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Investments (Summary Of Short-term Investments) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Investment [Line Items]    
Short-term investments, Cost Basis $ 12,648 $ 22,822
Short-term investment, Unrealized gains (losses) (6) (21)
Short-term investments, Fair value 12,642 22,801
Corporate Bonds [Member]    
Investment [Line Items]    
Short-term investments, Cost Basis 2,925 8,284
Short-term investment, Unrealized gains (losses) (5) (8)
Short-term investments, Fair value 2,920 8,276
U.S. Government Agencies and Securities [Member]    
Investment [Line Items]    
Short-term investments, Cost Basis 3,000 8,542
Short-term investment, Unrealized gains (losses) (1) (13)
Short-term investments, Fair value 2,999 8,529
Commercial Paper [Member]    
Investment [Line Items]    
Short-term investments, Cost Basis 6,723 5,996
Short-term investments, Fair value $ 6,723 $ 5,996