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Significant Accounting Policies - Concentrations (Details)
$ in Thousands
Sep. 30, 2021
USD ($)
issuer
Dec. 31, 2020
USD ($)
Sep. 30, 2020
USD ($)
Concentration risk      
Cash and cash equivalents $ 27,260 $ 106,794 $ 102,351
Investments - debt securities 102,986 48,051  
Investments - equity securities 375 2,623  
Preferred stock      
Concentration risk      
Investments - equity securities 400 $ 2,600  
Credit concentration risk | Assets | U.S. Treasury securities      
Concentration risk      
Investments - debt securities 103,000    
Credit concentration risk | Assets | Money market fund      
Concentration risk      
Cash and cash equivalents 9,400    
Credit concentration risk | Assets | Preferred stock | External Credit Rating, Non Investment Grade      
Concentration risk      
Investments - equity securities $ 400    
Number of issuers | issuer 2