Financing - Short-term Borrowings (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 2,439,000,000	$ 2,032,000,000
Short-term debt at the end of the period, weighted average interest rate	1.90%	1.10%
Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 1,832,000,000	$ 1,909,000,000
Short-term debt at the end of the period, weighted average interest rate	1.80%	1.10%
Short-term bank debt
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 607,000,000	$ 123,000,000
Short-term debt at the end of the period, weighted average interest rate	2.30%	1.70%
Southern Company Gas Capital | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 1,243,000,000	$ 733,000,000
Short-term debt at the end of the period, weighted average interest rate	1.73%	1.09%
GEORGIA POWER CO | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts		$ 392,000,000
Short-term debt at the end of the period, weighted average interest rate		1.10%
GEORGIA POWER CO | Short-term bank debt
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 150,000,000
Short-term debt at the end of the period, weighted average interest rate	2.1918%
GULF POWER CO
Short-term borrowings
Short term borrowings excluding other energy service contracts		$ 268,000,000
Short-term debt at the end of the period, weighted average interest rate		1.20%
GULF POWER CO | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 45,000,000	$ 168,000,000
Short-term debt at the end of the period, weighted average interest rate	2.00%	1.10%
GULF POWER CO | Short-term bank debt
Short-term borrowings
Short term borrowings excluding other energy service contracts		$ 100,000,000
Short-term debt at the end of the period, weighted average interest rate		1.50%
MISSISSIPPI POWER CO | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 0	$ 0
SOUTHERN POWER CO | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 105,000,000	0
Short-term debt at the end of the period, weighted average interest rate	2.00%
Nicor Gas | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 275,000,000	$ 524,000,000
Short-term debt at the end of the period, weighted average interest rate	1.83%	0.95%
SOUTHERN Co GAS | Commercial Paper
Short-term borrowings
Short term borrowings excluding other energy service contracts	$ 1,518,000,000	$ 1,257,000,000
Short-term debt at the end of the period, weighted average interest rate	1.75%	1.03%