Marketable Securities (Schedule Of Marketable Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Marketable Securities - Short-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	$ 6,688	$ 6,490
Unrealized Gains	0	5
Unrealized Losses	(18)	(2)
Fair Value	6,670	6,493
Marketable Securities - Short-Term [Member] | US Treasury and Government [Member]
Gain (Loss) on Investments [Line Items]
Cost	4,381	5,005
Unrealized Gains	0	4
Unrealized Losses	(13)	(1)
Fair Value	4,368	5,008
Marketable Securities - Short-Term [Member] | Corporate Debt Securities And Certificates Of Deposit [Member]
Gain (Loss) on Investments [Line Items]
Cost	1,792	1,476
Unrealized Gains	0	1
Unrealized Losses	(4)	(1)
Fair Value	1,788	1,476
Marketable Securities - Short-Term [Member] | Asset-backed Securities [Member]
Gain (Loss) on Investments [Line Items]
Cost	515	9
Unrealized Gains	0	0
Unrealized Losses	(1)	0
Fair Value	514	9
Marketable Securities - Long-Term [Member]
Gain (Loss) on Investments [Line Items]
Cost	9,077	8,750
Unrealized Gains	55	12
Unrealized Losses	(59)	(34)
Fair Value	9,073	8,728
Marketable Securities - Long-Term [Member] | US Treasury and Government [Member]
Gain (Loss) on Investments [Line Items]
Cost	4,801	4,815
Unrealized Gains	0	1
Unrealized Losses	(33)	(12)
Fair Value	4,768	4,804
Marketable Securities - Long-Term [Member] | Corporate Debt Securities And Certificates Of Deposit [Member]
Gain (Loss) on Investments [Line Items]
Cost	1,189	2,161
Unrealized Gains	0	0
Unrealized Losses	(10)	(17)
Fair Value	1,179	2,144
Marketable Securities - Long-Term [Member] | Asset-backed Securities [Member]
Gain (Loss) on Investments [Line Items]
Cost	3,045	1,732
Unrealized Gains	0	0
Unrealized Losses	(16)	(5)
Fair Value	3,029	1,727
Marketable Securities - Long-Term [Member] | Equity Security [Member]
Gain (Loss) on Investments [Line Items]
Cost	42	42
Unrealized Gains	55	11
Unrealized Losses	0	0
Fair Value	$ 97	$ 53