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	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	$ 1,028,781	$ 749,443
U S Government Corporations And Agencies Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	71,900	116,949
U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	38,165	45,167
Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	340,946	203,910
Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	75,837	31,642
U S States And Political Subdivisions [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	201,103	112,983
Mortgage Backed Securities [Member] | Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	267,056	205,473
Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	33,774	33,319
Securities Owned [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	1,028,781	749,443
Securities Owned [Member] | U S Government Corporations And Agencies Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	38,165	45,167
Securities Owned [Member] | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	71,900	116,949
Securities Owned [Member] | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	340,946	203,910
Securities Owned [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	75,837	31,642
Securities Owned [Member] | U S States And Political Subdivisions [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
State and municipal securities	201,103	112,983
Securities Owned [Member] | Mortgage Backed Securities [Member] | Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	267,056	205,473
Securities Owned [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	33,774	33,319
Securities Sold, But Not yet Purchased [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	674,841	521,744
Securities Sold, But Not yet Purchased [Member] | U S Government Corporations And Agencies Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	257,214	186,030
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	302,961	255,700
Securities Sold, But Not yet Purchased [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	27,999	29,184
Securities Sold, But Not yet Purchased [Member] | U S States And Political Subdivisions [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
State and municipal securities	43
Securities Sold, But Not yet Purchased [Member] | Mortgage Backed Securities [Member] | Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	$ 86,624	$ 50,830