Stock Compensation - Performance Shares, Compensation Costs and Related Tax Benefit (Details) - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	$ 91	$ 74	$ 96
Tax benefit of compensation cost recognized in income	24	29	37
Alabama Power
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	11	9	15
Tax benefit of compensation cost recognized in income	3	4	6
Georgia Power
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	11	10	15
Tax benefit of compensation cost recognized in income	3	4	6
Mississippi Power
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	3	2	4
Tax benefit of compensation cost recognized in income	1	1	$ 1
Southern Power
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	4
Tax benefit of compensation cost recognized in income	1
Southern Company Gas
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	11	8
Tax benefit of compensation cost recognized in income	$ 3	$ 3