INCOME TAXES - Reconciliation of Federal Statutory Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal deduction	4.90%	1.80%	12.50%
Employee stock plans' dividend deduction	(0.40%)	(1.00%)	(4.00%)
Non-deductible book depreciation	0.30%	0.80%	3.10%
Flowback of excess deferred income taxes	(2.10%)	(4.00%)	(0.30%)
AFUDC-Equity	(0.40%)	(1.00%)	(2.60%)
ITC basis difference	(0.10%)	(0.60%)	(1.70%)
AFUDC-Equity portion of Kemper IGCC charge			15.70%
Federal PTCs		(4.70%)	(12.10%)
Amortization of ITC	(0.80%)	(2.00%)	(4.20%)
Tax impact on sale of subsidiaries	5.10%	8.60%
Tax Reform Legislation		(1.40%)	(25.60%)
Noncontrolling interests	0.00%	(0.40%)	(1.40%)
Other	0.00%	(0.80%)	(1.10%)
Effective income tax (benefit) rate	27.50%	16.30%	13.30%
Alabama Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal deduction	4.90%	5.00%	4.50%
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.60%	0.60%	0.90%
Flowback of excess deferred income taxes	(5.30%)	(1.80%)	0.00%
AFUDC-Equity	(0.80%)	(1.00%)	(1.00%)
ITC basis difference	0.00%	0.00%	0.00%
AFUDC-Equity portion of Kemper IGCC charge			0.00%
Federal PTCs		0.00%	0.00%
Amortization of ITC	(0.10%)	(0.10%)	(0.20%)
Tax impact on sale of subsidiaries	0.00%	0.00%
Tax Reform Legislation		0.00%	0.30%
Noncontrolling interests	0.00%	0.00%	0.00%
Other	(0.40%)	(0.10%)	0.10%
Effective income tax (benefit) rate	19.90%	23.60%	39.60%
Georgia Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal deduction	1.00%	5.50%	2.00%
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.50%	1.20%	0.70%
Flowback of excess deferred income taxes	0.00%	0.00%	(0.10%)
AFUDC-Equity	(0.60%)	(1.40%)	(0.60%)
ITC basis difference	0.00%	0.00%	0.00%
AFUDC-Equity portion of Kemper IGCC charge			0.00%
Federal PTCs		0.00%	0.00%
Amortization of ITC	(0.10%)	(0.20%)	(0.10%)
Tax impact on sale of subsidiaries	0.00%	0.00%
Tax Reform Legislation		(4.90%)	(0.40%)
Noncontrolling interests	0.00%	0.00%	0.00%
Other	(0.30%)	0.10%	0.20%
Effective income tax (benefit) rate	21.50%	21.30%	36.70%
Mississippi Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal deduction	4.30%	(65.10%)	(0.60%)
Employee stock plans' dividend deduction	0.00%	0.00%	0.00%
Non-deductible book depreciation	0.40%	0.70%	(0.10%)
Flowback of excess deferred income taxes	(12.60%)	(4.10%)	0.00%
AFUDC-Equity	(0.10%)	0.00%	0.00%
ITC basis difference	0.00%	0.00%	0.00%
AFUDC-Equity portion of Kemper IGCC charge			(5.30%)
Federal PTCs		0.00%	0.00%
Amortization of ITC	(0.10%)	(0.20%)	0.00%
Tax impact on sale of subsidiaries	0.00%	0.00%
Tax Reform Legislation		(26.30%)	(11.90%)
Noncontrolling interests	0.00%	0.00%	0.00%
Other	4.90%	(1.40%)	0.00%
Effective income tax (benefit) rate	17.80%	(75.40%)	17.10%
Southern Power
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal deduction	4.00%	(90.80%)	(22.20%)
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	0.00%	0.00%	0.00%
AFUDC-Equity	0.00%	0.00%	0.00%
ITC basis difference	(1.90%)	(0.20%)	(10.00%)
AFUDC-Equity portion of Kemper IGCC charge			0.00%
Federal PTCs		(156.60%)	(72.50%)
Amortization of ITC	(16.10%)	(55.40%)	(20.60%)
Tax impact on sale of subsidiaries	(27.60%)	0.00%
Tax Reform Legislation		96.10%	(416.10%)
Noncontrolling interests	0.80%	(14.90%)	(8.60%)
Other	(0.60%)	2.00%	(10.70%)
Effective income tax (benefit) rate	(20.40%)	(198.80%)	(525.70%)
Southern Company Gas
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal deduction	6.10%	9.20%	10.00%
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	(6.00%)	(3.00%)	(0.20%)
AFUDC-Equity	0.00%	0.00%	0.00%
ITC basis difference	0.00%	0.00%	0.00%
AFUDC-Equity portion of Kemper IGCC charge			0.00%
Federal PTCs		0.00%	0.00%
Amortization of ITC	(0.10%)	(0.10%)	(0.20%)
Tax impact on sale of subsidiaries	(1.40%)	28.50%
Tax Reform Legislation		(0.40%)	15.00%
Noncontrolling interests	0.00%	0.00%	0.00%
Other	(1.40%)	0.30%	0.60%
Effective income tax (benefit) rate	18.20%	55.50%	60.20%