Revenue from Contracts with Customers and Lease Income - Schedule of Contract Balances (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Accounts Receivable	$ 2,343	$ 2,614
Contract Assets	165	158
Contract Liabilities	65	61
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivable	712	632
Contract Assets	5	2
Contract Liabilities	6	6
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivable	904	806
Contract Assets	103	71
Contract Liabilities	39	27
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivable	90	77
Contract Assets	0	0
Contract Liabilities	1	1
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Accounts Receivable	170	112
Contract Assets	1	0
Contract Liabilities	2	1
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Accounts Receivable	329	788
Contract Assets	0	0
Contract Liabilities	$ 0	$ 1