Marketable Securities (Schedule Of Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value
In Unrealized Loss Position For Less Than 12 Months	$ 1,104	$ 2,821
In Unrealized Loss Position For Greater Than 12 Months	0	7,923
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(2)	(7)
In Unrealized Loss Position For Greater Than 12 Months	0	(63)
U.S. Government And Agency Obligations [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	149	1,548
In Unrealized Loss Position For Greater Than 12 Months	0	4,608
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(1)	(4)
In Unrealized Loss Position For Greater Than 12 Months	0	(39)
Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	271	250
In Unrealized Loss Position For Greater Than 12 Months	0	1,178
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	0	0
In Unrealized Loss Position For Greater Than 12 Months	0	(8)
Asset Backed Securities [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	684	1,023
In Unrealized Loss Position For Greater Than 12 Months	0	2,137
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(1)	(3)
In Unrealized Loss Position For Greater Than 12 Months	$ 0	$ (16)