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	Jun. 30, 2016	Dec. 31, 2015
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	$ 1,086,446	$ 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	210,849	116,949
Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	278,643	203,910
Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	125,446	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	222,275	112,983
U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	11,258	45,167
Mortgage Backed Securities [Member] | Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	209,021	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	28,954	33,319
Securities Owned [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	1,086,446	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	11,258	45,167
Securities Owned [Member] | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	278,643	203,910
Securities Owned [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	125,446	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	222,275	112,983
Securities Owned [Member] | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	210,849	116,949
Securities Owned [Member] | Mortgage Backed Securities [Member] | Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	209,021	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	28,954	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	615,662	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	271,337	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	210,586	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	58,429	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	21
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	$ 75,289	$ 50,830