REVENUE FROM CONTRACTS WITH CUSTOMERS AND LEASE INCOME - Summary of Lease Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	$ 3	$ 2	$ 6	$ 5
Lease income - operating leases	47	67	97	139
Variable lease income	126	115	200	182
Total lease income	176	184	303	326
ALABAMA POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0	0	0
Lease income - operating leases	6	7	13	13
Variable lease income	0	0	0	0
Total lease income	6	7	13	13
GEORGIA POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0	0	0
Lease income - operating leases	15	19	30	39
Variable lease income	0	0	0	0
Total lease income	15	19	30	39
MISSISSIPPI POWER CO
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	3	2	5	5
Lease income - operating leases	0	0	0	0
Variable lease income	0	0	0	0
Total lease income	3	2	5	5
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	44	45	90
Variable lease income	136	129	215	204
Total lease income	157	173	260	294
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	17	17
Variable lease income	0	0	0	0
Total lease income	$ 9	$ 9	$ 17	$ 17