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Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Investment [Line Items]    
Estimated fair value less than 12 months $ 153,056 $ 3,638
Estimated fair value 12 months or more 1,208
Estimated fair value total 153,056 4,846
Unrealized losses, less than 12 months 3,731 5
Unrealized losses, 12 months or more 8
Unrealized losses, total 3,731 13
State and Political Subdivisions [Member]    
Investment [Line Items]    
Estimated fair value less than 12 months 140,384 3,638
Estimated fair value 12 months or more 1,208
Estimated fair value total 140,384 4,846
Unrealized losses, less than 12 months 3,328 5
Unrealized losses, 12 months or more 8
Unrealized losses, total 3,328 13
U.S. government agencies [Member]    
Investment [Line Items]    
Estimated fair value less than 12 months 12,672  
Estimated fair value 12 months or more  
Estimated fair value total 12,672  
Unrealized losses, less than 12 months 403  
Unrealized losses, 12 months or more  
Unrealized losses, total 403  
Certificates of Deposit [Member]    
Investment [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