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Fair Value Measurements (Fair Value of Marketable Securities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value Measurements    
Estimated fair value of marketable securities $ 25,706 $ 20,462
Fair value, inputs, Level 1 [Member]    
Fair Value Measurements    
Estimated fair value of marketable securities 25,706 20,462
Fair value, inputs, Level 2 [Member]    
Fair Value Measurements    
Estimated fair value of marketable securities 0 0
Fair value, inputs, Level 3 [Member]    
Fair Value Measurements    
Estimated fair value of marketable securities $ 0 $ 0