Summary of Significant Accounting Policies - Schedule of Specific Impacts of Applying ASC 606 to Revenues from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Statements of Income
Revenues										$ 23,495	$ 23,031	$ 19,896
Other operations and maintenance										5,889	5,739	5,382
Taxes other than income taxes										1,315	1,250	1,113
Operating Income	$ 578	$ 2,174	$ 63	$ 1,376	$ 739	$ 1,991	$ (1,649)	$ 1,252		4,191	2,333	4,486
Other income (expense), net										114	163	50
Earnings Before Income Taxes										2,749	1,068	3,480
Income taxes										449	142	951
Net Income										2,300	926	2,529
Net Income (Loss)	269	1,222	(127)	936	500	1,109	(1,348)	665		2,226	842	2,448
Changes in certain current assets and liabilities —
Receivables										(426)	(202)	(544)
Other current assets										(127)	(143)	(206)
Accrued taxes										267	(142)	1,456
Other current liabilities										63	(45)	250
Balance Sheets
Unbilled revenues	654				810					654	810
Other accounts and notes receivable	813				698					813	698
Other current assets	162				62					162	62
Accrued taxes	656				636					656	636
Other current liabilities	852				874					852	874
Total Stockholders' Equity	29,039				25,528				$ 26,612	29,039	25,528	26,612		$ 21,982
Natural gas revenues
Statements of Income
Revenues										3,854	3,791	1,596
Other revenues
Statements of Income
Revenues										1,239	803	442
Alabama Power
Statements of Income
Revenues										6,032	6,039	5,889
Other operations and maintenance										1,669	1,709	1,557
Taxes other than income taxes										389	384	380
Operating Income	164	561	380	372	255	601	440	361		1,477	1,657	1,618
Other income (expense), net										20	43	26
Earnings Before Income Taxes										1,236	1,434	1,370
Income taxes										291	568	531
Net Income										945	866	839
Net Income (Loss)	73	373	259	225	119	325	230	174		930	848	822
Changes in certain current assets and liabilities —
Receivables										(149)	(168)	94
Other current assets										30	20	38
Accrued taxes										10	(84)	93
Other current liabilities										128	3	11
Balance Sheets
Unbilled revenues	150				162					150	162
Other accounts and notes receivable	51				55					51	55
Other current assets	18				5					18	5
Accrued taxes	44				54					44	54
Other current liabilities	161				53					161	53
Total Stockholders' Equity	7,477				6,829				6,323	7,477	6,829	6,323		5,992
Alabama Power | Other revenues
Statements of Income
Revenues										267	208	215
Georgia Power
Statements of Income
Revenues										8,420	8,310	8,383
Other operations and maintenance										1,860	1,724	2,003
Taxes other than income taxes										437	409	405
Operating Income	257	991	(472)	513	452	1,017	621	483		1,289	2,573	2,434
Other income (expense), net										115	104	81
Earnings Before Income Taxes										1,007	2,258	2,127
Income taxes										214	830	780
Net Income										793	1,428	1,347
Net Income (Loss)	173	664	(396)	352	227	580	347	260		793	1,414	1,330
Changes in certain current assets and liabilities —
Receivables										8	(256)	60
Other current assets										(43)	(28)	(38)
Accrued taxes										58	1	131
Other current liabilities										(107)	(33)	28
Balance Sheets
Unbilled revenues	208				255					208	255
Other accounts and notes receivable	80				76					80	76
Other current assets	70				14					70	14
Other current liabilities	183				198					183	198
Total Stockholders' Equity	14,323				11,931				11,356	14,323	11,931	11,356		$ 10,719
Georgia Power | Other revenues
Statements of Income
Revenues										481	383	394
Southern Company Gas
Statements of Income
Revenues									1,652	3,909	3,920
Other operations and maintenance									480	981	945
Taxes other than income taxes									71	211	184
Operating Income	104	374	49	388	109	67	95	389	199	915	660
Other income (expense), net									12	1	44
Earnings Before Income Taxes									190	836	610
Income taxes									76	464	367
Net Income									114	372	243
Net Income (Loss)	78	$ 46	$ (31)	$ 279	(60)	$ 15	$ 49	$ 239	114	372	243
Changes in certain current assets and liabilities —
Receivables									(490)	(218)	(94)
Other current assets									(31)	4	(24)
Accrued taxes									8	10	110
Other current liabilities									24	(22)	(8)
Balance Sheets
Unbilled revenues	213				285					213	285
Other accounts and notes receivable	142				91					142	91
Other current assets	50				52					50	52
Accrued taxes	66				40					66	40
Other current liabilities	130				159					130	159
Total Stockholders' Equity	8,570				$ 9,022				9,109	8,570	9,022	$ 9,109	$ 8,001
Southern Company Gas | Natural gas revenues
Statements of Income
Revenues									1,591	3,874	3,787
Southern Company Gas | Other revenues
Statements of Income
