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, 2019	Dec. 31, 2018
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	$ 1,352,359	$ 1,267,449
U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	130,141	72,532
Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	326,407	310,457
Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	82,968	57,911
State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	193,859	154,622
U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	56,478	42,121
Sovereign Debt [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	800	14,063
Securities Sold, But Not yet Purchased [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Financial instruments owned, at fair value	956,344	947,306
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	4,011	32,755
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	248,962	208,725
Securities Sold, But Not yet Purchased [Member] | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	66,312	36,117
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		7
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	389,489	534,817
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	246,028	123,456
Securities Sold, But Not yet Purchased [Member] | Sovereign Debt [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Sovereign debt	1,542	11,429
Securities Owned
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Asset-backed securities	5,220	25,905
Financial instruments owned, at fair value	1,352,359	1,267,449
Securities Owned | U.S. Government Agency Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	130,141	72,532
Securities Owned | Corporate Fixed Income Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fixed income securities	326,407	310,457
Securities Owned | Corporate Equity Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities	82,968	57,911
Securities Owned | State and Municipal Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
State and municipal securities	193,859	154,622
Securities Owned | U.S. Government Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	56,478	42,121
Securities Owned | Mortgage Backed Securities [Member] | Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	545,340	564,111
Securities Owned | Mortgage Backed Securities [Member] | Non-Agency [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Trading securities owned	11,146	25,727
Securities Owned | Sovereign Debt [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Sovereign debt	$ 800	$ 14,063