Retirement Benefits - Components of Accumulated OCI and Changes in Regulatory Assets (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
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	25		21	25
Net (Gain) Loss	360		324	360
Other Regulatory Assets Deferred	419		382	419
Reclassification adjustments
Amortization of prior service costs			20	21	$ 21
Net periodic benefit cost			57	59	64
Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	0		0	0
Net (Gain) Loss	7		4	7
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		$ 8	7	8
Net (gain) loss			(3)	(1)
Change in prior service costs			0
Reclassification adjustments
Amortization of prior service costs			0	0
Amortization of net gain (loss)			0	0
Total reclassification adjustments			0	0
Net periodic benefit cost			(3)	(1)
Ending Balance	7		4	7	8
Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	25		21	25
Amortization prior service costs			7
Net (Gain) Loss	353		320	353
Amortization of gains (losses)			14
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		411	378	411
Net (gain) loss			(21)	(13)
Change in prior service costs			3
Reclassification adjustments
Amortization of prior service costs			(6)	(6)
Amortization of net gain (loss)			(13)	(14)
Total reclassification adjustments			(19)	(20)
Net periodic benefit cost			(37)	(33)
Ending Balance	378		341	378	411
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	55		17	55
Amortization prior service costs			5
Net (Gain) Loss	3,165		3,244	3,165
Amortization of gains (losses)			213
Other Regulatory Assets Deferred	3,207		3,273	3,207
Reclassification adjustments
Amortization of prior service costs			12	14	25
Net periodic benefit cost			25	66	218
Pension plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	4		3	4
Amortization prior service costs			1
Net (Gain) Loss	96		104	96
Amortization of gains (losses)			9
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		125	100	125
Net (gain) loss			15	(20)
Change in prior service costs			0	2
Reclassification adjustments
Amortization of prior service costs			(1)	(1)
Amortization of net gain (loss)			(7)	(6)
Total reclassification adjustments			(8)	(7)
Net periodic benefit cost			7	(25)
Ending Balance	100		107	100	125
Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	51		14	51
Amortization prior service costs			4
Net (Gain) Loss	3,069		3,140	3,069
Amortization of gains (losses)			204
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		2,998	3,120	2,998
Net (gain) loss			227	243
Change in prior service costs			(26)	37
Reclassification adjustments
Amortization of prior service costs			(11)	(13)
Amortization of net gain (loss)			(155)	(145)
Total reclassification adjustments			(166)	(158)
Net periodic benefit cost			35	122
Ending Balance	3,120		3,155	3,120	2,998
MISSISSIPPI POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	518		437	518
Reclassification adjustments
Amortization of prior service costs			1	1	1
Net periodic benefit cost			3	5	12
MISSISSIPPI POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	21		18	21
Reclassification adjustments
Amortization of prior service costs			1	1	1
Net periodic benefit cost			4	4	4
MISSISSIPPI POWER CO | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		18	19	18
Net (gain) loss			(1)	2
Reclassification adjustments
Amortization of net gain (loss)			(1)	(1)
Total reclassification adjustments			(1)	(1)
Net periodic benefit cost			(2)	1
Ending Balance	19		17	19	18
MISSISSIPPI POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	3		3	3
Amortization prior service costs			0
Net (Gain) Loss	151		155	151
Amortization of gains (losses)			10
Other Regulatory Assets Deferred	154		158	154
MISSISSIPPI POWER CO | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		144	154	144
Net (gain) loss			12	16
Change in prior service costs			0	2
Reclassification adjustments
Amortization of prior service costs			(1)	(1)
Amortization of net gain (loss)			(7)	(7)
Total reclassification adjustments			(8)	(8)
Net periodic benefit cost			4	10
Ending Balance	154		158	154	144
ALABAMA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	1,157		1,272	1,157
ALABAMA POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	86		63	86
Reclassification adjustments
Amortization of prior service costs			5	6	5
Net periodic benefit cost			3	4	5
ALABAMA POWER CO | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	15		11	15
Amortization prior service costs			4
Net (Gain) Loss	61		45	61
Amortization of gains (losses)			1
Other Regulatory Assets Deferred	76		56	76
