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Significant Accounting Policies - Concentrations (Details)
$ in Thousands
Mar. 31, 2020
USD ($)
issuer
Dec. 31, 2019
USD ($)
Mar. 31, 2019
USD ($)
Concentration risk      
Investments - debt securities (primarily U.S. Treasury Bills) $ 49,997 $ 53  
Investments - equity securities 2,432 9,746  
Cash and cash equivalents 106,427 185,716 $ 190,821
Preferred stock      
Concentration risk      
Investments - equity securities 2,400 $ 9,700  
Credit concentration risk | Assets | U.S. Treasury Bills      
Concentration risk      
Investments - debt securities (primarily U.S. Treasury Bills) 49,900    
Credit concentration risk | Assets | Preferred stock | Non-investment grade      
Concentration risk      
Investments - equity securities $ 2,400    
Number of issuers | issuer 3    
Credit concentration risk | Assets | Commercial paper      
Concentration risk      
Cash and cash equivalents $ 37,500    
Number of issuers | issuer 7