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Fair Value Measurements (Details 3) - USD ($) $ in Millions | Jun. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Debt Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost Basis | $ 1,698 | $ 1,778
Unrealized Gain | 62 | 16
Fair Value | 1,760 | 1,794
Equity securities |  |
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities |  |
Cost Basis | 681 | 542
Unrealized Gains | 89 | 34
Fair Value | 770 | 576
FES | Debt Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost Basis | 820 | 801
Unrealized Gain | 41 | 9
Fair Value | 861 | 810
FES | Equity securities |  |
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities |  |
Cost Basis | 434 | 354
Unrealized Gains | 61 | 24
Fair Value | $ 495 | $ 378

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