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Marketable Securities and Fair Value Measurements (Assets and Liabilities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities: $ 250 $ 6,975
Cash equivalents:    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Cash equivalents: 23,832 1,307
Cash equivalents: | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Cash equivalents: 23,832 1,307
Cash equivalents: | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Cash equivalents: 0 0
Cash equivalents: | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Cash equivalents: 0 0
U.S. government treasury bills    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   2,250
U.S. government treasury bills | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   2,250
U.S. government treasury bills | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   0
U.S. government treasury bills | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   0
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities: 250 2,238
Corporate debt securities | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities: 0 0
Corporate debt securities | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities: 250 2,238
Corporate debt securities | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities: $ 0 0
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   2,487
Commercial paper | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   0
Commercial paper | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   2,487
Commercial paper | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring [Line Items]    
Short-term marketable securities:   $ 0