Income Taxes (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Operating loss carryforwards			$ 24
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2014	$ 170
Tax positions from current periods	31
Tax positions from prior periods	456
Reductions due to settlements	0
Balance as of September 30, 2015	657
Tax positions impacting the effective tax rate	7			$ 10
Tax positions not impacting the effective tax rate	650			160
Kemper IGCC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of September 30, 2015	414
Unrecognized tax benefit, associated interest	7
Southern Power [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards	$ 212			$ 305
Effective tax rate	6.90%	14.40%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2014	$ 5
Tax positions from current periods	7
Tax positions from prior periods	(6)
Reductions due to settlements	0
Balance as of September 30, 2015	6
Tax positions impacting the effective tax rate	6
Tax positions not impacting the effective tax rate	$ 0
Mississippi Power [Member]
Income Tax Contingency [Line Items]
Effective tax rate	(20.90%)	(45.50%)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2014	$ 165
Tax positions from current periods	24
Tax positions from prior periods	459
Reductions due to settlements	0
Balance as of September 30, 2015	648
Tax positions impacting the effective tax rate	(2)
Tax positions not impacting the effective tax rate	650
Internal Revenue Code Section 48A tax credits (Phase II)	$ 279
Minimum percentage of carbon dioxide that must be capture and sequester to remain eligible for the phase II tax credits	65.00%
Tax credit carryforward	$ 276
Tax credit carryforward, utilized	235
Mississippi Power [Member] | Kemper IGCC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of September 30, 2015	414
Unrecognized tax benefit, associated interest	$ 7