Financial Instruments Owned And Financial Instruments Sold, But Not Yet Purchased (Components Of Trading Securities Owned And Trading Securities Sold, But Not Yet Purchased) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	$ 819,715	$ 972,932
U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	117,805	66,881
Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	219,734	250,783
Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	71,387	64,009
State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	143,201	137,211
Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	5,501	9,983
U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	42,444	9,266
Sovereign Debt [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	3,069	12,403
Securities Sold, But Not yet Purchased [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	599,934	662,852
Securities Sold, But Not yet Purchased [Member] | U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	6,265	1,000
Securities Sold, But Not yet Purchased [Member] | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	177,744	140,100
Securities Sold, But Not yet Purchased [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	42,017	32,047
Securities Sold, But Not yet Purchased [Member] | Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities		955
Securities Sold, But Not yet Purchased [Member] | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	220,510	243,570
Securities Sold, But Not yet Purchased [Member] | Mortgage Backed Securities [Member] | Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	135,005	231,909
Securities Sold, But Not yet Purchased [Member] | Sovereign Debt [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Sovereign debt	18,393	13,271
Securities Owned [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Asset-backed securities	44	28,385
Financial instruments owned, at fair value	819,715	972,932
Securities Owned [Member] | U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	117,805	66,881
Securities Owned [Member] | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	219,734	250,783
Securities Owned [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	71,387	64,009
Securities Owned [Member] | State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
State and municipal securities	143,201	137,211
Securities Owned [Member] | Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	5,501	9,983
Securities Owned [Member] | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	42,444	9,266
Securities Owned [Member] | Mortgage Backed Securities [Member] | Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	215,333	388,856
Securities Owned [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	1,197	5,155
Securities Owned [Member] | Sovereign Debt [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Sovereign debt	$ 3,069	$ 12,403