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Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Marketable Securities [Line Items]    
Estimated fair value, less than 12 months $ 1,793 $ 3,801
Estimated fair value, 12 months or more 17,593
Estimated fair value, total 1,793 21,394
Unrealized losses, less than 12 months 22 12
Unrealized losses, 12 months or more 157
Unrealized losses, total 22 169
U.S. government agencies [Member]    
Marketable Securities [Line Items]    
Estimated fair value, less than 12 months 1,793 3,801
Estimated fair value, 12 months or more 17,593
Estimated fair value, total 1,793 21,394
Unrealized losses, less than 12 months 22 12
Unrealized losses, 12 months or more 157
Unrealized losses, total $ 22 $ 169