v3.10.0.1
Regulatory Matters - MPC Regulatory Matters (Details)
customer in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
May 08, 2018
USD ($)
Feb. 14, 2018
USD ($)
Feb. 07, 2018
USD ($)
Feb. 06, 2018
USD ($)
Jan. 16, 2018
USD ($)
May 31, 2017
USD ($)
Aug. 31, 2016
USD ($)
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
filing
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jan. 10, 2019
USD ($)
Aug. 07, 2018
USD ($)
Aug. 03, 2018
USD ($)
Jul. 31, 2017
USD ($)
Jan. 31, 2017
USD ($)
Jul. 31, 2016
USD ($)
Dec. 31, 2013
USD ($)
customer
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Net regulatory assets                 $ 720,000,000 $ (1,894,000,000)                  
Major storms                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Storm damage reserves                 $ 74,000,000 41,000,000 $ 40,000,000                
Mississippi Power                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Performance evaluation plan, number of filings per calendar year | filing                 2                    
Annual PEP lookback refund to customers                                     $ 5,000,000
Gain contingency, surcharge revenue                     $ 5,000,000 $ 5,000,000              
Annual PEP filing rate increase                 4.00%                    
Annual PEP filing rate increase amount     $ 26,000,000           $ 38,000,000                    
Public utilities, approved return on equity     9.33%                                
Regulatory equity ratio     55.00%                                
Increase in annual base rate revenues                           $ 21,600,000 $ 17,000,000        
Amount approved for recovery                           $ 2,000,000          
Net regulatory assets                 $ (35,000,000) (42,000,000)                  
Retail rate of return on common equity                           9.31%          
Allowed equity ratio                 50.00%         50.00%          
Actual average equity ratio variance (more than)                 1.00%                    
Recognized revenues               $ 5,000,000                      
Required customers for energy efficiency programs | customer                                     25
Public utilities, approved rate increase (decrease), amount           $ 18,000,000 $ 18,000,000     $ 22,000,000                  
Regulatory amortization period                 5 years                    
Public utilities requested revenue requirement increase (decrease)           $ 26,000,000     $ 26,000,000                    
Percentage of PSC retail rate increase (decrease)                 0.80% 0.85% (0.07%)                
PSC retail increase (decrease)                 $ 7,000,000 $ 8,000,000 $ (1,000,000)           $ 55,000,000    
Threshold above which actual damages are charged to the reserve                 $ 50,000                    
Period to agree on system restoration rider                 120 days                    
Wholesale accrual per annual SSR rate                 $ 300,000                    
SRR rate                 0.00% 0.00% 0.00%                
Psc approved annual property damage reserve accrual                 $ 2,000,000 $ 4,000,000 $ 3,000,000                
Storm restoration costs                                 $ 9,000,000    
Mississippi Power | Retail electric revenues                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Proposed property damage reserve                 55,000,000                    
Mississippi Power | Wholesale                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Proposed property damage reserve                 1,000,000                    
Mississippi Power | Major storms                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Storm damage reserves                 $ 1,000,000 3,000,000 $ 4,000,000                
Mississippi Power | Mississippi Public Service Commission                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Period to agree on system restoration rider                 3 years                    
Mississippi Power | Retail electric revenues                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Public utilities, approved rate increase (decrease), amount $ 3,000,000       $ 39,000,000                            
Under recovered fuel cost                 $ 8,000,000 $ 6,000,000                  
Mississippi Power | Maximum                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Allowed equity ratio                           51.00%          
Public utilities approved rate increase (decrease)   2.00%       2.00% 2.00%     2.00%                  
Public utilities requested rate increase (decrease)   $ 17,000,000                                  
Mississippi Power | Kemper IGCC                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Public utilities, approved return on equity       8.60%                              
Public utilities, approved rate increase (decrease), amount       $ 99,300,000                              
Mississippi Power | Plant Greene County Units 1 And 2                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Regulatory asset                               $ 17,000,000      
Mississippi Power | Plant Watson                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Regulatory asset                                   $ 41,000,000  
Mississippi Power | Contested Compensation Costs                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Net regulatory assets                 4,000,000                    
Mississippi Power | Subsequent Event | Retail electric revenues                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
PSC retail increase (decrease)                         $ (35,000,000)            
Mississippi Power | Deferred credits related to income taxes                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Net regulatory assets                 $ (377,000,000) $ (377,000,000)                  
Regulatory amortization period                 8 years                    
Tax Cuts and Jobs Act of 2017, unprotected deferred tax assets                 $ 111,000,000                    
Mississippi Power | Deferred credits related to income taxes | Kemper IGCC                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Tax Cuts and Jobs Act of 2017, unprotected deferred tax assets                 $ 46,000,000                    
Mississippi Power | Deferred credits related to income taxes | Kemper IGCC | Retail electric revenues                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Regulatory amortization period                 8 years                    
Tax Cuts and Jobs Act of 2017, unprotected deferred tax assets                 $ 33,000,000                    
Mississippi Power | Deferred charges related to income taxes                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Regulatory liabilities                 44,000,000                    
Mississippi Power | Performance Settlement Agreement, Average Equity Ratio Differential                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Regulatory liabilities                 4,000,000                    
Mississippi Power | ECO Settlement Agreement, Average Equity Ratio Differential                                      
Schedule of Regulatory Assets and Liabilities [Line Items]                                      
Regulatory liabilities                 $ 2,000,000