Contingencies and Regulatory Matters - Current And Actual Cost Estimate (Details) - Mississippi Power [Member] - Kemper IGCC [Member] - USD ($) $ in Millions	1 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2013	Jan. 31, 2013	Sep. 30, 2015	Dec. 31, 2012
Loss Contingencies [Line Items]
Plant Subject to Cost Cap	$ 2,880	$ 2,880	$ 4,660	$ 2,880
Lignite Mine and Equipment			230
CO2 Pipeline Facilities			110
AFUDC			550
Combined Cycle and Related Assets Placed in Service – Incremental			0
General Exceptions			80
Deferred Costs			170
Total Kemper IGCC, Actual Costs			5,800
Project Estimate [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
Total Kemper IGCC, Actual Costs			2,970
Current Estimate [Member]
Loss Contingencies [Line Items]
Plant Subject to Cost Cap			5,110
Lignite Mine and Equipment			230
CO2 Pipeline Facilities			110
AFUDC			660
Combined Cycle and Related Assets Placed in Service – Incremental			20
General Exceptions			100
Deferred Costs			200
Total Kemper IGCC, Actual Costs			$ 6,430