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Fair Value Measurements (Details 3) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 783	$ 774
Unrealized Gains	2	11
Unrealized Losses	(27)	(17)
Fair Value	758	768
Equity securities
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities
Cost Basis	265	254
Unrealized Gains	31	40
Unrealized Losses	(1)	0
Fair Value	$ 295	$ 294

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