Derivatives - Interest Rate Derivatives (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Notional amount	$ 3,650,000,000
Fair value gain (loss)	(36,000,000)
Maturity Date March 2018 | Cash Flow Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 900,000,000
Notional amount of interest rate derivatives, interest rate paid	0.79%
Interest rate received	1-month LIBOR
Hedge maturity date	Mar. 01, 2018
Fair value gain (loss)	$ 1,000,000
Maturity Date August 2017 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Interest rate received
Basis spread on variable rate	0.17%
Maturity Date June 2018 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 250,000,000
Notional amount of interest rate derivatives, interest rate received	5.40%
Interest rate received	3-month LIBOR + 4.02%
Basis spread on variable rate	4.02%
Hedge maturity date	Jun. 01, 2018
Fair value gain (loss)	$ 0
Maturity Date December 2018 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 500,000,000
Notional amount of interest rate derivatives, interest rate received	1.95%
Interest rate received	3-month LIBOR + 0.76%
Basis spread on variable rate	0.76%
Hedge maturity date	Dec. 01, 2018
Fair value gain (loss)	$ (3,000,000)
Maturity Date December 2019 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 200,000,000
Notional amount of interest rate derivatives, interest rate received	4.25%
Interest rate received	3-month LIBOR + 2.46%
Basis spread on variable rate	2.46%
Hedge maturity date	Dec. 01, 2019
Fair value gain (loss)	$ (1,000,000)
Maturity Date June 2020 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 300,000,000
Notional amount of interest rate derivatives, interest rate received	2.75%
Interest rate received	3-month LIBOR + 0.92%
Basis spread on variable rate	0.92%
Hedge maturity date	Jun. 15, 2020
Fair value gain (loss)	$ (2,000,000)
Maturity Date June 2021 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 1,500,000,000
Notional amount of interest rate derivatives, interest rate received	2.35%
Interest rate received	1-month LIBOR + 0.87%
Basis spread on variable rate	0.87%
Hedge maturity date	Jul. 01, 2021
Fair value gain (loss)	$ (31,000,000)
GEORGIA POWER CO
Derivative [Line Items]
Notional amount	950,000,000
Fair value gain (loss)	(4,000,000)
GEORGIA POWER CO | Maturity Date June 2018 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 250,000,000
Notional amount of interest rate derivatives, interest rate paid	5.40%
Interest rate received	3-month LIBOR + 4.02%
Basis spread on variable rate	4.02%
Hedge maturity date	Jun. 01, 2018
Fair value gain (loss)	$ 0
GEORGIA POWER CO | Maturity Date December 2018 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 500,000,000
Notional amount of interest rate derivatives, interest rate paid	1.95%
Interest rate received	3-month LIBOR + 0.76%
Basis spread on variable rate	0.76%
Hedge maturity date	Dec. 01, 2018
Fair value gain (loss)	$ (3,000,000)
GEORGIA POWER CO | Maturity Date December 2019 | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional amount	$ 200,000,000
Notional amount of interest rate derivatives, interest rate paid	4.25%
Interest rate received	3-month LIBOR + 2.46%
Basis spread on variable rate	2.46%
Hedge maturity date	Dec. 01, 2019
Fair value gain (loss)	$ (1,000,000)
GULF POWER CO | Interest rate derivatives
Derivative [Line Items]
Notional amount	0
MISSISSIPPI POWER CO | Maturity Date March 2018 | Cash Flow Hedges of Existing Debt | Interest rate derivatives
Derivative [Line Items]
Notional amount	$ 900,000,000
Notional amount of interest rate derivatives, interest rate paid	0.79%
Fair value gain (loss)	$ 1,000,000