Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Estimated fair value, less than 12 months	$ 57,200	$ 153,056
Estimated fair value, 12 months or more
Estimated fair value, total	57,200	153,056
Unrealized losses, less than 12 months	1,404	3,731
Unrealized losses, 12 months or more
Unrealized losses, total	1,404	3,731
State and Political Subdivisions [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	45,082	140,384
Estimated fair value, 12 months or more
Estimated fair value, total	45,082	140,384
Unrealized losses, less than 12 months	1,076	3,328
Unrealized losses, 12 months or more
Unrealized losses, total	1,076	3,328
U.S. government agencies [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	12,118	12,672
Estimated fair value, 12 months or more
Estimated fair value, total	12,118	12,672
Unrealized losses, less than 12 months	328	403
Unrealized losses, 12 months or more
Unrealized losses, total	328	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