Investments In Securities Investments in Securities (Schedule of Carrying Value and Fair Value of Financial Assets Disclosed) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	$ 22,174	$ 20,214
Accrued interest and dividends	1,359	1,100
Common stocks	47,252	47,367
Commercial paper, money market funds and certificates of deposit	21,144	23,780
Equity investments in unconsolidated affiliates, equity method	6,134	6,593
Equity investments in unconsolidated affiliates, measurement alternative	5,467	5,439	$ 4,744
Assets, Fair Value Disclosure	103,530	104,493
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	22,174	20,214
Accrued interest and dividends	1,359	1,100
Common stocks	47,252	47,367
Commercial paper, money market funds and certificates of deposit	21,144	23,780
Equity investments in unconsolidated affiliates, equity method	0	0
Equity investments in unconsolidated affiliates, measurement alternative	0	0
Assets, Fair Value Disclosure	91,929	92,461
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	0	0
Accrued interest and dividends	0	0
Common stocks	0	0
Commercial paper, money market funds 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
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	0	0
Accrued interest and dividends	0	0
Common stocks	0	0
Commercial paper, money market funds and certificates of deposit	0	0
Equity investments in unconsolidated affiliates, equity method	6,134	6,593
Equity investments in unconsolidated affiliates, measurement alternative	5,467	5,439
Assets, Fair Value Disclosure	$ 11,601	$ 12,032