Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Estimated fair value, less than 12 months	$ 213,201	$ 68,938
Estimated fair value, 12 months or more	41,099	42,568
Estimated fair value, total	254,300	111,506
Unrealized losses, less than 12 months	3,356	499
Unrealized losses, 12 months or more	1,868	884
Unrealized losses, total	5,224	1,383
State and Political Subdivisions [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	171,575	34,755
Estimated fair value, 12 months or more	30,324	31,251
Estimated fair value, total	201,900	66,006
Unrealized losses, less than 12 months	2,364	123
Unrealized losses, 12 months or more	1,317	538
Unrealized losses, total	3,681	661
U.S. government agencies [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	41,626	34,183
Estimated fair value, 12 months or more	10,775	11,317
Estimated fair value, total	52,400	45,500
Unrealized losses, less than 12 months	992	376
Unrealized losses, 12 months or more	551	346
Unrealized losses, total	1,543	722
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