Regulatory Matters - Kemper Regulatory Matters (Details) $ in Millions				1 Months Ended				3 Months Ended						6 Months Ended	12 Months Ended						48 Months Ended	65 Months Ended	108 Months Ended
	Aug. 06, 2018	Feb. 07, 2018	Feb. 06, 2018 USD ($)	Apr. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	May 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) mi	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) mi MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2023 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2018 USD ($) mi	Jan. 31, 2017 USD ($)	Dec. 17, 2015 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
Period costs																$ 29.0	$ 16.0
Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
After tax charge to income																$ 722.0	2,400.0	$ 264.0
Mississippi Power
Schedule of Regulatory Assets and Liabilities [Line Items]
Plant capacity under coal gasification combined cycle technology | MW																582
Co two pipeline infrastructure | mi								61								61							61
Maximum cap construction costs																			$ 2,880.0
Pre-tax charge to income								$ (9.0)
After tax charge to income								(95.0)
Period costs																$ 0.0	0.0
PSC retail increase (decrease)								7.0		$ 8.0				$ 8.0		7.0	8.0	$ (1.0)					$ 7.0	$ 55.0
Public utilities, approved rate increase (decrease), amount						$ 18.0	$ 18.0										$ 22.0
Public utilities, approved return on equity		9.33%
Mississippi Power | Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
Maximum cap construction costs					$ 5,950.0														2,400.0
Cost cap exceptions																			$ 570.0
Estimated cost					7,380.0
Cost related to grant funding					137.0
Pre-tax charge to income					2,800.0				$ 44.0	208.0	$ 34.0	$ 3,000.0	$ 108.0	242.0		37.0						$ 3,070.0
After tax charge to income					$ 2,000.0			95.0	$ 33.0	185.0	$ 21.0	$ 2,100.0	$ 67.0	206.0								$ 1,890.0
Estimated cost								$ 10.0								10.0							10.0
Period costs, net of tax																$ 27.0
After tax charge to income, cost during suspension period														$ 164.0
Public utilities, approved rate increase (decrease), amount			$ 99.3
Public utilities, approved return on equity			8.60%
Amortization of regulatory assets																8 years
Amortization of regulatory liabilities																6 years
Disallowance pre-tax charge to income										78.0
Pre-tax charge to income before accumulated depreciation										85.0
Pre-tax charge to income, accumulated depreciation										7.0
Disallowance after tax charge to income										$ 48.0
Reduction in customer rates annually				$ 26.8
Government grants expected																				$ 270.0
Government grants received																							387.0
Mississippi Power | Kemper IGCC | In-Service Asset Proposal
Schedule of Regulatory Assets and Liabilities [Line Items]
PSC retail increase (decrease)																									$ 126.0
Mississippi Power | Kemper IGCC | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Period costs															$ 11.0
Mississippi Power | Plant Watson Unit 4
Schedule of Regulatory Assets and Liabilities [Line Items]
Accelerated retirement period	2 years
Mississippi Power | Plant Watson Unit 5
Schedule of Regulatory Assets and Liabilities [Line Items]
Accelerated retirement period	7 years
Mississippi Power | Plant Greene County Units 1 And 2
Schedule of Regulatory Assets and Liabilities [Line Items]
Accelerated retirement period	4 years
Mississippi Power | Mine
Schedule of Regulatory Assets and Liabilities [Line Items]
Term of management fee contract																				40 years
Minimum | Mississippi Power | Kemper IGCC | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Period costs																					$ 2.0
Maximum | Mississippi Power | Kemper IGCC | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Period costs																					$ 4.0
Department Of Energy, Initial Grants | Mississippi Power | Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
Government grants received																							382.0
Department Of Energy, Additional Grants For Future Rate Impacts | Mississippi Power | Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
Government grants received																							$ 5.0