Marketable Securities (Schedule Of Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value
In Unrealized Loss Position For Less Than 12 Months	$ 5,212	$ 741
In Unrealized Loss Position For Greater Than 12 Months	0	0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(7)	(1)
In Unrealized Loss Position For Greater Than 12 Months	0	0
U.S. government and agency obligations [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	3,637	330
In Unrealized Loss Position For Greater Than 12 Months	0	0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(5)	0
In Unrealized Loss Position For Greater Than 12 Months	0	0
Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	1,455	411
In Unrealized Loss Position For Greater Than 12 Months	0	0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(2)	(1)
In Unrealized Loss Position For Greater Than 12 Months	0	$ 0
Asset backed securities [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	120
In Unrealized Loss Position For Greater Than 12 Months	0
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	0
In Unrealized Loss Position For Greater Than 12 Months	$ 0