Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 08, 2017	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								$ 82	$ 21
Effective tax rate						42.60%	28.30%
Income benefit			$ (590)	$ (439)		$ (317)	$ (917)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016		$ 17				484
Tax positions from current periods						9
Tax positions from prior periods						186
Reductions due to settlements						290
Balance as of September 30, 2017			17			17
Tax positions impacting the effective tax rate								17	20
Tax positions not impacting the effective tax rate								0	464
Balance of unrecognized tax benefits		17	17			$ 484		17	484
Significantly increase (decrease) in amount of unrecognized tax benefits						12 months
Kemper IGCC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Tax positions from prior periods	$ 36
Balance as of September 30, 2017	176
Balance of unrecognized tax benefits	$ 176
MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								46
Effective tax rate						(30.30%)	(282.80%)
Income benefit			(24)	$ 2		$ 885	$ 29
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016		2				465
Tax positions from current periods						2
Tax positions from prior periods						175
Reductions due to settlements						290
Balance as of September 30, 2017			2			2
Tax positions impacting the effective tax rate								2
Tax positions not impacting the effective tax rate								0
Balance of unrecognized tax benefits		2	2			465		2	465
MISSISSIPPI POWER CO | Kemper IGCC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016
Balance as of September 30, 2017
Balance of unrecognized tax benefits
Interest on accrued income taxes
SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								19
Effective tax rate				60.30%		43.40%
Income benefit			(52)	$ (7)		$ (233)
SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Effective tax rate						(66.50%)	(88.90%)
Income benefit			39	$ 102		$ 129	$ 167
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits as of December 31, 2016		0				17
Tax positions from current periods						0
Tax positions from prior periods						17
Reductions due to settlements						0
Balance as of September 30, 2017			0			0
Balance of unrecognized tax benefits		0	$ 0			$ 17		0	17
Investment Tax Credit Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward, amount								1,900	$ 1,800
Federal
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								18
Federal | MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								0
Federal | SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								18
State (net of federal benefit)
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								64
State (net of federal benefit) | MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								46
State (net of federal benefit) | SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance								$ 1
Scenario, Forecast | SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Income benefit		$ 40
Predecessor | SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Effective tax rate					37.60%
Income benefit					$ (87)