Derivatives - Pre-Tax Effects of Derivatives Designated as Cash Flow Hedging (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
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			$ 91	$ (220)	$ (22)
Gain (loss) reclassified from AOCI into income (effective portion)			98	(112)	(9)
Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			(47)	18	0
Energy-related derivatives | Depreciation and Amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(16)	2	0
Energy-related derivatives | Cost of Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(2)	(1)	0
Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			(2)	(180)	(22)
Interest rate derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(21)	(18)	(9)
Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			140	(58)	0
Foreign currency derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(23)	(13)	0
Foreign currency derivatives | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			160	(82)	0
ALABAMA POWER CO | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(6)	(6)	(3)
ALABAMA POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			0	(3)	(7)
GEORGIA POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			1	0	(15)
GEORGIA POWER CO | Interest rate derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(4)	(4)	(3)
SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			102	(44)	0
Gain (loss) reclassified from AOCI into income (effective portion)			119	(94)	(1)
SOUTHERN POWER CO | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			(38)	14	0
SOUTHERN POWER CO | Energy-related derivatives | Amortization
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(17)	2	0
SOUTHERN POWER CO | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			0	0	0
SOUTHERN POWER CO | Interest rate derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			0	(1)	(1)
SOUTHERN POWER CO | Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives			140	(58)	0
SOUTHERN POWER CO | Foreign currency derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			(23)	(13)	0
SOUTHERN POWER CO | Foreign currency derivatives | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)			159	(82)	$ 0
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives	$ (3)
Gain (loss) reclassified from AOCI into income (effective portion)	(1)
SOUTHERN Co GAS | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives	2		(9)
SOUTHERN Co GAS | Energy-related derivatives | Cost of Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)	(1)		$ (2)
SOUTHERN Co GAS | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives	(5)
SOUTHERN Co GAS | Interest rate derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)	$ 0
Predecessor | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives		$ (64)		3
Gain (loss) reclassified from AOCI into income (effective portion)		(1)		(9)
Predecessor | SOUTHERN Co GAS | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives		0		3
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Cost of Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)		(1)		(10)
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Other Operations And Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)				(1)
Predecessor | SOUTHERN Co GAS | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on termination of interest rate derivatives		(64)		0
Predecessor | SOUTHERN Co GAS | Interest rate derivatives | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from AOCI into income (effective portion)		$ 0		$ 2