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Investment in Securities (Schedule of Investment Securities) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Investment [Line Items]    
Fair Value $ 446,440 $ 470,523
Gross Unrealized Gains 2,187 9,528
Gross Unrealized Losses 7,028 1,383
Amortized Cost 451,281 462,378
State and Political Subdivisions [Member]    
Investment [Line Items]    
Fair Value 334,874 417,032
Gross Unrealized Gains 2,187 9,528
Gross Unrealized Losses 4,691 661
Amortized Cost 337,378 408,165
U.S. government agencies [Member]    
Investment [Line Items]    
Fair Value 105,821 45,500
Gross Unrealized Gains
Gross Unrealized Losses 2,337 722
Amortized Cost 108,158 46,222
Certificates of Deposit [Member]    
Investment [Line Items]    
Fair Value 5,745 7,991
Gross Unrealized Gains
Gross Unrealized Losses
Amortized Cost $ 5,745 $ 7,991