Segment and Related Information - Financial Data for Business Segments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) state	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of States in which Entity Operates | state	4
Operating revenues	$ 3,965	$ 4,183
Segment net income (loss)	485	508
Total assets	78,735		$ 78,318
Kemper IGCC [Member]
Segment Reporting Information [Line Items]
Pre-Tax Charge To Income	53	9
After Tax Charge To Income	33	6
Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	3,954	4,172
Segment net income (loss)	514	510
Total assets	77,843		77,560
Intersegment Eliminations [Member] | Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	(103)	(124)
Segment net income (loss)	0	0
Total assets	(396)		(397)
Consolidation Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(36)	(29)
Segment net income (loss)	(3)	(5)
Total assets	(1,178)		(1,061)
Traditional Operating Companies [Member] | Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	3,742	3,948
Segment net income (loss)	464	477
Total assets	69,240		69,052
Southern Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	97	114
Southern Power [Member] | Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	315	348
Segment net income (loss)	50	33
Total assets	8,999		8,905
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	47	40
Segment net income (loss)	(26)	$ 3
Total assets	$ 2,070		$ 1,819