Marketable Securities (Schedule Of Schedule of Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Marketable Securities - Short-Term [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	$ 8,383
In Unrealized Loss Position For Greater Than 12 Months	6,766
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(40)
In Unrealized Loss Position For Greater Than 12 Months	(37)
Marketable Securities - Short-Term [Member] | US Treasury and Government [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	5,593
In Unrealized Loss Position For Greater Than 12 Months	3,543
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(29)
In Unrealized Loss Position For Greater Than 12 Months	(17)
Marketable Securities - Short-Term [Member] | Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	478
In Unrealized Loss Position For Greater Than 12 Months	1,991
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(2)
In Unrealized Loss Position For Greater Than 12 Months	(12)
Marketable Securities - Short-Term [Member] | Asset-backed Securities [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	2,312
In Unrealized Loss Position For Greater Than 12 Months	1,232
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(9)
In Unrealized Loss Position For Greater Than 12 Months	$ (8)
Marketable Securities - Long-Term [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months		$ 8,747
In Unrealized Loss Position For Greater Than 12 Months		0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months		(36)
In Unrealized Loss Position For Greater Than 12 Months		0
Marketable Securities - Long-Term [Member] | US Treasury and Government [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months		4,481
In Unrealized Loss Position For Greater Than 12 Months		0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months		(13)
In Unrealized Loss Position For Greater Than 12 Months		0
Marketable Securities - Long-Term [Member] | Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months		2,795
In Unrealized Loss Position For Greater Than 12 Months		0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months		(18)
In Unrealized Loss Position For Greater Than 12 Months		0
Marketable Securities - Long-Term [Member] | Asset-backed Securities [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months		1,471
In Unrealized Loss Position For Greater Than 12 Months		0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months		(5)
In Unrealized Loss Position For Greater Than 12 Months		$ 0