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Fair Value Measurements (Details 2) - Level 3
$ in Millions
 9 Months Ended
Sep. 30, 2017
USD ($)
MWh
$ / MWh
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ 3 $ 1 $ (5)
FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ 0 (1) (6)
NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (92) $ (107) $ (136)
Model | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ 3    
Model | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ 0    
Model | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ (92)    
Model | Minimum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) (5.00)    
Model | Minimum | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) (5.00)    
Model | Minimum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 400    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 29.90    
Model | Maximum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 4.60    
Model | Maximum | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 2.60    
Model | Maximum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 2,322,000    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 31.40    
Model | Weighted Average | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 0.60    
Model | Weighted Average | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 0.20    
Model | Weighted Average | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 470,000    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 29.90