Financial Instruments (Schedule Of Trading Securities Sold, Not Yet Purchased) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	$ 89,993	$ 85,183
U.S. Treasury securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	56,166	56,329
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	32,410	18,552
Municipal bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	21	20
Derivatives [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Trading securities sold, not yet purchased	$ 1,396	$ 10,282