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Pension and Other Postemployment Benefits (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Pension          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Service costs $ 52 $ 48 $ 156 $ 144  
Interest costs 97 99 291 298  
Expected return on plan assets (112) (100) (336) (297)  
Amortization of prior service costs (credits) 2 2 6 6  
Net periodic costs (credits) 39 49 117 151  
OPEB          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Service costs 1 2 3 4  
Interest costs 7 7 21 22  
Expected return on plan assets (7) (7) (22) (23)  
Amortization of prior service costs (credits) (20) (20) (60) (60)  
Net periodic costs (credits) (19) (18) (58) (57)  
FirstEnergy | Pension          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic benefit expense (credit) 30 35 89 107  
FirstEnergy | OPEB          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic benefit expense (credit) (14) (11) (43) (41)  
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 9 18  
Net periodic benefit expense (credit) 3 5 9 17  
FES | OPEB          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic costs (credits) (4) (4) (12) (12)  
Net periodic benefit expense (credit) (4) $ (4) (12) $ (12)  
FENOC          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Non-current liabilities $ 570   $ 570   $ 570