REVENUE FROM CONTRACTS WITH CUSTOMERS AND LEASE INCOME - Contract Balances (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Accounts Receivables	$ 2,344	$ 2,413
Contract Assets	159	117
Contract Liabilities	56	52
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivables	686	586
Contract Assets	5	0
Contract Liabilities	1	10
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivables	901	688
Contract Assets	103	69
Contract Liabilities	24	13
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivables	93	79
Contract Assets	0	0
Contract Liabilities	0	0
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivables	100	97
Contract Assets	0	0
Contract Liabilities	3	1
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Accounts Receivables	385	749
Contract Assets	0	0
Contract Liabilities	$ 1	$ 1