Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) $ in Thousands	Mar. 31, 2021 USD ($)
Marketable Securities [Line Items]
Estimated fair value, less than 12 months	$ 96,914
Estimated fair value, 12 months or more
Estimated fair value, total	96,914
Unrealized losses, less than 12 months	885
Unrealized losses, 12 months or more
Unrealized losses, total	885
State and political subdivisions [Member]
Marketable Securities [Line Items]
Estimated fair value, less than 12 months	36,423
Estimated fair value, 12 months or more
Estimated fair value, total	36,423
Unrealized losses, less than 12 months	174
Unrealized losses, 12 months or more
Unrealized losses, total	174
U.S. government agencies [Member]
Marketable Securities [Line Items]
Estimated fair value, less than 12 months	60,491
Estimated fair value, 12 months or more
Estimated fair value, total	60,491
Unrealized losses, less than 12 months	711
Unrealized losses, 12 months or more
Unrealized losses, total	$ 711