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Fair Value Measurements (Details 3) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Debt Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost Basis | $ 1,701 | $ 1,735
Unrealized Gain | 42 | 38
Fair Value | 1,743 | 1,773
Equity securities |  |
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities |  |
Cost Basis | 927 | 822
Unrealized Gains | 118 | 103
Fair Value | 1,045 | 925
FES | Debt Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost Basis | 942 | 847
Unrealized Gain | 27 | 27
Fair Value | 969 | 874
FES | Equity securities |  |
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities |  |
Cost Basis | 675 | 564
Unrealized Gains | 90 | 70
Fair Value | $ 765 | $ 634

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