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Significant Accounting Policies - Concentrations (Details)
$ in Thousands
Jun. 30, 2021
USD ($)
issuer
Dec. 31, 2020
USD ($)
Jun. 30, 2020
USD ($)
Concentration risk      
Cash and cash equivalents $ 33,336 $ 106,794 $ 106,039
Investments - debt securities 109,975 48,051  
Investments - equity securities 339 2,623  
Preferred stock      
Concentration risk      
Investments - equity securities 300 $ 2,600  
Credit concentration risk | Assets | U.S. Treasury Bills      
Concentration risk      
Investments - debt securities 110,000    
Credit concentration risk | Assets | Money Market Funds      
Concentration risk      
Cash and cash equivalents 5,300    
Credit concentration risk | Assets | Preferred stock | Non-investment grade      
Concentration risk      
Investments - equity securities $ 300    
Number of issuers | issuer 2