Revenue from Contracts with Customers - Contract Balances (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Receivables	$ 2,523	$ 2,630
Contract Assets	132	102
Contract Liabilities	58	32
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Receivables	713	520
Contract Assets	2	0
Contract Liabilities	12	12
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Receivables	993	721
Contract Assets	91	58
Contract Liabilities	15	7
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Receivables	94	100
Contract Assets	0	0
Contract Liabilities	6	0
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Receivables	132	118
Contract Assets	0	0
Contract Liabilities	3	11
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Receivables	441	952
Contract Assets	0	0
Contract Liabilities	$ 1	$ 2