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Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Investment [Line Items]    
Estimated fair value, less than 12 months $ 62,729 $ 153,056
Estimated fair value, 12 months or more 18,958
Estimated fair value, total 81,687 153,056
Unrealized losses, less than 12 months 674 3,731
Unrealized losses, 12 months or more 659
Unrealized losses, total 1,333 3,731
State and Political Subdivisions [Member]    
Investment [Line Items]    
Estimated fair value, less than 12 months 33,567 140,384
Estimated fair value, 12 months or more 16,288
Estimated fair value, total 49,855 140,384
Unrealized losses, less than 12 months 336 3,328
Unrealized losses, 12 months or more 588
Unrealized losses, total 924 3,328
U.S. government agencies [Member]    
Investment [Line Items]    
Estimated fair value, less than 12 months 29,162 12,672
Estimated fair value, 12 months or more 2,670
Estimated fair value, total 31,832 12,672
Unrealized losses, less than 12 months 338 403
Unrealized losses, 12 months or more 71
Unrealized losses, total 409 403
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