Retirement Benefits - Amounts Recognized in Balance Sheets and Amounts in AOCI (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	$ 6,851		$ 6,943	$ 6,851
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	1,406		1,577	1,406
Other current liabilities	(925)		(868)	(925)
Employee benefit obligations	(2,299)		(2,256)	(2,299)
Other regulatory liabilities, deferred	(258)		(239)	(258)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net regulatory assets	5,866		(1,894)	5,866
Accumulated Other Comprehensive Income (Loss), Net of Tax	(180)		(189)	(180)
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	3,207		3,273	3,207
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(53)		(53)	(53)
Employee benefit obligations	(749)		(763)	(749)
Other regulatory liabilities, deferred	(87)		(118)	(87)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	55		17	55
Amortization prior service costs			5
Net (Gain) Loss, Estimated			213
Accumulated Other Comprehensive Income (Loss), Net of Tax	100		107	100
Pension plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			7	6
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	100		107	100	$ 125
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	4		3	4
Amortization prior service costs			1
Net (Gain) Loss, Estimated			9
Defined Benefit Plan Reclassification Adjustments			(8)	(7)
Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			155	145
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	3,120		3,155	3,120	2,998
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	51		14	51
Amortization prior service costs			4
Net (Gain) Loss, Estimated			204
Defined Benefit Plan Reclassification Adjustments			(166)	(158)
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	419		382	419
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(4)		(5)	(4)
Employee benefit obligations	(1,349)		(1,281)	(1,349)
Other regulatory liabilities, deferred	(41)		(41)	(41)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	25		21	25
Accumulated Other Comprehensive Income (Loss), Net of Tax	7		4	7
Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			0	0
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	7		4	7	8
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	0		0	0
Defined Benefit Plan Reclassification Adjustments			0	0
Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			13	14
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	378		341	378	411
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	25		21	25
Amortization prior service costs			7
Net (Gain) Loss, Estimated			14
Defined Benefit Plan Reclassification Adjustments			(19)	(20)
ALABAMA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	1,157		1,272	1,157
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	163		189	163
Other current liabilities	(76)		(59)	(76)
Employee benefit obligations	(300)		(304)	(300)
Other regulatory liabilities, deferred	(100)		(84)	(100)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net regulatory assets	1,047		(1,088)	1,047
Accumulated Other Comprehensive Income (Loss), Net of Tax	(30)		(26)	(30)
ALABAMA POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	870		890	870
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(12)		(12)	(12)
Employee benefit obligations	(134)		(150)	(134)
ALABAMA POWER CO | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			42	40
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	870		890	870	822
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	10		8	10
Amortization prior service costs			1
Net (Gain) Loss, Estimated			54
Defined Benefit Plan Reclassification Adjustments			(44)	(43)
ALABAMA POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	86		63	86
Amounts recognized in the consolidated balance sheets related to company's pension plans
Employee benefit obligations	(134)		(111)	(134)
Other regulatory liabilities, deferred	(10)		(7)	(10)
ALABAMA POWER CO | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	76		56	76
Defined Benefit Plan, Amortization of Gain (Loss)			1	2
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	76		56	76	82
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	15		11	15
Amortization prior service costs			4
Net (Gain) Loss, Estimated			1
Defined Benefit Plan Reclassification Adjustments			(5)	(6)
GEORGIA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	2,774		2,932	2,774
Amounts recognized in the consolidated balance sheets related to company's pension plans
Prepaid pension costs	0		23	0
Other deferred charges and assets	417		548	417
Other current liabilities	(182)		(198)	(182)
Employee benefit obligations	(703)		(659)	(703)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net regulatory assets	3,506		232	3,506
Accumulated Other Comprehensive Income (Loss), Net of Tax	(13)		(10)	(13)
GEORGIA POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	1,129		1,105	1,129
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(14)		(15)	(14)
Employee benefit obligations	(165)		(138)	(165)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	17		14	17
Amortization prior service costs			2
Net (Gain) Loss, Estimated			69
GEORGIA POWER CO | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			57	55
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	1,129		1,105	1,129	1,076
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments			(60)	(60)
GEORGIA POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	213		202	213
Amounts recognized in the consolidated balance sheets related to company's pension plans
Employee benefit obligations	(493)		(477)	(493)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	6		5	6
Amortization prior service costs			1
Net (Gain) Loss, Estimated			9
GEORGIA POWER CO | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			8	9
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	213		202	213	223
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments			(9)	(10)
GULF POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	512		502	512
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	21		23	21
Other current liabilities	(24)		(27)	(24)
Employee benefit obligations	(96)		(102)	(96)
Other regulatory liabilities, deferred	(45)		(43)	(45)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net regulatory assets	320		(105)	320
Accumulated Other Comprehensive Income (Loss), Net of Tax	1		0	1
GULF POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	153		160	153
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(1)		(1)	(1)
Employee benefit obligations	(25)		(33)	(25)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	3		2	3
Amortization prior service costs			0
