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Fair Value Measurements (Details 1) - Level 3 - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
NUG contracts    
Reconciliation of changes in the fair value of FTRs contracts    
Beginning Balance, Derivative Assets $ 1 $ 1
Beginning Balance, Derivative Liabilities (108) (137)
Beginning Balance, Net (107) (136)
Unrealized gain (loss), Derivative Assets 0 2
Unrealized gain (loss), Derivative Liabilities (6) (17)
Unrealized gain (loss), Net (6) (15)
Purchases, Derivative Assets   0
Purchases, Derivative Liabilities   0
Purchases, Net   0
Settlements, Derivative Assets (1) (2)
Settlements, Derivative Liabilities 11 46
Settlements, Net 10 44
Ending Balance, Derivative Assets 0 1
Ending Balance, Derivative Liabilities (103) (108)
Ending Balance, Net (103) (107)
FTRs    
Reconciliation of changes in the fair value of FTRs contracts    
Beginning Balance, Derivative Assets 7 8
Beginning Balance, Derivative Liabilities (6) (13)
Beginning Balance, Net 1 (5)
Unrealized gain (loss), Derivative Assets 0 (6)
Unrealized gain (loss), Derivative Liabilities (1) (4)
Unrealized gain (loss), Net (1) (10)
Purchases, Derivative Assets   16
Purchases, Derivative Liabilities   (7)
Purchases, Net   9
Settlements, Derivative Assets (7) (11)
Settlements, Derivative Liabilities 3 18
Settlements, Net (4) 7
Ending Balance, Derivative Assets 0 7
Ending Balance, Derivative Liabilities (4) (6)
Ending Balance, Net (4) 1
FTRs | FES    
Reconciliation of changes in the fair value of FTRs contracts    
Beginning Balance, Derivative Assets 4 5
Beginning Balance, Derivative Liabilities (5) (11)
Beginning Balance, Net (1) (6)
Unrealized gain (loss), Derivative Assets   (4)
Unrealized gain (loss), Derivative Liabilities   (3)
Unrealized gain (loss), Net   (7)
Purchases, Derivative Assets   10
Purchases, Derivative Liabilities   (5)
Purchases, Net   5
Settlements, Derivative Assets (4) (7)
Settlements, Derivative Liabilities 3 14
Settlements, Net (1) 7
Ending Balance, Derivative Assets 0 4
Ending Balance, Derivative Liabilities (2) (5)
Ending Balance, Net $ (2) $ (1)