Marketable Securities (Schedule Of Schedule of Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
In Unrealized Loss Position For Less Than 12 Months	$ 8,156	$ 8,383
In Unrealized Loss Position For Greater Than 12 Months	7,120	6,766
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(53)	(40)
In Unrealized Loss Position For Greater Than 12 Months	(59)	(37)
U.S. government and agency obligations [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	5,185	5,593
In Unrealized Loss Position For Greater Than 12 Months	4,327	3,543
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(36)	(29)
In Unrealized Loss Position For Greater Than 12 Months	(39)	(17)
Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	628	478
In Unrealized Loss Position For Greater Than 12 Months	1,324	1,991
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(3)	(2)
In Unrealized Loss Position For Greater Than 12 Months	(8)	(12)
Asset backed securities [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	2,343	2,312
In Unrealized Loss Position For Greater Than 12 Months	1,469	1,232
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(14)	(9)
In Unrealized Loss Position For Greater Than 12 Months	$ (12)	$ (8)