Net (Gain) Loss, Estimated			10
GULF POWER CO | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			7	6
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	153		160	153	142
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments			(8)	(7)
GULF POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	11		8	11
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(1)		(1)	(1)
Employee benefit obligations	(64)		(62)	(64)
Other regulatory liabilities, deferred	(4)		(2)	(4)
GULF POWER CO | Other postretirement benefit plans | Regulatory Assets
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	7		6	7	5
MISSISSIPPI POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	518		437	518
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	56		33	56
Other current liabilities	(36)		(82)	(36)
Employee benefit obligations	(115)		(116)	(115)
Other regulatory liabilities, deferred	(77)		(79)	(77)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net regulatory assets	737		(42)	737
Accumulated Other Comprehensive Income (Loss), Net of Tax	(4)		(4)	(4)
MISSISSIPPI POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	154		158	154
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other current liabilities	(3)		(3)	(3)
Employee benefit obligations	(32)		(36)	(32)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	3		3	3
Amortization prior service costs			0
Net (Gain) Loss, Estimated			10
MISSISSIPPI POWER CO | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			7	7
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	154		158	154	144
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments			(8)	(8)
MISSISSIPPI POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	21		18	21
Amounts recognized in the consolidated balance sheets related to company's pension plans
Employee benefit obligations	(74)		(72)	(74)
Other regulatory liabilities, deferred	(2)		(1)	(2)
MISSISSIPPI POWER CO | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)			1	1
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	19		17	19	18
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments			(1)	(1)
SOUTHERN POWER CO
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	628		566	628
Other current liabilities	(152)		(143)	(152)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Accumulated Other Comprehensive Income (Loss), Net of Tax	35		(2)	35
SOUTHERN POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred			33
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost			1
Amortization prior service costs			0
Net (Gain) Loss, Estimated			2
SOUTHERN POWER CO | Other postretirement benefit plans
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred			3
Employee benefit obligations			(11)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net (Gain) Loss, Estimated			0
SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	973		901	973
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	170		218	170
Other current liabilities	(108)		(113)	(108)
Employee benefit obligations	(441)		(415)	(441)
Other regulatory liabilities, deferred	(29)		(30)	(29)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Net regulatory assets	(715)		(1,879)	(715)
Accumulated Other Comprehensive Income (Loss), Net of Tax	26		20	26
SOUTHERN Co GAS | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		40	0
Other Regulatory Assets Deferred	267		217	267
Defined Benefit Plan, Amortization of Gain (Loss)	(14)		(18)
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other deferred charges and assets	58		85	58
Other current liabilities	(2)		(3)	(2)
Employee benefit obligations	(206)		(198)	(206)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	(2)		(20)	(2)
SOUTHERN Co GAS | Pension plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		0	0
Defined Benefit Plan, Amortization of Gain (Loss)	0		0
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	(43)	$ 0	(42)	(43)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	0		0	0
Defined Benefit Plan Reclassification Adjustments	0		0
SOUTHERN Co GAS | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		40	0
Defined Benefit Plan, Amortization of Gain (Loss)	15		18
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	267	368	217	267
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	(2)		(20)	(2)
Amortization prior service costs			(2)
Net (Gain) Loss, Estimated			16
Defined Benefit Plan Reclassification Adjustments	(14)		(19)
SOUTHERN Co GAS | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		6	0
Other Regulatory Assets Deferred	52		46	52
Defined Benefit Plan, Amortization of Gain (Loss)	0		(4)
Amounts recognized in the consolidated balance sheets related to company's pension plans
Employee benefit obligations	(203)		(185)	(203)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	(12)		(7)	(12)
SOUTHERN Co GAS | Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		0	0
Defined Benefit Plan, Amortization of Gain (Loss)	0		0
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	(3)	0	(3)	(3)
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	0		0	0
Defined Benefit Plan Reclassification Adjustments	0		0
SOUTHERN Co GAS | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		6	0
Defined Benefit Plan, Amortization of Gain (Loss)	3		4
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred	52	77	46	52
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Prior Service Cost	(12)		(7)	$ (12)
Defined Benefit Plan Reclassification Adjustments	$ (2)		$ (1)
Predecessor | SOUTHERN Co GAS | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)		(13)			(31)
Predecessor | SOUTHERN Co GAS | Pension plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)		9
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred		274			282
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments		(8)
Predecessor | SOUTHERN Co GAS | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)		4
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred		84			88
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments		(4)
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)		(2)			(6)
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)		1
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred		35			36
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments		(1)
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)		1
Amounts recognized in the consolidated balance sheets related to company's pension plans
Other regulatory assets, deferred		30			$ 30
Amounts related to defined benefit pension Plans that had not yet been recognized in net periodic pension cost along with estimated amortization
Defined Benefit Plan Reclassification Adjustments		$ 0