Derivatives - Pre-Tax Effects of Derivatives Designated as Cash Flow and Fair Value Hedging (Details) - USD ($) $ in Millions	6 Months Ended			12 Months Ended
	Jun. 30, 2018	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				$ (62)	$ 91	$ (220)
Depreciation and amortization				3,131	3,010	2,502
Interest expense, net of amounts capitalized				(1,842)	(1,694)	(1,317)
Other income (expense), net				114	163	50
Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				17	(47)	18
Energy-related derivatives | Cost of Sales | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative		$ (1)		2	(2)	(1)
Energy-related derivatives | Depreciation and Amortization | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				7	(16)	2
Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(1)	(2)	(180)
Interest rate derivatives | Interest Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(21)	(21)	(18)
Interest rate derivatives | Interest Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(12)	(22)	(21)
Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(78)	140	(58)
Foreign currency derivatives | Interest Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(24)	(23)	(13)
Foreign currency derivatives | Other Income (Expense) | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(60)	160	(82)
Natural gas revenues
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenue				1,539	1,601	613
Alabama Power
Derivative Instruments, Gain (Loss) [Line Items]
Depreciation and amortization				764	736	703
Interest expense, net of amounts capitalized				(323)	(305)	(302)
Other income (expense), net				20	43	26
Alabama Power | Interest rate derivatives | Interest Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(6)	(6)	(6)
Georgia Power
Derivative Instruments, Gain (Loss) [Line Items]
Depreciation and amortization				923	895	855
Interest expense, net of amounts capitalized				(397)	(419)	(388)
Other income (expense), net				115	104	81
Georgia Power | Interest rate derivatives | Interest Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(4)	(4)	(4)
Georgia Power | Interest rate derivatives | Interest Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				2	(3)	(1)
Mississippi Power
Derivative Instruments, Gain (Loss) [Line Items]
Depreciation and amortization				169	161	132
Interest expense, net of amounts capitalized				(76)	(42)	(74)
Other income (expense), net				17	1	(2)
Mississippi Power | Interest rate derivatives | Interest Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(2)	(2)	3
Southern Power
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(68)	102	(44)
Depreciation and amortization				493	503	352
Interest expense, net of amounts capitalized				(183)	(191)	(117)
Other income (expense), net				23	1	6
Southern Power | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				10	(38)	14
Southern Power | Energy-related derivatives | Depreciation and Amortization | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				7	(17)	2
Southern Power | Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(78)	140	(58)
Southern Power | Foreign currency derivatives | Interest Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(24)	(23)	(13)
Southern Power | Foreign currency derivatives | Other Income (Expense) | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative				(60)	159	$ (82)
Southern Company Gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative		(3)		7	(9)
Interest expense, net of amounts capitalized		(81)		(228)	(200)
Other income (expense), net		12		1	44
Southern Company Gas | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative		2		7	(9)
Southern Company Gas | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative		(5)		0	0
Southern Company Gas | Natural gas revenues
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenue		$ 613		$ 1,539	$ 1,601
Predecessor | Energy-related derivatives | Cost of Sales | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	$ (1)
Predecessor | Southern Company Gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(64)
Interest expense, net of amounts capitalized			$ (96)
Other income (expense), net			3
Predecessor | Southern Company Gas | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	0
Predecessor | Southern Company Gas | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(64)
Predecessor | Southern Company Gas | Natural gas revenues
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenue	$ 755		$ 755