Summary of Significant Accounting Policies - Depreciation and Amortization (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2017
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 31,457.0	$ 29,852.0
Other cost of removal obligations		$ 2,684.0	$ 2,748.0
Utility Plant in Service
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		2.90%	3.00%	3.00%
Accumulated depreciation PPE		$ 30,800.0	$ 29,300.0
Other Plant in Service
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 673.0	$ 550.0
Minimum | Other Plant in Service
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		2 years
Maximum | Other Plant in Service
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		65 years
MISSISSIPPI POWER CO
Property, Plant and Equipment [Line Items]
Regulatory amortization period	5 years
Depreciation of cost of utility plant in service, composite straight-line rate		3.70%	4.20%	4.70%
Accumulated depreciation PPE		$ 1,325.0	$ 1,289.0
Other cost of removal obligations		$ 178.0	$ 170.0
GULF POWER CO
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		3.50%	3.50%	3.60%
Accumulated depreciation PPE		$ 1,461.0	$ 1,382.0
Other cost of removal obligations		221.0	249.0
SOUTHERN POWER CO
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 1,910.0	1,484.0
SOUTHERN POWER CO | Maximum | Natural Gas Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life		45 years
SOUTHERN POWER CO | Maximum | Solar Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life		35 years
SOUTHERN POWER CO | Maximum | Wind Generating Facility
Property, Plant and Equipment [Line Items]
Public utilities, property, plant and equipment, generation, useful life		30 years
SOUTHERN Co GAS
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 4,596.0	4,439.0
Other cost of removal obligations		$ 1,646.0	$ 1,616.0
SOUTHERN Co GAS | Utility Plant in Service
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		2.90%	2.80%
SOUTHERN Co GAS | Utility Plant in Service | Predecessor
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate				2.70%
SOUTHERN Co GAS | Minimum | Transportation Equipment
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		5 years
SOUTHERN Co GAS | Minimum | Storage Facilities
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		40 years
SOUTHERN Co GAS | Maximum | Transportation Equipment
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		15 years
SOUTHERN Co GAS | Maximum | Storage Facilities
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		60 years
SOUTHERN Co GAS | Maximum | Other Assets
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		65 years
GEORGIA POWER CO
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		2.70%	2.80%	2.70%
Regulatory liability amortization		$ 14.0	$ 14.0	$ 14.0
Accumulated depreciation PPE		11,704.0	11,317.0
ALABAMA POWER CO
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		9,563.0	9,112.0
Other cost of removal obligations		$ 609.0	$ 684.0
ALABAMA POWER CO | Utility Plant in Service
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		2.90%	3.00%	2.90%
Settlement Agreement | GULF POWER CO
Property, Plant and Equipment [Line Items]
Other cost of removal obligations					$ 62.5