Contingencies and Regulatory Matters - Current And Actual Cost Estimate (Details) - USD ($) $ in Millions			3 Months Ended		12 Months Ended	63 Months Ended
	Nov. 17, 2016	Apr. 08, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2012	Mar. 31, 2017
Kemper IGCC [Member]
Loss Contingencies [Line Items]
Pre-tax charge to income			$ 108	$ 53
Mississippi Power [Member]
Loss Contingencies [Line Items]
Additional DOE grants					$ 245
Total Kemper IGCC, Actual Costs					2,880
Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Plant Subject to Cost Cap	$ 68		5,570		$ 2,880
Lignite Mine and Equipment			230
CO2 Pipeline Facilities			120
AFUDC			800
Combined Cycle and Related Assets Placed in Service – Incremental			40
General Exceptions			90
Deferred Costs			220
Additional DOE grants			(140)
Total Kemper IGCC, Actual Costs			6,930
Government grants received		$ (137)	(140)
Pre-tax charge to income			108			$ 2,870
Recoverable cost above maximum cap construction cost			3,370
Mississippi Power [Member] | Project Estimate [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Plant Subject to Cost Cap			2,400
Lignite Mine and Equipment			210
CO2 Pipeline Facilities			140
AFUDC			170
Combined Cycle and Related Assets Placed in Service – Incremental			0
General Exceptions			50
Deferred Costs			0
Additional DOE grants			0
Total Kemper IGCC, Actual Costs			2,970
Mississippi Power [Member] | Current Estimate [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Plant Subject to Cost Cap			5,750
Lignite Mine and Equipment			230
CO2 Pipeline Facilities			120
AFUDC			830
Combined Cycle and Related Assets Placed in Service – Incremental			50
General Exceptions			100
Deferred Costs			220
Additional DOE grants			(140)
Total Kemper IGCC, Actual Costs			7,160
Gasifiers and Gas Clean-up Facilities [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			1,900
Lignite Mine Facility [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			310
CO2 Pipeline Facilities [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			110
Combined Cycle and Common Facilities [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			170
AFUDC [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			730
General Exceptions [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			70
Plant Inventory [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			40
Lignite Inventory [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			60
Regulatory and Other Deferred Assets [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			120
All Costs [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost			3,510
Investment Costs and Related Assets In Service [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Total Kemper IGCC, Actual Costs			830
Costs Previously Expensed and Related Assets In Service [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Total Kemper IGCC, Actual Costs			90
Post June 2013 Mine Operations, Lignite Fuel Inventory, and Nitrogen Plant Capital Lease [Member] | Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Total Kemper IGCC, Actual Costs			$ 230