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Fair Value Measurements (Details Textuals) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Fair Value of Financial Instruments [Line Items]    
Investment excludes receivables, payables and accrued income $ (14) $ (3)
Cash balance excluded from available for sale securities 53 61
Investments not required to be disclosed $ 269 266
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 $ (2) 2
Cash balance excluded from available for sale securities $ 43 $ 44