Derivatives - Pre-tax Effects of Derivatives Designated as Cash Flow Hedging Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	$ 76			$ 76
Southern Company
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	39	$ 31		86	$ (191)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	27	(4)		95	(32)
Southern Company | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(14)			(14)		$ 33
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(6)	0		(26)	(1)
Southern Company | Energy-related derivatives | Depreciation and amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(6)	1		(12)	1
Southern Company | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(18)			(18)		(16)
Southern Company | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(12)			(12)		(19)
Southern Company | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	14			14		56
Southern Company | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	2			2		12
Southern Company | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(1)	(6)		(2)	(189)
Southern Company | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5)	(6)		(15)	(13)
Southern Company | Foreign Exchange Contract
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	46	37		114	(1)
Southern Company | Foreign Exchange Contract | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5)	(6)		(17)	(7)
Southern Company | Foreign Exchange Contract | Other income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	43	7		139	(13)
ALABAMA POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	2			2		11
ALABAMA POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		(1)
ALABAMA POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		0
ALABAMA POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	3			3		8
ALABAMA POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	1			1		4
ALABAMA POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	0	0		0	(3)
ALABAMA POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(2)	(2)		(5)	(5)
GEORGIA POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	7			7		36
GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		0
GEORGIA POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	7			7		29
GEORGIA POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	1			1		7
GEORGIA POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	0	0		0	0
GEORGIA POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1)	(1)		(3)	(3)
GULF POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				(2)	(7)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	0
GULF POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(22)			(22)		(24)
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				(1)	0
GULF POWER CO | Energy-related derivatives | Depreciation and amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	0
GULF POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(13)			(13)		(9)
GULF POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(9)			(9)		(16)
GULF POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		1
GULF POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
GULF POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				(1)	(7)
GULF POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	0
MISSISSIPPI POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(4)			(4)		(7)
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(3)			(3)		(5)
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		(3)
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		1
MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
MISSISSIPPI POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(1)	(1)		0	(1)
MISSISSIPPI POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0	0		(1)	(1)
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	76			76		62
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		(5)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)		0
SOUTHERN Co GAS | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	3			3		17
Collateral already posted, aggregate fair value	1			1		8
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		0		(4)
SOUTHERN Co GAS | Energy-related derivatives | Cost of natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)		0		0
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	(1)			(1)		(1)
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		0
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	4			4		17
SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Regulatory hedge unrealized gain (loss) recognized on the balance sheet	0			0		$ 1
SOUTHERN Co GAS | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	0	(5)
SOUTHERN Co GAS | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0	0
SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	40	37		93	(2)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	32	2		110	(20)
SOUTHERN POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(6)	0		(21)	(1)
SOUTHERN POWER CO | Energy-related derivatives | Depreciation and amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(6)	1		(12)	1
SOUTHERN POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)				0	0
SOUTHERN POWER CO | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)				0	(1)
SOUTHERN POWER CO | Foreign Exchange Contract
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	46	37		114	(1)
SOUTHERN POWER CO | Foreign Exchange Contract | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5)	(6)		(17)	(7)
SOUTHERN POWER CO | Foreign Exchange Contract | Other income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 43	$ 7		$ 139	$ (13)
Predecessor | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)			$ (64)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			(1)
Predecessor | SOUTHERN Co GAS | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)			0
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Cost of natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			(1)
Predecessor | SOUTHERN Co GAS | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)			(64)
Predecessor | SOUTHERN Co GAS | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			$ 0