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, 2016	Dec. 31, 2015
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	$ 1,148,785	$ 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	226,632	116,949
Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	271,785	203,910
Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	27,995	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	271,722	112,983
U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	19,616	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	295,666	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	35,369	33,319
Securities Owned [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	1,148,785	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	19,616	45,167
Securities Owned [Member] | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	271,785	203,910
Securities Owned [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	27,995	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	271,722	112,983
Securities Owned [Member] | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	226,632	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	295,666	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	35,369	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	665,757	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	319,219	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	207,080	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	22,905	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	47
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	$ 116,506	$ 50,830