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Pension and Other Postemployment Benefits (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Pension          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Service costs $ 52 $ 48 $ 104 $ 96  
Interest costs 97 99 194 199  
Expected return on plan assets (112) (100) (224) (197)  
Amortization of prior service costs (credits) 2 2 4 4  
Net periodic costs (credits) 39 49 78 102  
OPEB          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Service costs 1 1 2 2  
Interest costs 7 8 14 15  
Expected return on plan assets (7) (8) (15) (16)  
Amortization of prior service costs (credits) (20) (20) (40) (40)  
Net periodic costs (credits) (19) (19) (39) (39)  
FirstEnergy | Pension          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic benefit expense (credit) 27 35 59 72  
FirstEnergy | OPEB          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic benefit expense (credit) (14) (15) (29) (30)  
FES          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Non-current liabilities 866   866   $ 866
FES | Pension          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic costs (credits) 3 6 6 12  
Net periodic benefit expense (credit) 3 6 6 12  
FES | OPEB          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic costs (credits) (4) (4) (8) (8)  
Net periodic benefit expense (credit) (4) $ (4) (8) $ (8)  
FENOC          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Non-current liabilities $ 570   $ 570   $ 570