Retirement Benefits - Components of Accumulated OCI and Changes in Regulatory Assets (Details) - USD ($)	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other postretirement benefit plans
Reclassification adjustments
Amortization of prior service costs			$ 21,000,000	$ 21,000,000	$ 6,000,000
Net periodic benefit cost			59,000,000	64,000,000	47,000,000
Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss) [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		$ 8,000,000	8,000,000	8,000,000
Net (gain) loss			(1,000,000)	0
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			(1,000,000)	0
Ending Balance	$ 7,000,000		7,000,000	8,000,000	8,000,000
Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		411,000,000	411,000,000	366,000,000
Net (gain) loss			(13,000,000)	33,000,000
Change in prior service costs				33,000,000
Reclassification adjustments
Amortization of prior service costs			(6,000,000)	(4,000,000)
Amortization of net gain (loss)			(14,000,000)	(17,000,000)
Total reclassification adjustments			(20,000,000)	(21,000,000)
Net periodic benefit cost			(33,000,000)	45,000,000
Ending Balance	378,000,000		378,000,000	411,000,000	366,000,000
Pension plans
Reclassification adjustments
Amortization of prior service costs			14,000,000	25,000,000	26,000,000
Net periodic benefit cost			66,000,000	218,000,000	139,000,000
Pension plans | Accumulated Other Comprehensive Income (Loss) [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		125,000,000	125,000,000	134,000,000
Net (gain) loss			(20,000,000)	1,000,000
Change in prior service costs			2,000,000
Reclassification adjustments
Amortization of prior service costs			(1,000,000)	(1,000,000)
Amortization of net gain (loss)			(6,000,000)	(9,000,000)
Total reclassification adjustments			(7,000,000)	(10,000,000)
Net periodic benefit cost			(25,000,000)	(9,000,000)
Ending Balance	100,000,000		100,000,000	125,000,000	134,000,000
Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		2,998,000,000	2,998,000,000	3,073,000,000
Net (gain) loss			243,000,000	155,000,000
Change in prior service costs			37,000,000
Reclassification adjustments
Amortization of prior service costs			(13,000,000)	(24,000,000)
Amortization of net gain (loss)			(145,000,000)	(206,000,000)
Total reclassification adjustments			(158,000,000)	(230,000,000)
Net periodic benefit cost			122,000,000	(75,000,000)
Ending Balance	3,120,000,000		3,120,000,000	2,998,000,000	3,073,000,000
Mississippi Power [Member]
Reclassification adjustments
Amortization of prior service costs			1,000,000	1,000,000	1,000,000
Net periodic benefit cost			5,000,000	12,000,000	7,000,000
Mississippi Power [Member] | Other postretirement benefit plans
Reclassification adjustments
Amortization of prior service costs			1,000,000	1,000,000	0
Net periodic benefit cost			4,000,000	4,000,000	3,000,000
Mississippi Power [Member] | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		18,000,000	18,000,000	16,000,000
Net (gain) loss			2,000,000	0
Change in prior service costs			0	3,000,000
Reclassification adjustments
Amortization of net gain (loss)			(1,000,000)	(1,000,000)
Total reclassification adjustments			(1,000,000)	(1,000,000)
Net periodic benefit cost			1,000,000	2,000,000
Ending Balance	19,000,000		19,000,000	18,000,000	16,000,000
Mississippi Power [Member] | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		144,000,000	144,000,000	151,000,000
Net (gain) loss			16,000,000	4,000,000
Change in prior service costs			2,000,000	0
Reclassification adjustments
Amortization of prior service costs			(1,000,000)	(1,000,000)
Amortization of net gain (loss)			(7,000,000)	(10,000,000)
Total reclassification adjustments			(8,000,000)	(11,000,000)
Net periodic benefit cost			10,000,000	(7,000,000)
Ending Balance	154,000,000		154,000,000	144,000,000	151,000,000
Georgia Power [Member] | Other postretirement benefit plans
Reclassification adjustments
Amortization of prior service costs			10,000,000	11,000,000	2,000,000
