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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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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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