Income Taxes - Reconciliation of Federal Statutory Income Tax Rate (Details)	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate			35.00%	35.00%	35.00%
State income tax, net of federal deduction			12.50%	2.10%	1.90%
Employee stock plans dividend deduction			4.10%	1.20%	1.20%
Non-deductible book depreciation			3.10%	0.90%	1.20%
AFUDC-equity			(2.60%)	(2.00%)	(2.20%)
Non-deductible equity portion on Kemper IGCC write-off			15.70%	(0.00%)	(0.00%)
ITC basis difference			(1.70%)	(5.00%)	(1.50%)
Federal PTCs			(12.10%)	(1.20%)	(0.00%)
Amortization of ITC			(4.20%)	(0.90%)	(0.50%)
Tax Reform Legislation			(25.60%)	0.00%	0.00%
Other			(2.70%)	(0.40%)	0.20%
Effective income tax rate			13.30%	27.30%	32.90%
ALABAMA POWER CO
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate			35.00%	35.00%	35.00%
State income tax, net of federal deduction			4.40%	4.20%	3.80%
Non-deductible book depreciation			0.90%	1.00%	1.20%
AFUDC-equity			(1.00%)	(0.70%)	(1.60%)
Tax Reform Legislation			0.30%	0.00%	0.00%
Other			0.00%	(0.70%)	0.00%
Effective income tax rate			39.60%	38.80%	38.40%
GEORGIA POWER CO
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate			35.00%	35.00%	35.00%
State income tax, net of federal deduction			2.00%	2.10%	2.50%
Non-deductible book depreciation			0.70%	0.80%	1.20%
AFUDC-equity			(0.60%)	(0.80%)	(0.70%)
Tax Reform Legislation			(0.40%)	0.00%	0.00%
Other			0.00%	(0.40%)	(0.40%)
Effective income tax rate			36.70%	36.70%	37.60%
GULF POWER CO
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate			35.00%	35.00%	35.00%
State income tax, net of federal deduction			3.70%	3.40%	3.90%
Non-deductible book depreciation			0.20%	0.60%	0.50%
Differences in prior years' deferred and current tax rates			0.00%	(0.10%)	(0.10%)
AFUDC-equity			(0.00%)	(0.00%)	(1.80%)
Other			0.50%	0.60%	(0.60%)
Effective income tax rate			39.40%	39.50%	36.90%
MISSISSIPPI POWER CO
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate			(35.00%)	(35.00%)	(35.00%)
State income tax, net of federal deduction			0.60%	(5.70%)	(6.30%)
Non-deductible book depreciation			0.10%	0.70%	1.30%
AFUDC-equity			(0.00%)	(28.50%)	(49.60%)
Non-deductible equity portion on Kemper IGCC write-off			5.30%	(0.00%)	(0.00%)
Tax Reform Legislation			11.90%	0.00%	0.00%
Other			0.00%	0.00%	(2.90%)
Effective income tax rate			(17.10%)	(68.50%)	(92.50%)
SOUTHERN POWER CO
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate			35.00%	35.00%	35.00%
State income tax, net of federal deduction			(22.20%)	(9.10%)	(0.30%)
Non-deductible equity portion on Kemper IGCC write-off			(10.00%)	(89.00%)	(21.50%)
Federal PTCs			(72.50%)	(23.30%)	(0.60%)
Amortization of ITC			(31.80%)	(20.60%)	(5.00%)
Tax Reform Legislation			(416.10%)	0.00%	0.00%
Other			0.50%	4.60%	2.50%
Noncontrolling interests			(8.60%)	(6.20%)	(1.70%)
Effective income tax rate			(525.70%)	(108.60%)	8.40%
SOUTHERN Co GAS
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate	35.00%		35.00%
State income tax, net of federal deduction	4.00%		4.00%
Other	1.00%		0.00%
Effective income tax rate	40.00%		60.20%
Predecessor | SOUTHERN Co GAS
Reconciliation of federal statutory income tax rate to effective income tax rate
Federal statutory rate		35.00%			35.00%
State income tax, net of federal deduction		3.50%			3.40%
Other		(0.90%)			(2.00%)
Effective income tax rate		37.60%			36.40%
Domestic Tax Authority [Member] | SOUTHERN Co GAS
Reconciliation of federal statutory income tax rate to effective income tax rate
Tax Reform Legislation			15.00%
State and Local Jurisdiction | SOUTHERN Co GAS
Reconciliation of federal statutory income tax rate to effective income tax rate
Tax Reform Legislation			6.20%