Trading Securities Owned And Trading Securities Sold, But Not Yet Purchased (Components Of Trading Securities Owned And Trading Securities Sold, But Not Yet Purchased) (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Trading securities	$ 750,947,000	$ 474,951,000
Trading securities pledged	565,800,000	392,400,000
Securities Owned [Member]
Trading securities	750,947,000	474,951,000
Securities Owned [Member] | U.S. Government Agency Securities [Member]
Trading securities owned	80,673,000	66,424,000
Securities Owned [Member] | U.S. Government Securities [Member]
Trading securities owned	31,879,000	32,845,000
Securities Owned [Member] | Corporate Fixed Income Securities [Member]
Fixed income securities	432,086,000	244,535,000
Securities Owned [Member] | Corporate Equity Securities [Member]
Equity securities	43,997,000	19,859,000
Securities Owned [Member] | State And Municipal Securities [Member]
State and municipal securities	162,312,000	111,288,000
Securities Sold, But Not yet Purchased [Member]
Trading securities	411,680,000	266,833,000
Securities Sold, But Not yet Purchased [Member] | U.S. Government Agency Securities [Member]
Trading securities owned	6,658,000	954,000
Securities Sold, But Not yet Purchased [Member] | U.S. Government Securities [Member]
Trading securities owned	208,786,000	109,776,000
Securities Sold, But Not yet Purchased [Member] | Corporate Fixed Income Securities [Member]
Fixed income securities	177,976,000	149,460,000
Securities Sold, But Not yet Purchased [Member] | Corporate Equity Securities [Member]
Equity securities	18,090,000	6,060,000
Securities Sold, But Not yet Purchased [Member] | State And Municipal Securities [Member]
State and municipal securities	$ 170,000	$ 583,000