REVENUE FROM CONTRACTS WITH CUSTOMERS AND LEASE INCOME - Summary of Lease Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	$ 3	$ 2	$ 8	$ 7
Lease income - operating leases	50	64	148	216
Variable lease income	145	141	345	324
Total lease income	198	207	501	547
ALABAMA POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0	0	0
Lease income - operating leases	11	5	24	19
Variable lease income	0	0	0	0
Total lease income	11	5	24	19
GEORGIA POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0	0	0
Lease income - operating leases	14	18	44	57
Variable lease income	0	0	0	0
Total lease income	14	18	44	57
MISSISSIPPI POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	3	2	8	7
Lease income - operating leases	0	0	1	0
Variable lease income	0	0	0	0
Total lease income	3	2	9	7
SOUTHERN POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0	0	0
Lease income - operating leases	21	31	66	111
Variable lease income	153	151	368	349
Total lease income	174	182	434	460
SOUTHERN Co GAS
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0	0	0
Lease income - operating leases	9	9	26	26
Variable lease income	0	0	0	0
Total lease income	$ 9	$ 9	$ 26	$ 26