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Summary of Significant Accounting Policies - Concentrations (Details)
$ in Thousands
Sep. 30, 2020
USD ($)
issuer
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Concentration risk      
Investments - debt securities $ 50,035 $ 53  
Investments - equity securities 2,677 9,746  
Cash and cash equivalents 102,351 185,716 $ 165,311
Preferred stock      
Concentration risk      
Investments - equity securities 2,700 $ 9,700  
Assets | Money market fund      
Concentration risk      
Cash and cash equivalents 84,000    
Credit concentration risk | Assets | U.S. Treasury Bills      
Concentration risk      
Investments - debt securities 49,900    
Credit concentration risk | Assets | Preferred stock | Non-investment grade      
Concentration risk      
Investments - equity securities $ 2,700    
Number of issuers | issuer 3