Contingencies and Regulatory Matters - IGCC (Details) $ in Millions												1 Months Ended		2 Months Ended	3 Months Ended		5 Months Ended	6 Months Ended		9 Months Ended		12 Months Ended		38 Months Ended	65 Months Ended
	Dec. 31, 2017 USD ($)	Aug. 21, 2017 USD ($)	Jun. 28, 2017 USD ($)	Nov. 17, 2016 USD ($)	Jun. 09, 2016 USD ($)	Jun. 03, 2016	Dec. 03, 2015 USD ($)	Jul. 07, 2015 USD ($)	Jun. 03, 2015 USD ($)	Jan. 01, 2014 USD ($)	Mar. 19, 2013	Jun. 30, 2017 USD ($)	Jan. 31, 2013 USD ($)	May 31, 2017 USD ($)	Sep. 30, 2017 USD ($) mi	Sep. 30, 2016 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) mi MW	Sep. 30, 2016 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2010 USD ($)	Jun. 30, 2017 USD ($)	May 31, 2017 USD ($)	May 20, 2015 USD ($)
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Plant capacity under coal gasification combined cycle technology | MW																				582
Cost related to grant funding																						$ 245
Maximum cap construction cost																						2,880
Reduced percentage interest transferred under asset purchase agreement											15.00%												15.00%
Regulatory liabilities															$ 10					$ 10
Public utilities, approved rate increase (decrease), percentage										3.00%
Settlement agreement collection amount to mitigate rate impact, year two										$ 156
Retail rate recovery								$ 342
Carrying costs associated with retail rate recovery								$ 29
AFUDC cost																				22
Interest bearing refundable deposit related to assets sale																										$ 275
Promissory note									$ 301
Kemper IGCC
Loss Contingencies [Line Items]
Pre-tax charge to income															34	$ 88				3,200	$ 222
After tax charge to income															$ 21	$ 54				$ 2,200	$ 137
Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Pipeline infrastructure | mi															61					61
Estimated cost			$ 7,380	$ 68								$ 7,380										2,880	$ 2,970	$ 2,880
Cost related to grant funding			137									137
Maximum cap construction cost			5,950									5,950											$ 2,400
Period of commercial operations established by discovery docket																				5 years
Pre-tax charge to income												2,800		$ 196	$ 34		$ 305			$ 3,200					$ 3,070
After tax charge to income												$ 2,000		$ 121	21		$ 188			2,200					$ 1,890
Construction and development costs																			$ 1,300	1,300
Average annual increase (decrease) in operations and maintenance expenses																				105
Average annual increase (decrease) in maintenance capital																				44
Approved increase (decrease) in fuel rates amount		$ 126
Regulatory assets															113					113
Regulatory liabilities															7					$ 7
AFUDC cost																								494
Kemper IGCC | MISSISSIPPI POWER CO | Minimum
Loss Contingencies [Line Items]
Amortization period of regulatory assets and liabilities																				2 years
Kemper IGCC | MISSISSIPPI POWER CO | Maximum
Loss Contingencies [Line Items]
Amortization period of regulatory assets and liabilities																				10 years
Kemper IGCC | MISSISSIPPI POWER CO | Pending Litigation
Loss Contingencies [Line Items]
Loss contingency, value of damages sought			$ 500		$ 100
Kemper IGCC | MISSISSIPPI POWER CO | Property, Plant and Equipment
Loss Contingencies [Line Items]
Construction and development costs																				$ 1,100
Kemper IGCC | MISSISSIPPI POWER CO | Property, Plant, and Equipment Accumulated Depreciation
Loss Contingencies [Line Items]
Construction and development costs																				80
Kemper IGCC | MISSISSIPPI POWER CO | Materials And Supplies
Loss Contingencies [Line Items]
Construction and development costs																				15
Kemper IGCC | MISSISSIPPI POWER CO | Other deferred charges and assets
Loss Contingencies [Line Items]
Construction and development costs																				10
Kemper IGCC | MISSISSIPPI POWER CO | Regulatory Assets
Loss Contingencies [Line Items]
Construction and development costs																				113
Kemper IGCC | MISSISSIPPI POWER CO | Regulatory Assets Accumulated Depreciation
Loss Contingencies [Line Items]
Construction and development costs																				63
Kemper IGCC | MISSISSIPPI POWER CO | Other regulatory assets current
Loss Contingencies [Line Items]
Construction and development costs																				21
Kemper IGCC | MISSISSIPPI POWER CO | Other regulatory assets, deferred
Loss Contingencies [Line Items]
Construction and development costs																				92
Kemper IGCC | MISSISSIPPI POWER CO | Transmission and Related Regulatory Assets
Loss Contingencies [Line Items]
Construction and development costs																				800
Kemper IGCC | MISSISSIPPI POWER CO | Assets, Current
Loss Contingencies [Line Items]
Regulatory assets															$ 21					21
Electricity Generation Plant, Non-Nuclear | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Estimated cost													$ 2,400									$ 2,400
Kemper IGCC - Combined Cycle | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
CIP expenditures incurred but not yet paid																				$ 500
Percentage of costs contracted to third party																				15.00%
Kemper IGCC - Gasification | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
AFUDC cost																								$ 460
Mine | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Term of management fee contract																							40 years
Monthly payments of contributed capital																							$ 2
Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Additional suspension costs	$ 5
Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO | Minimum
Loss Contingencies [Line Items]
Pre-tax charge to income																		$ 100
Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO | Maximum
Loss Contingencies [Line Items]
Pre-tax charge to income																		$ 200
In-Service Asset Proposal | Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount							$ 126
Public utilities, approved equity capital structure, percentage							49.733%
Public utilities, approved return on equity percentage							9.225%
Denbury Onshore, Contractor | Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Long term contract for purchase of percentage of carbon dioxide captured from plant						100.00%									70.00%					70.00%
Treetop Midstream Services, LLC, Contractor | Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Long term contract for purchase of percentage of carbon dioxide captured from plant															30.00%					30.00%
Denbury Onshore, Contractor | Electricity Generation Plant, Non-Nuclear | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Long-term contract for purchase of carbon dioxide, term						16 years