Marketable Securities (Schedule Of Marketable Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Short-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	$ 8,268	$ 5,797
Unrealized Gains	27	3
Unrealized Losses	0	(29)
Fair Value	8,295	5,771
Long-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	12,105	10,332
Unrealized Gains	65	31
Unrealized Losses	(2)	(41)
Fair Value	12,168	10,322
U.S. Government And Agency Obligations [Member] | Short-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	5,766	3,377
Unrealized Gains	22	0
Unrealized Losses	0	(20)
Fair Value	5,788	3,357
U.S. Government And Agency Obligations [Member] | Long-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	6,524	6,114
Unrealized Gains	30	10
Unrealized Losses	(1)	(23)
Fair Value	6,553	6,101
Corporate Debt Securities And Certificates Of Deposit [Member] | Short-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	1,085	1,787
Unrealized Gains	1	3
Unrealized Losses	0	(5)
Fair Value	1,086	1,785
Corporate Debt Securities And Certificates Of Deposit [Member] | Long-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	3,004	754
Unrealized Gains	14	1
Unrealized Losses	0	(3)
Fair Value	3,018	752
Asset Backed Securities [Member] | Short-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	1,417	633
Unrealized Gains	4	0
Unrealized Losses	0	(4)
Fair Value	1,421	629
Asset Backed Securities [Member] | Long-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	2,535	3,422
Unrealized Gains	15	2
Unrealized Losses	(1)	(15)
Fair Value	2,549	3,409
Equity Security [Member] | Long-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	42	42
Unrealized Gains	6	18
Unrealized Losses	0	0
Fair Value	$ 48	$ 60