INCOME TAXES - Reconciliation of Federal Statutory Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal deduction	2.80%	4.90%	1.80%
Employee stock plans' dividend deduction	(0.70%)	(0.40%)	(1.00%)
Non-deductible book depreciation	0.70%	0.30%	0.80%
Flowback of excess deferred income taxes	(8.80%)	(2.10%)	(4.00%)
AFUDC-Equity	(0.80%)	(0.40%)	(1.00%)
ITC basis difference		(0.10%)	(0.60%)
Federal PTCs	0.00%		(4.70%)
Amortization of ITC	(1.60%)	(0.80%)	(2.00%)
Tax impact from sale of subsidiaries		5.10%	8.60%
Tax Reform Legislation			(1.40%)
Noncontrolling interests	0.00%	0.00%	(0.40%)
Leveraged lease impairments	(1.60%)
Other	0.20%	0.00%	(0.80%)
Effective income tax (benefit) rate	11.20%	27.50%	16.30%
Alabama Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal deduction	5.00%	4.90%	5.00%
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.60%	0.60%	0.60%
Flowback of excess deferred income taxes	(3.10%)	(5.30%)	(1.80%)
AFUDC-Equity	(0.60%)	(0.80%)	(1.00%)
ITC basis difference		0.00%	0.00%
Federal PTCs	0.00%		0.00%
Amortization of ITC	(0.10%)	(0.10%)	(0.10%)
Tax impact from sale of subsidiaries		0.00%	0.00%
Tax Reform Legislation			0.00%
Noncontrolling interests	0.00%	0.00%	0.00%
Leveraged lease impairments	0.00%
Other	(0.30%)	(0.40%)	(0.10%)
Effective income tax (benefit) rate	22.50%	19.90%	23.60%
Georgia Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal deduction	0.50%	1.00%	5.50%
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.80%	0.50%	1.20%
Flowback of excess deferred income taxes	(12.00%)	0.00%	0.00%
AFUDC-Equity	(1.10%)	(0.60%)	(1.40%)
ITC basis difference		0.00%	0.00%
Federal PTCs	0.00%		0.00%
Amortization of ITC	(0.10%)	(0.10%)	(0.20%)
Tax impact from sale of subsidiaries		0.00%	0.00%
Tax Reform Legislation			(4.90%)
Noncontrolling interests	0.00%	0.00%	0.00%
Leveraged lease impairments	0.00%
Other	(0.30%)	(0.30%)	0.10%
Effective income tax (benefit) rate	8.80%	21.50%	21.30%
Mississippi Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal deduction	4.80%	4.30%	(65.10%)
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.50%	0.40%	0.70%
Flowback of excess deferred income taxes	(18.50%)	(12.60%)	(4.10%)
AFUDC-Equity	(0.10%)	(0.10%)	0.00%
ITC basis difference		0.00%	0.00%
Federal PTCs	0.00%		0.00%
Amortization of ITC	(0.10%)	(0.10%)	(0.20%)
Tax impact from sale of subsidiaries		0.00%	0.00%
Tax Reform Legislation			(26.30%)
Noncontrolling interests	0.00%	0.00%	0.00%
Leveraged lease impairments	0.00%
Other	0.90%	4.90%	(1.40%)
Effective income tax (benefit) rate	8.50%	17.80%	(75.40%)
Southern Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal deduction	2.70%	4.00%	(90.80%)
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.00%	0.00%	0.00%
Flowback of excess deferred income taxes	0.00%	0.00%	0.00%
AFUDC-Equity	0.00%	0.00%	0.00%
ITC basis difference		(1.90%)	(0.20%)
Federal PTCs	(2.50%)		(156.60%)
Amortization of ITC	(22.10%)	(16.10%)	(55.40%)
Tax impact from sale of subsidiaries		(27.60%)	0.00%
Tax Reform Legislation			96.10%
Noncontrolling interests	3.10%	0.80%	(14.90%)
Leveraged lease impairments	0.00%
Other	(0.90%)	(0.60%)	2.00%
Effective income tax (benefit) rate	1.30%	(20.40%)	(198.80%)
Southern Company Gas
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal deduction	4.00%	6.10%	9.20%
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.00%	0.00%	0.00%
Flowback of excess deferred income taxes	(2.70%)	(6.00%)	(3.00%)
AFUDC-Equity	0.00%	0.00%	0.00%
ITC basis difference		0.00%	0.00%
Federal PTCs	0.00%		0.00%
Amortization of ITC	(0.10%)	(0.10%)	(0.10%)
Tax impact from sale of subsidiaries		(1.40%)	28.50%
Tax Reform Legislation			(0.40%)
Noncontrolling interests	0.00%	0.00%	0.00%
Leveraged lease impairments	0.00%
Other	0.50%	(1.40%)	0.30%
Effective income tax (benefit) rate	22.70%	18.20%	55.50%