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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Cost of natural gas	$ 104	$ 134	$ 1,053	$ 1,085
Gain (loss) recognized in income	(15)	39	(26)	86
Depreciation and amortization	787	767	2,338	2,236
Interest expense, net of amounts capitalized	(458)	(407)	(1,386)	(1,248)
Other income (expense), net	57	65	195	165
Energy-related derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(5)	(6)	7	(26)
Interest rate derivatives
Derivative [Line Items]
Gain (loss) recognized in income	0	(1)	(2)	(2)
Foreign currency derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(10)	46	(31)	114
Cash Flow Hedging | Energy-related derivatives | Cost of natural gas
Derivative [Line Items]
Gain (loss) recognized in income	0	0	(2)	0
Cash Flow Hedging | Energy-related derivatives | Depreciation and amortization
Derivative [Line Items]
Gain (loss) recognized in income	0	(6)	2	(12)
Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(5)	(5)	(16)	(15)
Cash Flow Hedging | Foreign currency derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(6)	(5)	(18)	(17)
Cash Flow Hedging | Foreign currency derivatives | Other income (expense), net
Derivative [Line Items]
Gain (loss) recognized in income	(9)	43	(46)	139
Fair Value Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(4)	(5)	(35)	(6)
ALABAMA POWER CO
Derivative [Line Items]
Depreciation and amortization	192	185	570	549
Interest expense, net of amounts capitalized	(82)	(76)	(240)	(229)
Other income (expense), net	9	10	24	35
ALABAMA POWER CO | Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(1)	(2)	(4)	(5)
GEORGIA POWER CO
Derivative [Line Items]
Depreciation and amortization	232	225	690	669
Interest expense, net of amounts capitalized	(95)	(105)	(303)	(310)
Other income (expense), net	30	22	104	95
GEORGIA POWER CO | Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(1)	(1)	(4)	(3)
GEORGIA POWER CO | Fair Value Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	0	0	(1)	(1)
MISSISSIPPI POWER CO
Derivative [Line Items]
Depreciation and amortization	42	39	126	120
Interest expense, net of amounts capitalized	(19)	13	(59)	(23)
Other income (expense), net	0	1	28	4
MISSISSIPPI POWER CO | Cash Flow Hedging | Interest rate derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	0	0	(1)	(1)
SOUTHERN POWER CO
Derivative [Line Items]
Gain (loss) recognized in income	(15)	40	(26)	93
Depreciation and amortization	130	131	370	379
Interest expense, net of amounts capitalized	(45)	(47)	(138)	(144)
Other income (expense), net	17	3	22	3
SOUTHERN POWER CO | Energy-related derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(5)	(6)	5	(21)
SOUTHERN POWER CO | Foreign currency derivatives
Derivative [Line Items]
Gain (loss) recognized in income	(10)	46	(31)	114
SOUTHERN POWER CO | Cash Flow Hedging | Energy-related derivatives | Depreciation and amortization
Derivative [Line Items]
Gain (loss) recognized in income	0	(6)	2	(12)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) recognized in income	(6)	(5)	(18)	(17)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Other income (expense), net
Derivative [Line Items]
Gain (loss) recognized in income	(9)	43	(46)	139
SOUTHERN Co GAS
Derivative [Line Items]
Cost of natural gas	104	134	1,053	1,085
Depreciation and amortization	119	125	374	370
Interest expense, net of amounts capitalized	(52)	(51)	(170)	(145)
Other income (expense), net	6	19	21	30
SOUTHERN Co GAS | Energy-related derivatives
Derivative [Line Items]
Gain (loss) recognized in income	0	0	2	(4)
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Cost of natural gas
Derivative [Line Items]
Gain (loss) recognized in income	$ 0	$ 0	$ (2)	$ 0