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Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Marketable Securities [Line Items]    
Estimated fair value less than 12 months $ 3,801 $ 121,657
Estimated fair value 12 months or more 17,593 98,551
Estimated fair value total 21,394 220,208
Unrealized losses, less than 12 months 12 686
Unrealized losses, 12 months or more 157 2,537
Unrealized losses, total 169 3,223
State and Political Subdivisions [Member]    
Marketable Securities [Line Items]    
Estimated fair value less than 12 months 91,248
Estimated fair value 12 months or more 60,546
Estimated fair value total 151,794
Unrealized losses, less than 12 months 556
Unrealized losses, 12 months or more 1,250
Unrealized losses, total 1,806
U.S. government agencies [Member]    
Marketable Securities [Line Items]    
Estimated fair value less than 12 months 3,801 30,409
Estimated fair value 12 months or more 17,593 38,005
Estimated fair value total 21,394 68,414
Unrealized losses, less than 12 months 12 130
Unrealized losses, 12 months or more 157 1,287
Unrealized losses, total 169 1,417
Certificates of Deposit [Member]    
Marketable Securities [Line Items]    
Estimated fair value less than 12 months
Estimated fair value 12 months or more
Estimated fair value total
Unrealized losses, less than 12 months
Unrealized losses, 12 months or more
Unrealized losses, total