Revenues									$ 56	55	$ 129
Balances Without Adoption of ASC 606
Statements of Income
Other operations and maintenance										5,830
Operating Income										4,243
Other income (expense), net										60
Earnings Before Income Taxes										2,747
Income taxes										448
Net Income										2,299
Net Income (Loss)										2,225
Changes in certain current assets and liabilities —
Receivables										(472)
Other current assets										(81)
Accrued taxes										268
Other current liabilities										61
Balance Sheets
Unbilled revenues	728									728
Other accounts and notes receivable	814									814
Other current assets	87									87
Accrued taxes	655									655
Other current liabilities	854									854
Total Stockholders' Equity	29,038									29,038
Balances Without Adoption of ASC 606 | Natural gas revenues
Statements of Income
Revenues										3,852
Balances Without Adoption of ASC 606 | Other revenues
Statements of Income
Revenues										1,234
Balances Without Adoption of ASC 606 | Alabama Power
Statements of Income
Other operations and maintenance										1,625
Taxes other than income taxes										388
Operating Income										1,485
Other income (expense), net										12
Balances Without Adoption of ASC 606 | Alabama Power | Other revenues
Statements of Income
Revenues										230
Balances Without Adoption of ASC 606 | Georgia Power
Statements of Income
Other operations and maintenance										1,772
Operating Income										1,283
Other income (expense), net										121
Changes in certain current assets and liabilities —
Receivables										1
Other current assets										(36)
Balance Sheets
Unbilled revenues	243									243
Other accounts and notes receivable	81									81
Other current assets	34									34
Balances Without Adoption of ASC 606 | Georgia Power | Other revenues
Statements of Income
Revenues										387
Balances Without Adoption of ASC 606 | Southern Company Gas
Statements of Income
Operating Income										913
Earnings Before Income Taxes										834
Income taxes										463
Net Income										371
Net Income (Loss)										371
Changes in certain current assets and liabilities —
Accrued taxes										11
Other current liabilities										(24)
Balance Sheets
Accrued taxes	65									65
Other current liabilities	132									132
Total Stockholders' Equity	8,569									8,569
Balances Without Adoption of ASC 606 | Southern Company Gas | Natural gas revenues
Statements of Income
Revenues										3,872
Effect of Change
Statements of Income
Other operations and maintenance										59
Operating Income										(52)
Other income (expense), net										54
Earnings Before Income Taxes										2
Income taxes										1
Net Income										1
Net Income (Loss)										1
Changes in certain current assets and liabilities —
Receivables										46
Other current assets										(46)
Accrued taxes										(1)
Other current liabilities										2
Balance Sheets
Unbilled revenues	(74)									(74)
Other accounts and notes receivable	(1)									(1)
Other current assets	75									75
Accrued taxes	1									1
Other current liabilities	(2)									(2)
Total Stockholders' Equity	1									1
Effect of Change | Natural gas revenues
Statements of Income
Revenues										2
Effect of Change | Other revenues
Statements of Income
Revenues										5
Effect of Change | Alabama Power
Statements of Income
Other operations and maintenance										44
Taxes other than income taxes										1
Operating Income										(8)
Other income (expense), net										8
Effect of Change | Alabama Power | Other revenues
Statements of Income
Revenues										37
Effect of Change | Georgia Power
Statements of Income
Other operations and maintenance										88
Operating Income										6
Other income (expense), net										(6)
Changes in certain current assets and liabilities —
Receivables										7
Other current assets										(7)
Balance Sheets
Unbilled revenues	(35)									(35)
Other accounts and notes receivable	(1)									(1)
Other current assets	36									36
Effect of Change | Georgia Power | Other revenues
Statements of Income
Revenues										94
Effect of Change | Southern Company Gas
Statements of Income
Operating Income										2
Earnings Before Income Taxes										2
Income taxes										1
Net Income										1
Net Income (Loss)										1
Changes in certain current assets and liabilities —
Accrued taxes										(1)
Other current liabilities										2
Balance Sheets
Accrued taxes	1									1
Other current liabilities	(2)									(2)
Total Stockholders' Equity	$ 1									1
Effect of Change | Southern Company Gas | Natural gas revenues
Statements of Income
Revenues										$ 2