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		82	76	82
Net (gain) loss			(15)	0
Reclassification adjustments
Amortization of prior service costs			(4)	(4)
Amortization of net gain (loss)			(1)	(2)
Total reclassification adjustments			(5)	(6)
Net periodic benefit cost			(20)	(6)
Ending Balance	76		56	76	82
ALABAMA POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	870		890	870
Reclassification adjustments
Amortization of prior service costs			2	3	6
Net periodic benefit cost			9	11	48
ALABAMA POWER CO | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	10		8	10
Amortization prior service costs			1
Net (Gain) Loss	860		882	860
Amortization of gains (losses)			54
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		822	870	822
Net (gain) loss			64	84
Change in prior service costs			0	7
Reclassification adjustments
Amortization of prior service costs			(2)	(3)
Amortization of net gain (loss)			(42)	(40)
Total reclassification adjustments			(44)	(43)
Net periodic benefit cost			20	48
Ending Balance	870		890	870	822
GEORGIA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	2,774		2,932	2,774
GEORGIA POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	6		5	6
Amortization prior service costs			1
Net (Gain) Loss	207		197	207
Amortization of gains (losses)			9
Other Regulatory Assets Deferred	213		202	213
Reclassification adjustments
Amortization of prior service costs			9	10	11
Net periodic benefit cost			20	24	28
GEORGIA POWER CO | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		223	213	223
Net (gain) loss			(2)	0
Reclassification adjustments
Amortization of prior service costs			(1)	(1)
Amortization of net gain (loss)			(8)	(9)
Total reclassification adjustments			(9)	(10)
Net periodic benefit cost			(11)	(10)
Ending Balance	213		202	213	223
GEORGIA POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	17		14	17
Amortization prior service costs			2
Net (Gain) Loss	1,112		1,091	1,112
Amortization of gains (losses)			69
Other Regulatory Assets Deferred	1,129		1,105	1,129
Reclassification adjustments
Amortization of prior service costs			3	5	9
Net periodic benefit cost			(11)	8	61
GEORGIA POWER CO | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		1,076	1,129	1,076
Net (gain) loss			36	99
Change in prior service costs			0	14
Reclassification adjustments
Amortization of prior service costs			(3)	(5)
Amortization of net gain (loss)			(57)	(55)
Total reclassification adjustments			(60)	(60)
Net periodic benefit cost			(24)	53
Ending Balance	1,129		1,105	1,129	1,076
GULF POWER CO
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	512		502	512
Reclassification adjustments
Amortization of prior service costs			1	1	1
Net periodic benefit cost			2	4	10
GULF POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Net (Gain) Loss	(7)		(6)	(7)
Other Regulatory Assets Deferred	11		8	11
Reclassification adjustments
Net periodic benefit cost			3	3	3
GULF POWER CO | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		5	7	5
Net (gain) loss			(1)	2
Reclassification adjustments
Ending Balance	7		6	7	5
GULF POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	3		2	3
Amortization prior service costs			0
Net (Gain) Loss	150		158	150
Amortization of gains (losses)			10
Other Regulatory Assets Deferred	153		160	153
GULF POWER CO | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		142	153	142
Net (gain) loss			15	16
Change in prior service costs			0	2
Reclassification adjustments
Amortization of prior service costs			(1)	(1)
Amortization of net gain (loss)			(7)	(6)
Total reclassification adjustments			(8)	(7)
Net periodic benefit cost			7	11
Ending Balance	153		160	153	142
SOUTHERN POWER CO | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Net (Gain) Loss			3
Amortization of gains (losses)			0
Reclassification adjustments
Ending Balance			3
SOUTHERN POWER CO | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost			1
Amortization prior service costs			0
Net (Gain) Loss			32
Amortization of gains (losses)			2
Other Regulatory Assets Deferred			33
SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Other Regulatory Assets Deferred	973		901	973
SOUTHERN Co GAS | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	(12)		(7)	(12)
Net (Gain) Loss	(61)		(44)	(61)
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		6	0
Other Regulatory Assets Deferred	52		46	52
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Amortization of regulatory assets	2		0
Reclassification adjustments
Amortization of prior service costs	0		(3)