Net periodic benefit cost			24,000,000	28,000,000	17,000,000
Georgia Power [Member] | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		223,000,000	223,000,000	213,000,000
Net (gain) loss			0	9,000,000
Change in prior service costs			0	12,000,000
Reclassification adjustments
Amortization of prior service costs			(1,000,000)	0
Amortization of net gain (loss)			(9,000,000)	(11,000,000)
Total reclassification adjustments			(10,000,000)	(11,000,000)
Net periodic benefit cost			(10,000,000)	10,000,000
Ending Balance	213,000,000		213,000,000	223,000,000	213,000,000
Georgia Power [Member] | Pension plans
Reclassification adjustments
Amortization of prior service costs			5,000,000	9,000,000	10,000,000
Net periodic benefit cost			8,000,000	61,000,000	38,000,000
Georgia Power [Member] | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		1,076,000,000	1,076,000,000	1,102,000,000
Net (gain) loss			99,000,000	59,000,000
Change in prior service costs			14,000,000	0
Reclassification adjustments
Amortization of prior service costs			(5,000,000)	(9,000,000)
Amortization of net gain (loss)			(55,000,000)	(76,000,000)
Total reclassification adjustments			(60,000,000)	(85,000,000)
Net periodic benefit cost			53,000,000	(26,000,000)
Ending Balance	1,129,000,000		1,129,000,000	1,076,000,000	1,102,000,000
Alabama Power [Member] | Other postretirement benefit plans
Reclassification adjustments
Amortization of prior service costs			6,000,000	5,000,000	4,000,000
Net periodic benefit cost			4,000,000	5,000,000	4,000,000
Alabama Power [Member] | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		82,000,000	82,000,000	54,000,000
Net (gain) loss			0	25,000,000
Change in prior service costs			0	8,000,000
Reclassification adjustments
Amortization of prior service costs			(4,000,000)	(3,000,000)
Amortization of net gain (loss)			(2,000,000)	(2,000,000)
Total reclassification adjustments			(6,000,000)	(5,000,000)
Net periodic benefit cost			(6,000,000)	28,000,000
Ending Balance	76,000,000		76,000,000	82,000,000	54,000,000
Alabama Power [Member] | Pension plans
Reclassification adjustments
Amortization of prior service costs			3,000,000	6,000,000	7,000,000
Net periodic benefit cost			11,000,000	48,000,000	21,000,000
Alabama Power [Member] | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		822,000,000	822,000,000	827,000,000
Net (gain) loss			84,000,000	56,000,000
Change in prior service costs			7,000,000	0
Reclassification adjustments
Amortization of prior service costs			(3,000,000)	(6,000,000)
Amortization of net gain (loss)			(40,000,000)	(55,000,000)
Total reclassification adjustments			(43,000,000)	(61,000,000)
Net periodic benefit cost			48,000,000	(5,000,000)
Ending Balance	870,000,000		870,000,000	822,000,000	827,000,000
Gulf Power [Member]
Reclassification adjustments
Amortization of prior service costs			1,000,000	1,000,000	1,000,000
Net periodic benefit cost			4,000,000	10,000,000	7,000,000
Gulf Power [Member] | Other postretirement benefit plans
Reclassification adjustments
Net periodic benefit cost			3,000,000	3,000,000	3,000,000
Gulf Power [Member] | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		5,000,000	5,000,000	2,000,000
Net (gain) loss			2,000,000	1,000,000
Change in prior service costs			0	2,000,000
Reclassification adjustments
Net periodic benefit cost			2,000,000	3,000,000
Ending Balance	7,000,000		7,000,000	5,000,000	2,000,000
Gulf Power [Member] | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance		142,000,000	142,000,000	146,000,000
Net (gain) loss			16,000,000	6,000,000
Change in prior service costs			2,000,000	0
Reclassification adjustments
Amortization of prior service costs			(1,000,000)	(1,000,000)
Amortization of net gain (loss)			(6,000,000)	(9,000,000)
Total reclassification adjustments			(7,000,000)	(10,000,000)
Net periodic benefit cost			11,000,000	(4,000,000)
Ending Balance	153,000,000		153,000,000	142,000,000	146,000,000
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans
Reclassification adjustments
