LEASES - Rent Expense and PPA Capacity Expense Related to Leases (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Lessee, Lease, Description [Line Items]
Rent expense	$ 192	$ 176
PPA capacity expense	231	235
Alabama Power
Lessee, Lease, Description [Line Items]
Rent expense	23	25
PPA capacity expense	44	41
Georgia Power
Lessee, Lease, Description [Line Items]
Rent expense	34	31
PPA capacity expense	206	225
Operating leases, contingent rent expense (2018 - Immaterial)	72,000,000	73,000,000
Mississippi Power
Lessee, Lease, Description [Line Items]
Rent expense	4	3
PPA capacity expense	0	0
Operating leases, contingent rent expense (2018 - Immaterial)	10,000,000	5,000,000
Southern Power
Lessee, Lease, Description [Line Items]
Rent expense	31	29
PPA capacity expense	0	0
Southern Company Gas
Lessee, Lease, Description [Line Items]
Rent expense	15	15
PPA capacity expense	0	0
Affiliate Operating Lease PPA | Georgia Power
Lessee, Lease, Description [Line Items]
Operating leases, contingent rent expense (2018 - Immaterial)	$ 29,000,000	$ 29,000,000