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Marketable Securities and Fair Value Measurements (Short Term Marketable Securities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost $ 253 $ 6,972
Unrealized Gain (Loss) (3) 3
Fair Value 250 6,975
Corporate debt securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost 253 2,236
Unrealized Gain (Loss) (3) 2
Fair Value $ 250 2,238
U.S. government treasury bills    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost   2,249
Unrealized Gain (Loss)   1
Fair Value   2,250
Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost   2,487
Unrealized Gain (Loss)   0
Fair Value   $ 2,487