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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 |  |

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