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Organization and Basis of Presentation (Details Textuals)
customer in Millions, MWh in Millions
 1 Months Ended 2 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Aug. 30, 2017
Aug. 30, 2017
MW
Aug. 30, 2017
Natural_gas_plant
Aug. 30, 2017
transmission_center
Aug. 30, 2017
USD ($)
Aug. 30, 2017
Aug. 30, 2017
limited_guaranty
Dec. 31, 2017
USD ($)
Mar. 31, 2017
MWh
Jan. 31, 2017
USD ($)
MW
Mar. 06, 2017
USD ($)
MWh
MW
Mar. 31, 2018
USD ($)
Sep. 30, 2017
USD ($)
transmission_center
MW
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
customer
transmission_center
company
mi
MW
Sep. 30, 2016
USD ($)
Dec. 31, 2018
MWh
Dec. 31, 2017
MWh
Dec. 31, 2016
USD ($)
MWh
Dec. 31, 2015
USD ($)
MW
Dec. 31, 2020
MW
Mar. 07, 2017
MW
Property, Plant and Equipment [Line Items]                                            
Aggregate amount of capacity | MW                             17,000              
Length of transmission lines | mi                             24,500              
Number of regional transmission centers | transmission_center                         2   2              
Capitalized cost of equity                         $ 8,000,000 $ 11,000,000 $ 25,000,000 $ 28,000,000            
Capitalized interest                         $ 11,000,000 17,000,000 34,000,000 51,000,000            
Net cash used in operating activities                             (2,762,000,000) (2,592,000,000)            
Net cash provided by financing activities                             $ (381,000,000) 304,000,000            
Utilization of Accelerated Useful Life                                            
Property, Plant and Equipment [Line Items]                                            
Reduction to depreciation                           $ 21,000,000           $ 19,000,000    
Accounting Standards Update 2016-09                                            
Property, Plant and Equipment [Line Items]                                            
Net cash used in operating activities                                     $ 12,000,000      
Net cash provided by financing activities                                     12,000,000      
Accounting Standards Update 2016-09 | Retained Earnings                                            
Property, Plant and Equipment [Line Items]                                            
Cumulative effect of new accounting principle                                     $ 6,000,000      
Regulated Distribution                                            
Property, Plant and Equipment [Line Items]                                            
Number of existing utility operating companies | company                             10              
Number of customers served by utility operating companies | customer                             6              
Plant capacity (in MW's) | MW                         3,790   3,790              
CES                                            
Property, Plant and Equipment [Line Items]                                            
Capacity in deactivated power plants (in MW's) | MW                                       6,770    
Plant capacity (in MW's) | MW                     13,000   13,162   13,162             10,000
Purchase power agreements additional capacity (in MWh) | MWh                     5                      
Contract sales (in MWh) | MWh                                     53      
FES                                            
Property, Plant and Equipment [Line Items]                                            
Net cash used in operating activities                             $ (458,000,000) (605,000,000)            
Net cash provided by financing activities                             (83,000,000) $ 61,000,000            
FES | CES                                            
Property, Plant and Equipment [Line Items]                                            
Inability to extend or refinance debt next year                         $ 515,000,000   515,000,000              
Scenario, Forecast | CES                                            
Property, Plant and Equipment [Line Items]                                            
Capacity in deactivated power plants (in MW's) | MW                                         856  
Scenario, Forecast | FES | CES                                            
Property, Plant and Equipment [Line Items]                                            
Interest and lease payments               $ 48,000,000                            
Interest payments                       $ 38,000,000                    
Minimum | CES                                            
Property, Plant and Equipment [Line Items]                                            
Annual generation capacity (in MW) | MWh                     70                      
Minimum | Scenario, Forecast | CES                                            
Property, Plant and Equipment [Line Items]                                            
Expected contract sales (in MWh) | MWh                                 30 40        
Maximum | CES                                            
Property, Plant and Equipment [Line Items]                                            
Annual generation capacity (in MW) | MWh                 65   75                      
Maximum | Scenario, Forecast | CES                                            
Property, Plant and Equipment [Line Items]                                            
Expected contract sales (in MWh) | MWh                                 35 45        
Line of Credit | Revolving Credit Facility                                            
Property, Plant and Equipment [Line Items]                                            
Maximum amount borrowed under revolving credit facility                         $ 500,000,000   $ 500,000,000              
Purchase Agreement with Aspen Generating, LLC                                            
Property, Plant and Equipment [Line Items]                                            
Plant capacity (in MW's) | MW   1,615               1,572                        
Cash purchase price         $ 825,000,000         $ 925,000,000                        
Number of limited guaranties       2     2                              
Term of guaranties 3 years                                          
Purchase Agreement with Aspen Generating, LLC | AE Supply                                            
Property, Plant and Equipment [Line Items]                                            
Number of gas generating plants | Natural_gas_plant     4                                      
Discharge of note indenture         305,000,000                                  
Make-whole premiums         $ 100,000,000                                  
Purchase Agreement with Aspen Generating, LLC | AGC                                            
Property, Plant and Equipment [Line Items]                                            
Ownership percentage           59.00%                                
Purchase Agreement with Aspen Generating, LLC | BU Energy                                            
Property, Plant and Equipment [Line Items]                                            
Ownership percentage           50.00%                                
Pleasants Power Station                                            
Property, Plant and Equipment [Line Items]                                            
Plant capacity (in MW's) | MW                     1,300                      
Pleasants Power Station | CES                                            
Property, Plant and Equipment [Line Items]                                            
Plant capacity (in MW's) | MW                         1,300   1,300              
Held-for-sale | Pleasants Power Station                                            
Property, Plant and Equipment [Line Items]                                            
Assets held for sale                     $ 195,000,000