LEASES - Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease cost
Operating lease cost	$ 309	$ 310
Finance lease cost:
Amortization of ROU assets	26	28
Interest on lease obligations	11	12
Total finance lease cost	37	40
Short-term lease costs	39	48
Variable lease cost	91	105
Sublease income	0	0
Total lease cost	476	503
Alabama Power
Lease cost
Operating lease cost	55	54
Finance lease cost:
Amortization of ROU assets	1	1
Interest on lease obligations	0	0
Total finance lease cost	1	1
Short-term lease costs	11	19
Variable lease cost	4	6
Sublease income	(1)	(1)
Total lease cost	70	79
Georgia Power
Lease cost
Operating lease cost	212	206
Finance lease cost:
Amortization of ROU assets	15	15
Interest on lease obligations	16	18
Total finance lease cost	31	33
Short-term lease costs	26	22
Variable lease cost	76	85
Sublease income	0	0
Total lease cost	345	346
Mississippi Power
Lease cost
Operating lease cost	3	3
Finance lease cost:
Amortization of ROU assets	0	0
Interest on lease obligations	0	0
Total finance lease cost	0	0
Short-term lease costs	0	0
Variable lease cost	0	0
Sublease income	0	0
Total lease cost	3	3
Southern Power
Lease cost
Operating lease cost	29	28
Finance lease cost:
Amortization of ROU assets	0	0
Interest on lease obligations	0	0
Total finance lease cost	0	0
Short-term lease costs	0	0
Variable lease cost	7	7
Sublease income	0	0
Total lease cost	36	35
Southern Company Gas
Lease cost
Operating lease cost	19	18
Finance lease cost:
Amortization of ROU assets	0	0
Interest on lease obligations	0	0
Total finance lease cost	0	0
Short-term lease costs	0	0
Variable lease cost	0	0
Sublease income	0	0
Total lease cost	$ 19	$ 18