Revenue from Contracts with Customers - Contract Balances (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Receivables	$ 2,522	$ 2,630
Contract Assets	84	102
Contract Liabilities	62	32
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Receivables	514	520
Contract Assets	1	0
Contract Liabilities	10	12
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Receivables	668	721
Contract Assets	41	58
Contract Liabilities	25	7
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Receivables	85	100
Contract Assets	0	0
Contract Liabilities	0	0
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Receivables	99	118
Contract Assets	0	0
Contract Liabilities	4	11
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Receivables	948	952
Contract Assets	0	0
Contract Liabilities	$ 1	$ 2