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Fair Value Measurements (Unrealized Losses And Fair Value Of Investments With Unrealized Losses) (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Total, number of investments 115
Total, fair value $ 70
Corporate and Asset-Backed  
Total, number of investments 75
Total, fair value $ 34
Foreign government  
Total, number of investments 0
Total, fair value $ 0
United States Agency  
Total, number of investments 8
Total, fair value $ 10
United States Treasury  
Total, number of investments 17
Total, fair value $ 19
Certificate of Deposit  
Total, number of investments 15
Total, fair value $ 7