Effective Tax Rate and Unrecognized Tax Benefits - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Contingency [Line Items]
Effective tax rate	34.30%	32.30%
Kemper Igcc [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefit	211
Unrecognized tax benefit, associated interest	3
Mississippi Power [Member]
Income Tax Contingency [Line Items]
Effective tax rate	10.00%	(43.00%)
Mississippi Power [Member] | Kemper Igcc [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefit	211
Unrecognized tax benefit, associated interest	3
Southern Power [Member]
Income Tax Contingency [Line Items]
Effective tax rate	25.80%	8.50%