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