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Fair Value Measurements (Unrealized Losses And Fair Value Of Investments With Unrealized Losses) (Details) $ in Millions | Dec. 31, 2017 USD ($)
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Total, number of investments | 182
Total, fair value | $ 223
Corporate and Asset-Backed |
Total, number of investments | 118
Total, fair value | $ 108
Foreign government |
Total, number of investments | 1
Total, fair value | $ 2
United States Agency |
Total, number of investments | 15
Total, fair value | $ 20
United States Treasury |
Total, number of investments | 20
Total, fair value | $ 70
Certificate of Deposit |
Total, number of investments | 28
Total, fair value | $ 23

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