Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Estimated fair value, less than 12 months	$ 7,084	$ 121,657
Estimated fair value, 12 months or more	25,809	98,551
Estimated fair value, total	32,893	220,208
Unrealized losses, less than 12 months	19	686
Unrealized losses, 12 months or more	225	2,537
Unrealized losses, total	244	3,223
State and Political Subdivisions [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	1,117	91,248
Estimated fair value, 12 months or more		60,546
Estimated fair value, total	1,117	151,794
Unrealized losses, less than 12 months	1	556
Unrealized losses, 12 months or more		1,250
Unrealized losses, total	1	1,806
U.S. government agencies [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	5,967	30,409
Estimated fair value, 12 months or more	25,809	38,005
Estimated fair value, total	31,776	68,414
Unrealized losses, less than 12 months	18	130
Unrealized losses, 12 months or more	225	1,287
Unrealized losses, total	$ 243	$ 1,417