Contingencies and Regulatory Matters - Textual - MPC (Details)															1 Months Ended		2 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended									65 Months Ended
	Feb. 14, 2018 USD ($)	Feb. 07, 2018 USD ($)	Feb. 06, 2018 USD ($)	Jan. 01, 2018 USD ($)	Sep. 28, 2017 MW	Sep. 18, 2017	Jun. 28, 2017 USD ($)	May 04, 2017	Jan. 12, 2017 USD ($)	Aug. 17, 2016 USD ($)	Jun. 09, 2016 USD ($)	May 01, 2016 USD ($)	Mar. 31, 2016	Dec. 03, 2015 USD ($)	Jun. 30, 2017 USD ($)	Jul. 31, 2016 USD ($)	Mar. 15, 2017 USD ($)	Dec. 31, 2017 USD ($) mi	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) mi	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) rate_plan_filing mi MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($) Customer	Dec. 31, 2012 USD ($)	Dec. 31, 2011	Dec. 31, 2010 USD ($)	Dec. 31, 2008 MW	May 31, 2017 USD ($)	Feb. 01, 2018 USD ($)	Jan. 26, 2018 day	Feb. 03, 2017 USD ($)	Jan. 21, 2017 USD ($)	Jun. 17, 2016 USD ($)	May 03, 2016 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2014 MW
Schedule of Regulatory Assets and Liabilities [Line Items]
Storm restoration costs																																								$ 9,000,000
MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Increase (Decrease) Cash Collateral from Counterparties																												$ 0
Increase in base rate under cost based electric tariff due to settlement												$ 7,000,000
AFUDC cost																	$ 22,000,000
Period of notice to cancel counterparty						10 years
Period of notice required for contract termination						1 year
Public utilities, approved rate increase (decrease), amount										$ 18,000,000																		$ 22,000,000
Percentage of PSC retail rate incrase																																									0.85%
Energy power supply agreement term																																	10 years		10 years
Energy power supply agreement capacity | MW					286																							86							152
Performance evaluation plan, number of filings per calendar year | rate_plan_filing																												2
Annual PEP lookback refund to customers																															$ 5,000,000
Annual PEP filing rate increase																															1.90%
Annual PEP filing rate increase amount																															$ 15,000,000
Anticipates of elimination adjustment will result in additional revenue																															$ 3,000,000
Gain contingency, surcharge revenue																													$ 5,000,000
Required customers for energy efficiency programs | Customer																															25,000
PSC retail increase (decrease)																																									$ (8,000,000)	$ (2,000,000)
Regulatory amortization period																5 years
Psc approved annual property damage reserve accrual																																							$ 1,000,000				$ 3,000,000
Plant capacity under coal gasification combined cycle technology | MW																												582
Co two pipeline infrastructure | mi																		61								61		61
Maximum cap construction costs																																$ 2,880,000,000
Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
Pre-Tax charge to income																		$ 208,000,000	$ 34,000,000	$ 3,000,000,000	$ 108,000,000	$ 206,000,000	$ 88,000,000	$ 81,000,000	$ 53,000,000
After tax charge to income																		185,000,000	21,000,000	2,100,000,000	67,000,000	127,000,000	54,000,000	50,000,000	33,000,000			$ 2,400,000,000	264,000,000	$ 226,000,000
Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Period of amortization of regulatory assets													36 months
Regulatory asset																		114,000,000								$ 114,000,000		114,000,000
Regulatory liabilities																		26,000,000								26,000,000		$ 26,000,000
Maximum cap construction costs															$ 5,950,000,000																			$ 2,400,000,000
Cost cap exceptions																																		$ 570,000,000
Period of commercial operations established by discovery docket																												5 years
Estimated cost															7,380,000,000
Cost related to grant funding															137,000,000
Pre-Tax charge to income															2,800,000,000			208,000,000	34,000,000	3,000,000,000	108,000,000	206,000,000	88,000,000	81,000,000	53,000,000	242,000,000										$ 3,070,000,000
After tax charge to income															$ 2,000,000,000			185,000,000	$ 21,000,000	$ 2,100,000,000	$ 67,000,000	127,000,000	$ 54,000,000	$ 50,000,000	$ 33,000,000	206,000,000										$ 1,890,000,000
After tax charge to income, cost during suspension period																										164,000,000
Amortization of regulatory assets																												8 years
Amortization of regulatory liabilities																												6 years
Disallowance pre-tax charge to income																		78,000,000
Pre-tax charge to income before accumulated depreciation																		85,000,000
Pre-Tax charge to income, accumulated depreciation																		7,000,000
Disallowance after tax charge to income																		48,000,000
Reduction in customer rates annually																		26,800,000
Mine | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Term of management fee contract																																		40 years
Plant Sweatt Units 1 And 2 | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity unit conversation to non-fossil fuel source | MW																																												80
Plant Watson Units 4 And 5 | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity unit conversation to non-fossil fuel source | MW																																												750
Plant Greene County Units 1 And 2 | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity unit conversation to non-fossil fuel source | MW																																												200
Regulatory asset																		17,000,000								17,000,000		$ 17,000,000
Plant Watson | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset																$ 41,000,000
MRA Revenue | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Over recovered fuel cost																						13,000,000							13,000,000
Retail | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Over recovered fuel cost																						$ 37,000,000							$ 37,000,000
Public utilities, approved rate increase (decrease), amount									$ 55,000,000
Approved increase (decrease) in annual billing based on fuel cost recovery rate										$ (39,000,000)
Under recovered fuel cost																		$ 6,000,000								$ 6,000,000		$ 6,000,000
Subsequent Event | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of days to file information on impact of tax reform legislation | day																																						30
Annual PEP filing rate increase amount		$ 26,000,000
Public utilities, approved return on equity		9.33%
Regulatory equity ratio		55.00%
Requested increase in property damage reserve accrual																																					$ 2,000,000
Subsequent Event | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount			$ 99,300,000
Public utilities, approved return on equity			8.60%
Subsequent Event | MRA Revenue | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount				$ 11,000,000
Scenario, Forecast | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount																											$ 13,000,000
Annual PEP filing rate increase																											4.00%
Annual PEP filing rate increase amount																											$ 38,000,000
Scenario, Forecast | Subsequent Event | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities requested revenue requirement increase (decrease)																											26,000,000
Plant Daniel Units 1 & 2 (coal) | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Percent ownership																		50.00%								50.00%		50.00%
Maximum | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities approved rate increase (decrease)								2.00%		2.00%
Maximum | Subsequent Event | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, requested rate increase (decrease)	2.00%
Public utilities requested rate increase (decrease)	$ 17,000,000
Maximum | Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Pre-Tax charge to income																											100,000,000
Minimum | Scenario, Forecast | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Pre-Tax charge to income																											$ 50,000,000
In-Service Asset Proposal | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity														9.225%
PSC retail increase (decrease)														$ 126,000,000
Public utilities approved equity capital structure percentage														49.733%
In-Service Asset Proposal | Maximum | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Amortization period of regulatory assets and liabilities																													10 years
In-Service Asset Proposal | Minimum | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Amortization period of regulatory assets and liabilities																													2 years
Pending Litigation | Kemper IGCC | MISSISSIPPI POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Loss contingency, damages sought, value							$ 500,000,000				$ 100,000,000