Net periodic benefit cost	5,000,000
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss) [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	0
Net (gain) loss	(3,000,000)
Reclassification adjustments
Amortization of prior service costs	0
Amortization of net gain (loss)	0
Total reclassification adjustments	0
Net periodic benefit cost	(3,000,000)
Ending Balance	(3,000,000)	0	(3,000,000)
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	77,000,000
Net (gain) loss	(23,000,000)
Reclassification adjustments
Amortization of prior service costs	1,000,000
Amortization of net gain (loss)	(3,000,000)
Total reclassification adjustments	(2,000,000)
Net periodic benefit cost	(25,000,000)
Ending Balance	52,000,000	77,000,000	52,000,000
Successor [Member] | Southern Company Gas [Member] | Pension plans
Reclassification adjustments
Net periodic benefit cost	13,000,000
Successor [Member] | Southern Company Gas [Member] | Pension plans | Accumulated Other Comprehensive Income (Loss) [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	0
Net (gain) loss	(43,000,000)
Reclassification adjustments
Amortization of prior service costs	0
Amortization of net gain (loss)	0
Total reclassification adjustments	0
Net periodic benefit cost	(43,000,000)
Ending Balance	(43,000,000)	0	(43,000,000)
Successor [Member] | Southern Company Gas [Member] | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	368,000,000
Net (gain) loss	(87,000,000)
Reclassification adjustments
Amortization of prior service costs	1,000,000
Amortization of net gain (loss)	(15,000,000)
Total reclassification adjustments	(14,000,000)
Net periodic benefit cost	(101,000,000)
Ending Balance	267,000,000	368,000,000	267,000,000
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans
Reclassification adjustments
Net periodic benefit cost		4,000,000		11,000,000	13,000,000
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Accumulated Other Comprehensive Income (Loss) [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	35,000,000	36,000,000	36,000,000	36,000,000
Net (gain) loss				2,000,000
Reclassification adjustments
Amortization of prior service costs		0		0
Amortization of net gain (loss)		(1,000,000)		(2,000,000)
Total reclassification adjustments		(1,000,000)		(2,000,000)
Net periodic benefit cost		(1,000,000)		0
Ending Balance		35,000,000		36,000,000	36,000,000
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	30,000,000	30,000,000	30,000,000	39,000,000
Net (gain) loss				(8,000,000)
Reclassification adjustments
Amortization of prior service costs		1,000,000		2,000,000
Amortization of net gain (loss)		(1,000,000)		(3,000,000)
Total reclassification adjustments		0		(1,000,000)
Net periodic benefit cost		0		(9,000,000)
Ending Balance		30,000,000		30,000,000	39,000,000
Predecessor [Member] | Southern Company Gas [Member] | Pension plans
Reclassification adjustments
Net periodic benefit cost		13,000,000		37,000,000	26,000,000
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Accumulated Other Comprehensive Income (Loss) [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	274,000,000	282,000,000	282,000,000	301,000,000
Net (gain) loss				0
Reclassification adjustments
Amortization of prior service costs		1,000,000		2,000,000
Amortization of net gain (loss)		(9,000,000)		(21,000,000)
Total reclassification adjustments		(8,000,000)		(19,000,000)
Net periodic benefit cost		(8,000,000)		(19,000,000)
Ending Balance		274,000,000		282,000,000	301,000,000
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Regulatory Assets
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	$ 84,000,000	88,000,000	$ 88,000,000	76,000,000
Net (gain) loss				22,000,000
Reclassification adjustments
Amortization of prior service costs		0		0
Amortization of net gain (loss)		(4,000,000)		(10,000,000)
Total reclassification adjustments		(4,000,000)		(10,000,000)
Net periodic benefit cost		(4,000,000)		12,000,000
Ending Balance		$ 84,000,000		$ 88,000,000	$ 76,000,000