Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Estimated fair value less than 12 months	$ 68,938	$ 153,056
Estimated fair value 12 months or more	42,568
Estimated fair value total	111,506	153,056
Unrealized losses, less than 12 months	499	3,731
Unrealized losses, 12 months or more	884
Unrealized losses, total	1,383	3,731
State and Political Subdivisions [Member]
Investment [Line Items]
Estimated fair value less than 12 months	34,755	140,384
Estimated fair value 12 months or more	31,251
Estimated fair value total	66,006	140,384
Unrealized losses, less than 12 months	123	3,328
Unrealized losses, 12 months or more	538
Unrealized losses, total	661	3,328
U.S. government agencies [Member]
Investment [Line Items]
Estimated fair value less than 12 months	34,183	12,672
Estimated fair value 12 months or more	11,317
Estimated fair value total	45,500	12,672
Unrealized losses, less than 12 months	376	403
Unrealized losses, 12 months or more	346
Unrealized losses, total	722	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