STOCK COMPENSATION - Performance Shares, Compensation Costs and Related Tax Benefit (Details) - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	$ 77	$ 91	$ 74
Tax benefit of compensation cost recognized in income	20	24	29
Southern Company Gas
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized in income	14	11	8
Tax benefit of compensation cost recognized in income	$ 4	$ 3	$ 3