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Marketable Securities (Summary of Investment Securities in an Unrealized Loss Position) (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Investments, Debt and Equity Securities [Abstract]  
Unrealized Losses, Less than 12 months $ (95)
Unrealized Losses, 12 months or more (49)
Unrealized Losses, Total (144)
Fair value, Less than 12 months 5,881
Fair value, 12 months or more 7,749
Fair value, Total $ 13,630