Derivatives - Financial Statement Presentation (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair value of energy-related derivatives and interest rate derivatives
Collateral already posted, aggregate fair value	$ 193	$ 62
Weather derivative premium	11	4
Energy-related derivatives
Fair value of energy-related derivatives and interest rate derivatives
Collateral already posted, aggregate fair value	6	8
Parent Company
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	700	690
Derivative asset, gross amount offset	(405)	(462)
Derivative Asset	295	228
Derivative liability, gross	794	718
Derivative liability, gross amounts offset	(598)	(524)
Derivative Liability	196	194
Parent Company | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	550	556
Derivative liability, gross	652	564
Parent Company | Energy-related derivatives | Other current assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	380	489
Parent Company | Energy-related derivatives | Liabilities from risk management activities, net of collateral | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	437	483
Parent Company | Energy-related derivatives | Other deferred charges and assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	170	66
Parent Company | Energy-related derivatives | Other deferred credits and liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	215	81
Parent Company | Interest rate derivatives | Other current assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	1
ALABAMA POWER CO
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	4	20
Derivative asset, gross amount offset	(4)	(8)
Derivative Asset	0	12
Derivative liability, gross	10	9
Derivative liability, gross amounts offset	(4)	(8)
Derivative Liability	6	1
GEORGIA POWER CO
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	6	46
Derivative asset, gross amount offset	(6)	(8)
Derivative Asset	0	38
Derivative liability, gross	(24)	11
Derivative liability, gross amounts offset	6	(8)
Derivative Liability	(18)	3
GULF POWER CO
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	5
Derivative asset, gross amount offset	0	(4)
Derivative Asset	0	1
Derivative liability, gross	21	29
Derivative liability, gross amounts offset	0	(4)
Derivative Liability	21	25
MISSISSIPPI POWER CO
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	3	7
Derivative asset, gross amount offset	(2)	(3)
Derivative Asset	1	4
Derivative liability, gross	9	11
Derivative liability, gross amounts offset	(2)	(3)
Derivative Liability	7	8
SOUTHERN POWER CO | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	4
Derivative liability, gross	2	1
SOUTHERN POWER CO | Energy-related derivatives | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	132	22
Derivative asset, gross amount offset	(3)	(5)
Derivative Asset	129	17
Derivative liability, gross	36	63
Derivative liability, gross amounts offset	(3)	(5)
Derivative Liability	33	58
SOUTHERN POWER CO | Energy-related derivatives | Other deferred charges and assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	3
SOUTHERN POWER CO | Energy-related derivatives | Other deferred credits and liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	2	1
SOUTHERN POWER CO | Interest rate derivatives | Other current assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	1
SOUTHERN POWER CO | Interest rate derivatives | Other current liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	0	0
SOUTHERN Co GAS
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	554
Derivative asset, gross amount offset	(390)
Derivative Asset	164
Derivative liability, gross	581
Derivative liability, gross amounts offset	(435)
Derivative Liability	146
Collateral already posted, aggregate fair value	193	62
Weather derivative premium	11	4
SOUTHERN Co GAS | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	549
Derivative liability, gross	552
SOUTHERN Co GAS | Energy-related derivatives
Fair value of energy-related derivatives and interest rate derivatives
Collateral already posted, aggregate fair value	6
SOUTHERN Co GAS | Energy-related derivatives | Liabilities from risk management activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	486
SOUTHERN Co GAS | Energy-related derivatives | Other deferred charges and assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	170
SOUTHERN Co GAS | Energy-related derivatives | Assets From Risk Management Activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	379
SOUTHERN Co GAS | Energy-related derivatives | Other deferred credits and liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	66
Hedging Instruments for Regulatory Purposes | Parent Company | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	17	98
Derivative liability, gross	67	60
Hedging Instruments for Regulatory Purposes | Parent Company | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	10	73
Hedging Instruments for Regulatory Purposes | Parent Company | Energy-related derivatives | Liabilities from risk management activities, net of collateral | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	43	27
Hedging Instruments for Regulatory Purposes | Parent Company | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	7	25
Hedging Instruments for Regulatory Purposes | Parent Company | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	24	33
Hedging Instruments for Regulatory Purposes | Parent Company | Interest rate derivatives | Liabilities from risk management activities
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	0	0
Hedging Instruments for Regulatory Purposes | ALABAMA POWER CO | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	4	20
Derivative liability, gross	10	9
Hedging Instruments for Regulatory Purposes | ALABAMA POWER CO | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	2	13
Hedging Instruments for Regulatory Purposes | ALABAMA POWER CO | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	2	7
Hedging Instruments for Regulatory Purposes | ALABAMA POWER CO | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	6	5
Hedging Instruments for Regulatory Purposes | ALABAMA POWER CO | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	4	4
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	6	44
Derivative liability, gross	(19)	8
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	2	30
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	4	14
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	(9)	1
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	(10)	7
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	5
Derivative liability, gross	21	29
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other current assets
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	4
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Liabilities from risk management activities
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	14	12
