Introduction - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Net income (loss)		$ 1,222,000,000	$ 1,109,000,000	$ 2,031,000,000	$ 427,000,000
ALABAMA POWER CO
Related Party Transaction [Line Items]
Net income (loss)		377,000,000	330,000,000	$ 868,000,000	743,000,000
Increase in ARO due to new CCR rule	$ 1,200,000,000
Inflation rate assumption				4.50%
Trust earnings rate assumption				7.00%
MISSISSIPPI POWER CO
Related Party Transaction [Line Items]
Net income (loss)		47,000,000	40,000,000	$ 87,000,000	(2,033,000,000)
Increase in ARO due to new CCR rule	11,000,000
SOUTHERN Co GAS
Related Party Transaction [Line Items]
Net income (loss)		46,000,000	15,000,000	294,000,000	303,000,000
Inventory decrement		0		0
Wholesale Gas Services | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Inventory write-down		0	$ 0	0	$ 0
Plant Farley | ALABAMA POWER CO
Related Party Transaction [Line Items]
Increase in ARO	$ 300,000,000
Accounting Standards Update 2014-09 | Effect of Change
Related Party Transaction [Line Items]
Net income (loss)		(4,000,000)		1,000,000
Accounting Standards Update 2014-09 | Effect of Change | SOUTHERN Co GAS
Related Party Transaction [Line Items]
Net income (loss)		$ (4,000,000)		$ 1,000,000