Derivatives - Narrative (Details) MMBTU in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) MMBTU	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative, gain (loss) on derivative, net	$ 60,000,000	$ 81,000,000
Collateral already posted, aggregate fair value	$ 223,000,000
Southern Company
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	31
Cash flow hedge gain (loss) to be reclassified within twelve months	$ (20,000,000)
GEORGIA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	9
ALABAMA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	5
Gulf Power and Mississippi Power
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	3
MISSISSIPPI POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	4
SOUTHERN POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	10
Parent Company and Southern Power
Derivative [Line Items]
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months	$ (22,000,000)
SOUTHERN Co GAS
Derivative [Line Items]
Derivative, gain (loss) on derivative, net	59,000,000	82,000,000
Collateral already posted, aggregate fair value	223,000,000		$ 193,000,000
Weather derivative premium	4,000,000		11,000,000
Derivative Counterparties | Registrants
Derivative [Line Items]
Collateral already posted, aggregate fair value	0
Energy-related derivatives | SOUTHERN Co GAS
Derivative [Line Items]
Collateral already posted, aggregate fair value	8,000,000		$ 6,000,000
Weather Derivatives | Southern Company
Derivative [Line Items]
Derivative, gain (loss) on derivative, net	$ 14,000,000	$ 14,000,000
Long | Derivatives not designated as hedging instruments | Energy-related, Natural Gas | SOUTHERN Co GAS
Derivative [Line Items]
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,600
Short | Derivatives not designated as hedging instruments | Energy-related, Natural Gas | SOUTHERN Co GAS
Derivative [Line Items]
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,300