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other deferred charges and assets
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	1
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other deferred credits and liabilities
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	7	17
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	2	4
Derivative liability, gross	9	11
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	1	2
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	1	2
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	6	6
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	3	5
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	5
Derivative liability, gross	25
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	24
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Assets From Risk Management Activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	5
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	1
Cash Flow and Fair Value Hedging | Parent Company | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	133	36
Derivative liability, gross	75	94
Cash Flow and Fair Value Hedging | Parent Company | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	3	23
Cash Flow and Fair Value Hedging | Parent Company | Energy-related derivatives | Liabilities from risk management activities, net of collateral | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	14	7
Cash Flow and Fair Value Hedging | Parent Company | Foreign currency derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	0
Cash Flow and Fair Value Hedging | Parent Company | Foreign currency derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	129	0
Cash Flow and Fair Value Hedging | Parent Company | Foreign currency derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	23	25
Cash Flow and Fair Value Hedging | Parent Company | Foreign currency derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	0	33
Cash Flow and Fair Value Hedging | Parent Company | Interest rate derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	1	12
Cash Flow and Fair Value Hedging | Parent Company | Interest rate derivatives | Liabilities from risk management activities, net of collateral | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	4	1
Cash Flow and Fair Value Hedging | Parent Company | Interest rate derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	1
Cash Flow and Fair Value Hedging | Parent Company | Interest rate derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	34	28
Cash Flow and Fair Value Hedging | GEORGIA POWER CO | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	2
Derivative liability, gross	(5)	3
Cash Flow and Fair Value Hedging | GEORGIA POWER CO | Interest rate derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	2
Cash Flow and Fair Value Hedging | GEORGIA POWER CO | Interest rate derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	0
Cash Flow and Fair Value Hedging | GEORGIA POWER CO | Interest rate derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	(4)	0
Cash Flow and Fair Value Hedging | GEORGIA POWER CO | Interest rate derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	(1)	3
Cash Flow and Fair Value Hedging | MISSISSIPPI POWER CO | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	1	3
Derivative liability, gross	0	0
Cash Flow and Fair Value Hedging | MISSISSIPPI POWER CO | Interest rate derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	1	2
Cash Flow and Fair Value Hedging | MISSISSIPPI POWER CO | Interest rate derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	1
Cash Flow and Fair Value Hedging | MISSISSIPPI POWER CO | Interest rate derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	0	0
Cash Flow and Fair Value Hedging | MISSISSIPPI POWER CO | Interest rate derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	0	0
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	132	18
Derivative liability, gross	34	62
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	3	18
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	11	4
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Foreign currency derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	0	0
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Foreign currency derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	129	0
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Foreign currency derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	23	25
Cash Flow and Fair Value Hedging | SOUTHERN POWER CO | Foreign currency derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	0	33
Cash Flow and Fair Value Hedging | SOUTHERN Co GAS | Energy-related derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross	4
Cash Flow and Fair Value Hedging | SOUTHERN Co GAS | Energy-related derivatives | Assets From Risk Management Activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross	$ 0
Predecessor | SOUTHERN Co GAS
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		660
Derivative asset, gross amount offset		(583)
Derivative Asset		77
Derivative liability, gross		569
Derivative liability, gross amounts offset		(497)
Derivative Liability		72
Predecessor | SOUTHERN Co GAS | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		649
Derivative liability, gross		563
Predecessor | SOUTHERN Co GAS | Energy-related derivatives
Fair value of energy-related derivatives and interest rate derivatives
Collateral already posted, aggregate fair value		8
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Liabilities from risk management activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross		482
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Other deferred charges and assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		215
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Assets From Risk Management Activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		434
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Other deferred credits and liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross		81
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		8
Derivative liability, gross		3
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross		3
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		0
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Assets From Risk Management Activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		8
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross		0
Predecessor | Cash Flow and Fair Value Hedging | SOUTHERN Co GAS | Energy-related derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross		3
Predecessor | Cash Flow and Fair Value Hedging | SOUTHERN Co GAS | Energy-related derivatives | Assets From Risk Management Activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross		$ 3