Investments in Securities and Fair Value Investments in Securities (Schedule of Carrying Value and Fair Value of Financial Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial Assets
Cash	$ 13,723	$ 25,949
Accrued interest and dividends	1,038	1,033
Equity securities, at fair value	64,919	61,108
Money market funds, Treasury bills, commercial paper and certificates of deposit	15,170	13,134
Equity investments in unconsolidated affiliates, equity method	6,752	6,083
Equity investments in unconsolidated affiliates, measurement alternative	8,741	7,899	$ 6,589
Assets, Fair Value Disclosure	110,343	115,206
Level 1
Financial Assets
Cash	13,723	25,949
Accrued interest and dividends	1,038	1,033
Equity securities, at fair value	64,919	61,108
Money market funds, Treasury bills, commercial paper and certificates of deposit	15,170	13,134
Equity investments in unconsolidated affiliates, equity method	0	0
Equity investments in unconsolidated affiliates, measurement alternative	0	0
Assets, Fair Value Disclosure	94,850	101,224
Level 2
Financial Assets
Cash	0	0
Accrued interest and dividends	0	0
Equity securities, at fair value	0	0
Money market funds, Treasury bills, commercial paper and certificates of deposit	0	0
Equity investments in unconsolidated affiliates, equity method	0	0
Equity investments in unconsolidated affiliates, measurement alternative	0	0
Assets, Fair Value Disclosure	0	0
Level 3
Financial Assets
Cash	0	0
Accrued interest and dividends	0	0
Equity securities, at fair value	0	0
Money market funds, Treasury bills, commercial paper and certificates of deposit	0	0
Equity investments in unconsolidated affiliates, equity method	6,752	6,083
Equity investments in unconsolidated affiliates, measurement alternative	8,741	7,899
Assets, Fair Value Disclosure	$ 15,493	$ 13,982