Contingencies and Regulatory Matters - Textual - SO Kemper (Details) $ in Millions							1 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended						65 Months Ended
	Feb. 07, 2018	Feb. 06, 2018 USD ($)	Jun. 28, 2017 USD ($)	Aug. 17, 2016 USD ($)	Jun. 09, 2016 USD ($)	Dec. 03, 2015 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) mi	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) mi	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) mi MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2010 USD ($)	May 31, 2017 USD ($)	Jun. 17, 2016 USD ($)	May 03, 2016 USD ($)
MISSISSIPPI POWER CO
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
Public utilities, approved rate increase (decrease), amount				$ 18.0														$ 22.0
PSC retail increase (decrease)																								$ (8.0)	$ (2.0)
Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
Pre-Tax charge to income								$ 208.0	$ 34.0	$ 3,000.0	$ 108.0	$ 206.0	$ 88.0	$ 81.0	$ 53.0
After tax charge to income								185.0	21.0	2,100.0	67.0	127.0	54.0	50.0	33.0			$ 2,400.0	$ 264.0	$ 226.0
Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Maximum cap construction costs							$ 5,950.0															$ 2,400.0
Cost cap exceptions																						$ 570.0
Period of commercial operations established by discovery docket																		5 years
Estimated cost							7,380.0
Cost related to grant funding							137.0
Pre-Tax charge to income							2,800.0	208.0	34.0	3,000.0	108.0	206.0	88.0	81.0	53.0	$ 242.0							$ 3,070.0
After tax charge to income							$ 2,000.0	185.0	$ 21.0	$ 2,100.0	$ 67.0	$ 127.0	$ 54.0	$ 50.0	$ 33.0	206.0							$ 1,890.0
Reduction in customer rates annually								26.8
After tax charge to income, cost during suspension period																164.0
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
Regulatory asset								114.0								114.0		$ 114.0
Regulatory liabilities								$ 26.0								$ 26.0		$ 26.0
Mine | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Term of management fee contract																						40 years
Scenario, Forecast | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount																	$ 13.0
Minimum | Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Pre-Tax charge to income																	50.0
Maximum | Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Pre-Tax charge to income																	$ 100.0
In-Service Asset Proposal | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity						9.225%
PSC retail increase (decrease)						$ 126.0
Public utilities approved equity capital structure percentage						49.733%
In-Service Asset Proposal | Minimum | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Amortization period of regulatory assets and liabilities																			2 years
In-Service Asset Proposal | Maximum | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Amortization period of regulatory assets and liabilities																			10 years
Pending Litigation | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Loss contingency, damages sought, value			$ 500.0		$ 100.0
Subsequent Event | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity	9.33%
Subsequent Event | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount		$ 99.3
Public utilities, approved return on equity		8.60%