Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) $ in Thousands	Jun. 30, 2021 USD ($)
Marketable Securities [Line Items]
Estimated fair value, less than 12 months	$ 68,182
Estimated fair value, 12 months or more
Estimated fair value, total	68,182
Unrealized losses, less than 12 months	472
Unrealized losses, 12 months or more
Unrealized losses, total	472
State and political subdivisions [Member]
Marketable Securities [Line Items]
Estimated fair value, less than 12 months	8,184
Estimated fair value, 12 months or more
Estimated fair value, total	8,184
Unrealized losses, less than 12 months	48
Unrealized losses, 12 months or more
Unrealized losses, total	48
U.S. government agencies [Member]
Marketable Securities [Line Items]
Estimated fair value, less than 12 months	59,998
Estimated fair value, 12 months or more
Estimated fair value, total	59,998
Unrealized losses, less than 12 months	424
Unrealized losses, 12 months or more
Unrealized losses, total	$ 424