Financial Instruments (Schedule Of Trading Securities Sold, Not Yet Purchased) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	$ 81,220	$ 91,887
U.S. government agency MBS and CMOs [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	17
U.S. Treasury securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	42,015	62,798
Corporate bonds and redeemable preferred stock [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	34,462	28,445
Municipal bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	20	37
Derivatives [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	$ 4,706	$ 607