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, 2017	Dec. 31, 2016
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	$ 1,063,651	$ 925,045
U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	119,703	89,833
Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	367,866	299,946
Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	43,671	32,044
State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	147,458	159,095
U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	16,924	9,951
Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	54,566	28,402
Mortgage Backed Securities [Member] | Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	313,463	305,774
Securities Sold, But Not yet Purchased [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	705,577	699,032
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	1,995	20,549
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	245,148	202,968
Securities Sold, But Not yet Purchased [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	41,091	18,395
Securities Sold, But Not yet Purchased [Member] | State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
State and municipal securities	37	31
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	298,832	362,536
Securities Sold, But Not yet Purchased [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	16	1
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	118,458	94,552
Securities Owned
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	1,063,651	925,045
Securities Owned | U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	119,703	89,833
Securities Owned | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	367,866	299,946
Securities Owned | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	43,671	32,044
Securities Owned | State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
State and municipal securities	147,458	159,095
Securities Owned | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	16,924	9,951
Securities Owned | Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	54,566	28,402
Securities Owned | Mortgage Backed Securities [Member] | Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	$ 313,463	$ 305,774