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Fair Value Measurements (Details Textuals) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2017
Sep. 30, 2017
Dec. 31, 2016
Fair Value of Financial Instruments [Line Items]      
Investment excludes receivables, payables and accrued income   $ (13) $ (3)
Cash balance excluded from available for sale securities   100 61
Investments not required to be disclosed   $ 255 266
NUG contracts      
Fair Value of Financial Instruments [Line Items]      
Period of future observable data to determine contract price   2 years  
FES      
Fair Value of Financial Instruments [Line Items]      
Investment excludes receivables, payables and accrued income   $ (8) 2
Cash balance excluded from available for sale securities   $ 89 $ 44
NG | Beaver Valley Unit 2      
Fair Value of Financial Instruments [Line Items]      
NDT assets transferred $ 189