Derivatives - Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Southern Company
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	$ 25	$ 2
Southern Company | Depreciation and amortization
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	769	716
Southern Company | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	(458)	(416)
Southern Company | Interest expense, net of amounts capitalized | Foreign currency derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	36	17
Southern Company | Other income (expense), net | Foreign currency derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	36	17
Cash Flow Hedging | Southern Company | Amortization | Energy-related derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	1	(4)
Cash Flow Hedging | Southern Company | Interest expense, net of amounts capitalized | Interest rate derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	(5)	(5)
Cash Flow Hedging | Southern Company | Interest expense, net of amounts capitalized | Foreign currency derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	(5)	(6)
Fair Value Hedging | Southern Company | Interest expense, net of amounts capitalized | Interest rate derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	(24)	(8)
Fair Value Hedging | Southern Company | Other income (expense), net
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	60	48
SOUTHERN POWER CO
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	32	7
SOUTHERN POWER CO | Depreciation and amortization
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	114	119
SOUTHERN POWER CO | Cash Flow Hedging | Amortization | Energy-related derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	1	(4)
SOUTHERN POWER CO | Cash Flow Hedging | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	(47)	(50)
SOUTHERN POWER CO | Cash Flow Hedging | Interest expense, net of amounts capitalized | Foreign currency derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	(5)	(6)
SOUTHERN POWER CO | Cash Flow Hedging | Other income (expense), net
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	3	(1)
SOUTHERN POWER CO | Cash Flow Hedging | Other income (expense), net | Foreign currency derivatives
Derivative [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income	$ 36	$ 17