LEASES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 1,852	$ 1,852	$ 1,844
Maximum | Real estate
Lessee, Lease, Description [Line Items]
Remaining terms		24 years
Maximum | Land
Lessee, Lease, Description [Line Items]
Lessee, lease term		46 years
Maximum | Communication towers
Lessee, Lease, Description [Line Items]
Lessee, lease term		10 years
Lessee, renewal period		20 years
Maximum | Fuel cells
Lessee, Lease, Description [Line Items]
Lessor, lease term, operating lease	14 years	14 years
Alabama Power
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 170	$ 170	156
Alabama Power | Maximum | Electric generating units
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	17 years	17 years
Alabama Power | Minimum | Electric generating units
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	3 years	3 years
Georgia Power
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 1,307	$ 1,307	1,426
Variable lease, payment		39	42
Georgia Power | Purchased power, affiliates
Lessee, Lease, Description [Line Items]
Variable lease, payment		20	20
Southern Power
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 451	$ 451	398
Southern Power | Maximum | Electric generating units
Lessee, Lease, Description [Line Items]
Lessor, lease term, operating lease	26 years	26 years
Mississippi Power
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 9	$ 9	6
Transfer of construction work in progress to lease receivables	26
Sales-type lease, lease receivable	26	26
Mississippi Power | Other Property and Investments
Lessee, Lease, Description [Line Items]
Sales-type lease, lease receivable	24	24
Mississippi Power | Other Accounts, Notes, and Loans Receivable
Lessee, Lease, Description [Line Items]
Sales-type lease, lease receivable	$ 2	$ 2
Mississippi Power | Maximum | Electric generating units
Lessee, Lease, Description [Line Items]
Lessor, lease term, sales-type lease	18 years	18 years
Southern Company Gas
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 82	$ 82	92
Southern Company Gas | Maximum
Lessee, Lease, Description [Line Items]
Lessor, remaining term		22 years
Affiliate | Georgia Power | Electric generating units
Lessee, Lease, Description [Line Items]
Operating lease obligations	$ 575	$ 575	$ 624
Traditional Electric Operating Companies | Maximum | Outdoor lighting
Lessee, Lease, Description [Line Items]
Lessor, lease term, operating lease	7 years	7 years