Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Estimated fair value, less than 12 months	$ 1,007	$ 121,657
Estimated fair value, 12 months or more	36,448	98,551
Estimated fair value, total	37,455	220,208
Unrealized losses, less than 12 months		686
Unrealized losses, 12 months or more	354	2,537
Unrealized losses, total	354	3,223
State and Political Subdivisions [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	1,007	91,248
Estimated fair value, 12 months or more	2,976	60,546
Estimated fair value, total	3,983	151,794
Unrealized losses, less than 12 months		556
Unrealized losses, 12 months or more	7	1,250
Unrealized losses, total	7	1,806
U.S. government agencies [Member]
Investment [Line Items]
Estimated fair value, less than 12 months		30,409
Estimated fair value, 12 months or more	33,472	38,005
Estimated fair value, total	33,472	68,414
Unrealized losses, less than 12 months		130
Unrealized losses, 12 months or more	347	1,287
Unrealized losses, total	$ 347	$ 1,417