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Fair Value Measurements (Fair Value of Marketable Securities) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value Measurements    
Estimated fair value of marketable securities $ 28,514 $ 25,706
Fair value, inputs, Level 1 [Member]    
Fair Value Measurements    
Estimated fair value of marketable securities 28,514 25,706
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