Summary of Significant Accounting Policies - Depreciation and Amortization (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 29,852.0	$ 24,253.0
Other Cost of Removal Obligations		$ 2,748.0	$ 1,162.0
Utility Plant in Service
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		3.00%	3.00%	3.10%
Accumulated depreciation PPE		$ 29,300.0	$ 23,700.0
Other Plant in Service
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 550.0	$ 510.0
Minimum [Member] | Other Plant in Service
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		3 years
Maximum [Member] | Other Plant in Service
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		65 years
Mississippi Power [Member]
Property, Plant and Equipment [Line Items]
Regulatory amortization period	5 years
Depreciation of cost of utility plant in service, composite straight-line rate		4.20%	4.70%	3.30%
Accumulated depreciation PPE		$ 1,289.0	$ 1,262.0
Other Cost of Removal Obligations		$ 170.0	$ 165.0
Gulf Power [Member]
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		$ 1,382.0	$ 1,296.0
Other Cost of Removal Obligations		249.0	233.0
Southern Power [Member]
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 1,484.0	1,248.0
Southern Power [Member] | Maximum [Member] | Natural Gas Generating Facility [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Generation, Useful Life		45 years
Southern Power [Member] | Maximum [Member] | Solar Generating Facility [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Generation, Useful Life		35 years
Southern Power [Member] | Maximum [Member] | Wind Generating Facility [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Generation, Useful Life		30 years
Southern Company Gas [Member] | Successor [Member]
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE		$ 4,439.0
Other Cost of Removal Obligations		$ 1,616.0
Southern Company Gas [Member] | Predecessor [Member]
Property, Plant and Equipment [Line Items]
Accumulated depreciation PPE			2,775.0
Other Cost of Removal Obligations			$ 1,538.0
Southern Company Gas [Member] | Utility Plant in Service | Successor [Member]
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		2.80%
Southern Company Gas [Member] | Utility Plant in Service | Predecessor [Member]
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate			2.70%	2.70%
Southern Company Gas [Member] | Minimum [Member] | Transportation Equipment [Member]
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		5 years
Southern Company Gas [Member] | Minimum [Member] | Storage Facilities [Member]
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		40 years
Southern Company Gas [Member] | Maximum [Member] | Transportation Equipment [Member]
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		15 years
Southern Company Gas [Member] | Maximum [Member] | Storage Facilities [Member]
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		60 years
Southern Company Gas [Member] | Maximum [Member] | Other Assets [Member]
Property, Plant and Equipment [Line Items]
Plant in service, estimated useful lives		65 years
Georgia Power [Member]
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		2.80%	2.70%	2.70%
Regulatory liability amortization		$ 14.0	$ 14.0	$ 14.0
Accumulated depreciation PPE		11,317.0	10,903.0
Alabama Power [Member]
Property, Plant and Equipment [Line Items]
Regulatory liability amortization				$ 120.0
Accumulated depreciation PPE		9,112.0	8,736.0
Other Cost of Removal Obligations		$ 684.0	$ 722.0
Alabama Power [Member] | Utility Plant in Service
Property, Plant and Equipment [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate		3.00%	2.90%	3.30%
Settlement Agreement [Member] | Gulf Power [Member]
Property, Plant and Equipment [Line Items]
Other Cost of Removal Obligations		$ 62.5
Kemper IGCC [Member] | Mississippi Power [Member]
Property, Plant and Equipment [Line Items]
Regulatory amortization period			10 years