Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		$ 981,282	$ 694,028
Available-for-sale securities		2,190,128	2,230,297
Investments		119,545	113,862
Cash equivalents measured at NAV		22,590	34,192
Total Assets		3,313,545	3,072,379
Financial instruments sold, but not yet purchased, at fair value		644,762	437,978
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		61,918	46,900
Financial instruments sold, but not yet purchased, at fair value		287,820	91,974
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		166,501	56,450
Available-for-sale securities		3,645	4,361
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		223,545	194,575
Available-for-sale securities		711,559	631,758
Financial instruments sold, but not yet purchased, at fair value		185,728	162,626
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		82,402	67,593
Investments		35,518	29,496
Financial instruments sold, but not yet purchased, at fair value		34,443	30,848
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		162,083	96,150
Available-for-sale securities		2,428	2,453
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		377,708	698,133
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		58,515	59,048
Investments in Funds and Partnerships Measured at NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		12,553	12,385
Agency Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		254,203
Agency Mortgage-Backed Securities [Member] | Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned			216,434
Available-for-sale securities		1,003,691	793,410
Financial instruments sold, but not yet purchased, at fair value		121,648	141,227
Agency Mortgage-Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		87,439	95,613
Agency Mortgage-Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		3,658	4,569
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	[1]	30,630	15,926
Financial instruments sold, but not yet purchased, at fair value	[2]	15,123	11,303
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		133,852	114,353
Investments		43,866	39,838
Total Assets		177,718	154,191
Financial instruments sold, but not yet purchased, at fair value		322,943	126,916
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		61,918	46,900
Financial instruments sold, but not yet purchased, at fair value		287,820	91,974
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		220	4,474
Financial instruments sold, but not yet purchased, at fair value		680	4,094
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		71,714	62,979
Investments		33,517	29,496
Financial instruments sold, but not yet purchased, at fair value		34,443	30,848
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		10,349	10,342
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		833,153	568,391
Available-for-sale securities		2,190,128	2,230,297
Investments		6,339	6,593
Total Assets		3,029,620	2,805,281
Financial instruments sold, but not yet purchased, at fair value		321,819	311,062
Level 2 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		166,501	56,450
Available-for-sale securities		3,645	4,361
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		223,325	190,101
Available-for-sale securities		711,559	631,758
Financial instruments sold, but not yet purchased, at fair value		185,048	158,532
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		10,688	4,614
Investments		1
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		162,083	96,150
Available-for-sale securities		2,428	2,453
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		377,708	698,133
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		6,338	6,593
Level 2 [Member] | Agency Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		254,203
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned			216,434
Available-for-sale securities		1,003,691	793,410
Financial instruments sold, but not yet purchased, at fair value		121,648	141,227
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		87,439	95,613
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		3,658	4,569
Level 2 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	[1]	16,353	4,642
Financial instruments sold, but not yet purchased, at fair value	[2]	15,123	11,303
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		14,277	11,284
Investments		56,787	55,046
Total Assets		71,064	66,330
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		2,000
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		41,828	42,113
Level 3 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	[1]	$ 14,277	$ 11,284

[1] Includes loans, asset-backed securities, sovereign debt, and non-agency mortgage-backed securities.

[2] Includes U.S. government agency securities, sovereign debt, and state and municipal securities.