Marketable Securities (Schedule Of Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value
In Unrealized Loss Position For Less Than 12 Months	$ 0	$ 2,821
In Unrealized Loss Position For Greater Than 12 Months	5,176	7,923
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	0	(7)
In Unrealized Loss Position For Greater Than 12 Months	(11)	(63)
U.S. government and agency obligations [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	0	1,548
In Unrealized Loss Position For Greater Than 12 Months	3,286	4,608
Gross Unrealized Losses
In Unrealized Loss Position For Less Than 12 Months	0	(4)
In Unrealized Loss Position For Greater Than 12 Months	(7)	(39)
Corporate Debt Securities And Certificates Of Deposit [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	0	250
In Unrealized Loss Position For Greater Than 12 Months	534	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	(1)	(8)
Asset backed securities [Member]
Fair Value
In Unrealized Loss Position For Less Than 12 Months	0	1,023
In Unrealized Loss Position For Greater Than 12 Months	1,356	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	$ (3)	$ (16)