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Fair Value Measurements (Details Textuals) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Fair Value of Financial Instruments [Line Items]      
Investment excludes receivables, payables and accrued income $ (18) $ (18) $ (3)
Cash balance excluded from available for sale securities 87 87 61
Investments not required to be disclosed 254 $ 254 266
NUG contracts      
Fair Value of Financial Instruments [Line Items]      
Period of future observable data to determine contract price   3 years  
FES      
Fair Value of Financial Instruments [Line Items]      
Investment excludes receivables, payables and accrued income (3) $ (3) 2
Cash balance excluded from available for sale securities 80 $ 80 $ 44
NG | Beaver Valley Unit 2      
Fair Value of Financial Instruments [Line Items]      
NDT assets transferred $ 189