Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Estimated fair value, less than 12 months	$ 4,945	$ 121,657
Estimated fair value, 12 months or more	66,868	98,551
Estimated fair value, total	71,813	220,208
Unrealized losses, less than 12 months	5	686
Unrealized losses, 12 months or more	996	2,537
Unrealized losses, total	1,001	3,223
State and Political Subdivisions [Member]
Investment [Line Items]
Estimated fair value, less than 12 months		91,248
Estimated fair value, 12 months or more	22,617	60,546
Estimated fair value, total	22,617	151,794
Unrealized losses, less than 12 months		556
Unrealized losses, 12 months or more	181	1,250
Unrealized losses, total	181	1,806
U.S. government agencies [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	4,945	30,409
Estimated fair value, 12 months or more	44,251	38,005
Estimated fair value, total	49,196	68,414
Unrealized losses, less than 12 months	5	130
Unrealized losses, 12 months or more	815	1,287
Unrealized losses, total	820	1,417
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