Amortization of net gain (loss)	0		4
Net periodic benefit cost	5		6
SOUTHERN Co GAS | Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	0		0	0
Net (Gain) Loss	3		3	3
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		0	0
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	0		(3)
Net (gain) loss	(3)		0
Reclassification adjustments
Amortization of prior service costs	0		0
Amortization of net gain (loss)	0		0
Total reclassification adjustments	0		0
Net periodic benefit cost	(3)		0
Ending Balance	(3)	0	(3)	(3)
SOUTHERN Co GAS | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	(12)		(7)	(12)
Net (Gain) Loss	(64)		(47)	(64)
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		6	0
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	77		52
Net (gain) loss	(23)		(5)
Reclassification adjustments
Amortization of prior service costs	1		3
Amortization of net gain (loss)	(3)		(4)
Total reclassification adjustments	(2)		(1)
Net periodic benefit cost	(25)		(6)
Ending Balance	52	77	46	52
SOUTHERN Co GAS | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	(2)		(20)	(2)
Net (Gain) Loss	226		155	226
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		40	0
Other Regulatory Assets Deferred	267		217	267
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Amortization of regulatory assets	0		1
Reclassification adjustments
Amortization of prior service costs	(1)		0
Amortization of net gain (loss)	14		18
Net periodic benefit cost	13		14
SOUTHERN Co GAS | Pension plans | Accumulated Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	0		0	0
Net (Gain) Loss	(43)		(42)	(43)
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		0	0
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	0		(43)
Net (gain) loss	(43)		1
Amortization of regulatory assets			0
Reclassification adjustments
Amortization of prior service costs	0
Amortization of net gain (loss)	0		0
Total reclassification adjustments	0		0
Net periodic benefit cost	(43)		1
Ending Balance	(43)	0	(42)	(43)
SOUTHERN Co GAS | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost	(2)		(20)	(2)
Amortization prior service costs			(2)
Net (Gain) Loss	269		197	269
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization	0		40	0
Amortization of gains (losses)			16
Defined Benefit Plan, Accumulated Other Comprehensive Income, Regulatory Amortization, Next Twelve Months			3
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	368		267
Net (gain) loss	(87)		(31)
Amortization of regulatory assets			(1)
Reclassification adjustments
Amortization of prior service costs	1
Amortization of net gain (loss)	(15)		(18)
Total reclassification adjustments	(14)		(19)
Net periodic benefit cost	(101)		(50)
Ending Balance	267	368	$ 217	267
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Amortization of regulatory assets		0			0
Reclassification adjustments
Amortization of prior service costs		(1)			(3)
Amortization of net gain (loss)		2			6
Net periodic benefit cost		4			11
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss)
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	35	36		36
Net (gain) loss		0
Reclassification adjustments
Amortization of prior service costs		0
Amortization of net gain (loss)		(1)
Total reclassification adjustments		(1)
Net periodic benefit cost		(1)
Ending Balance		35			36
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	30	30		30
Net (gain) loss		0
Reclassification adjustments
Amortization of prior service costs		1
Amortization of net gain (loss)		(1)
Total reclassification adjustments		0
Net periodic benefit cost		0
Ending Balance		30			30
Predecessor | SOUTHERN Co GAS | Pension plans
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Amortization of regulatory assets		0			0
Reclassification adjustments
Amortization of prior service costs		(1)			(2)
Amortization of net gain (loss)		13			31
Net periodic benefit cost		13			37
Predecessor | SOUTHERN Co GAS | Pension plans | Accumulated Other Comprehensive Income (Loss)
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	274	282		282
Reclassification adjustments
Amortization of prior service costs		1
Amortization of net gain (loss)		(9)
Total reclassification adjustments		(8)
Net periodic benefit cost		(8)
Ending Balance		274			282
Predecessor | SOUTHERN Co GAS | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	$ 84	88		$ 88
Reclassification adjustments
Amortization of prior service costs		0
Amortization of net gain (loss)		(4)
Total reclassification adjustments		(4)
Net periodic benefit cost		(4)
Ending Balance		$ 84			$ 88