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Fair Value Measurements (Details 2) - Level 3
$ in Millions
 3 Months Ended
Mar. 31, 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 | $ $ (4) $ 1 $ (5)
FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (2) (1) (6)
NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (103) $ (107) $ (136)
Model | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (4)    
Model | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (2)    
Model | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ (103)    
Model | Minimum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) (2.70)    
Model | Minimum | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) (2.70)    
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) 31.70    
Model | Maximum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 2.90    
Model | Maximum | FTRs | FES      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 2.40    
Model | Maximum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 2,766,000    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 33.60    
Model | Weighted Average | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 0.30    
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 560,000    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 31.70