322)			$ (325)

Segment and Related Informati_5

Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Total operating revenues	$ 6,159	$ 6,201	$ 18,158	$ 17,403
Retail
Revenue from External Customer [Line Items]
Total operating revenues	4,605	4,615	11,913	11,786
Electric Utilities' Revenues
Revenue from External Customer [Line Items]
Total operating revenues	5,509	5,523	14,456	14,239
Electric Utilities' Revenues | Retail
Revenue from External Customer [Line Items]
Total operating revenues	4,605	4,615	11,913	11,786
Electric Utilities' Revenues | Wholesale
Revenue from External Customer [Line Items]
Total operating revenues	693	718	1,923	1,867
Electric Utilities' Revenues | Other
Revenue from External Customer [Line Items]
Total operating revenues	211	190	620	586
Southern Company Gas' Revenues
Revenue from External Customer [Line Items]
Total operating revenues	492	565	2,861	2,841
Southern Company Gas' Revenues | Gas Distribution Operations
Revenue from External Customer [Line Items]
Total operating revenues	438	430	2,276	2,119
Southern Company Gas' Revenues | Gas Marketing Services
Revenue from External Customer [Line Items]
Total operating revenues	44	143	403	597
Southern Company Gas' Revenues | Other
Revenue from External Customer [Line Items]
Total operating revenues	$ 10	$ (8)	$ 182	$ 125