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Significant Accounting Policies - Concentrations (Details)
$ in Thousands
Sep. 30, 2019
USD ($)
issuer
item
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Concentration risk      
Investments - debt securities $ 35 $ 8,958  
Investments - equity securities 37,074 36,132  
Cash and cash equivalents 165,311 195,155 $ 200,884
U.S. Treasury securities | Credit concentration risk      
Concentration risk      
Cash and cash equivalents 7,000    
Preferred stock      
Concentration risk      
Investments - equity securities 35,200 $ 36,100  
Preferred stock | Credit concentration risk      
Concentration risk      
Investments - equity securities 35,200    
Common Stock      
Concentration risk      
Investments - equity securities 1,900    
Common Stock | Credit concentration risk      
Concentration risk      
Investments - equity securities $ 1,900    
Number of issuers | item 1    
Commercial paper | Credit concentration risk      
Concentration risk      
Cash and cash equivalents $ 138,400    
Number of issuers | issuer 22    
Non-investment grade | Preferred stock | Credit concentration risk      
Concentration risk      
Number of issuers | issuer 7    
Investment grade | Preferred stock | Credit concentration risk      
Concentration risk      
Number of issuers | issuer 1