LEASES - Balance Sheet Amounts Recorded for Operating and Financing Leases (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Leases
Operating lease ROU assets, net	$ 1,802	$ 1,800
Operating lease obligations - current	241	229
Operating lease obligations - non-current	1,611	1,615
Total operating lease obligations	$ 1,852	$ 1,844
Finance Leases
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	so:PlantInServiceNetOfDepreciation	so:PlantInServiceNetOfDepreciation
Finance lease ROU assets, net	$ 218	$ 216
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Finance lease obligations - current	$ 17	$ 21
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligations	us-gaap:LongTermDebtAndCapitalLeaseObligations
Finance lease obligations - non-current	$ 214	$ 205
Total finance lease obligations	231	226
Alabama Power
Operating Leases
Operating lease ROU assets, net	$ 151	$ 132
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating lease obligations - current	$ 51	$ 49
Operating lease obligations - non-current	119	107
Total operating lease obligations	$ 170	$ 156
Finance Leases
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	so:PlantInServiceNetOfDepreciation	so:PlantInServiceNetOfDepreciation
Finance lease ROU assets, net	$ 5	$ 4
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Finance lease obligations - current	$ 1	$ 1
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligations	us-gaap:LongTermDebtAndCapitalLeaseObligations
Finance lease obligations - non-current	$ 4	$ 3
Total finance lease obligations	5	4
Georgia Power
Operating Leases
Operating lease ROU assets, net	1,308	1,428
Operating lease obligations - current	151	144
Operating lease obligations - non-current	1,156	1,282
Total operating lease obligations	$ 1,307	$ 1,426
Finance Leases
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	so:PlantInServiceNetOfDepreciation	so:PlantInServiceNetOfDepreciation
Finance lease ROU assets, net	$ 115	$ 130
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Finance lease obligations - current	$ 9	$ 11
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligations	us-gaap:LongTermDebtAndCapitalLeaseObligations
Finance lease obligations - non-current	$ 136	$ 145
Total finance lease obligations	$ 145	$ 156
Mississippi Power
Operating Leases
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Operating lease ROU assets, net	$ 9	$ 6
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating lease obligations - current	$ 2	$ 2
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Operating lease obligations - non-current	$ 7	$ 4
Total operating lease obligations	$ 9	6
Finance Leases
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	so:PlantInServiceNetOfDepreciation
Finance lease ROU assets, net	$ 19	0
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Finance lease obligations - current	$ 1	0
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligations
Finance lease obligations - non-current	$ 18	0
Total finance lease obligations	19	0
Southern Power
Operating Leases
Operating lease ROU assets, net	$ 415	$ 369
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating lease obligations - current	$ 25	$ 22
Operating lease obligations - non-current	426	376
Total operating lease obligations	451	398
Finance Leases
Finance lease ROU assets, net	0	0
Finance lease obligations - current	0	0
Finance lease obligations - non-current	0	0
Total finance lease obligations	0	0
Southern Company Gas
Operating Leases
Operating lease ROU assets, net	$ 81	$ 93
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating lease obligations - current	$ 15	$ 14
Operating lease obligations - non-current	67	78
Total operating lease obligations	82	92
Finance Leases
Finance lease ROU assets, net	0	0
Finance lease obligations - current	0	0
Finance lease obligations - non-current	0	0
Total finance lease obligations	$ 0	$ 0