Marketable Securities (Schedule Of Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value
In Unrealized Loss Position For Less Than 12 Months	$ 1,127	$ 2,821
In Unrealized Loss Position For Greater Than 12 Months	3,095	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	(1)	(63)
U.S. government and agency obligations [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	1,057	1,548
In Unrealized Loss Position For Greater Than 12 Months	2,282	4,608
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	(2)	(4)
In Unrealized Loss Position For Greater Than 12 Months	(1)	(39)
Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	20	250
In Unrealized Loss Position For Greater Than 12 Months	80	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	50	1,023
In Unrealized Loss Position For Greater Than 12 Months	733	2,137
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	0	(3)
In Unrealized Loss Position For Greater Than 12 Months	$ 0	$ (16)