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Fair Value Measurements (Details 3) - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Debt Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost Basis $ 1,728 $ 1,778
Unrealized Gain 69 16
Fair Value 1,797 1,794
Equity securities    
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities    
Cost Basis 816 542
Unrealized Gains 92 34
Fair Value 908 576
FES | Debt Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost Basis 834 801
Unrealized Gain 45 9
Fair Value 879 810
FES | Equity securities    
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities    
Cost Basis 561 354
Unrealized Gains 63 24
Fair Value $ 624 $ 378