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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 |  |

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