Investments (Cost And Estimated Fair Value Of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 8,052	$ 8,052
Gross unrealized gain	2	1
Gross unrealized loss	(12)	(12)
Fair value, investments	8,042	8,041
Cash equivalents, Carrying amount	1,045	1,090	$ 7,850	$ 8,785
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	719	718
Gross unrealized gain	0	0
Gross unrealized loss	0	0
Fair value, investments	719	718
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	7,998	7,998
Gross unrealized gain	2	1
Gross unrealized loss	0	0
Fair value, investments	8,000	7,999
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	54	54
Gross unrealized gain	0	0
Gross unrealized loss	(12)	(12)
Fair value, investments	42	42
Debt and Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	8,771	8,770
Gross unrealized gain	2	1
Gross unrealized loss	(12)	(12)
Fair value, investments	8,761	8,759
Less Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gain	0	0
Gross unrealized loss	0	0
Cash equivalents, Carrying amount	719	718
Cash equivalents, Fair value	$ 719	$ 718