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Investments (Summary Of Short-term Investments) (Details) - USD ($) $ in Thousands | 12 Months Ended
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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

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