Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		$ 1,245,013	$ 694,028
Available-for-sale securities		2,280,157	2,230,297
Investments		110,165	113,862
Cash equivalents measured at NAV		8,493	34,192
Total Assets		3,643,828	3,072,379
Financial instruments sold, but not yet purchased, at fair value		848,910	437,978
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		19,998	46,900
Financial instruments sold, but not yet purchased, at fair value		370,149	91,974
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		171,470	56,450
Available-for-sale securities		3,180	4,361
Financial instruments sold, but not yet purchased, at fair value		60,502
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		25,780
Available-for-sale securities		370,789	698,133
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		377,349	194,575
Available-for-sale securities		809,569	631,758
Financial instruments sold, but not yet purchased, at fair value		175,477	162,626
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		102,567	67,593
Investments		26,137	29,496
Financial instruments sold, but not yet purchased, at fair value		40,931	30,848
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		106,967	96,150
Available-for-sale securities		2,430	2,453
Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		22,839
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		51,340	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		19,729	12,385
Agency Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		389,828
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		1,011,876	793,410
Financial instruments sold, but not yet purchased, at fair value		188,946	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		80,561	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		1,752	4,569
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		26,375
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		1,840	15,926	[1]
Financial instruments sold, but not yet purchased, at fair value	[2]	12,905	11,303
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		121,563	114,353
Investments		27,950	39,838
Total Assets		149,513	154,191
Financial instruments sold, but not yet purchased, at fair value		410,258	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		19,998	46,900
Financial instruments sold, but not yet purchased, at fair value		370,149	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		700	4,474
Financial instruments sold, but not yet purchased, at fair value		137	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		100,865	62,979
Investments		17,382	29,496
Financial instruments sold, but not yet purchased, at fair value		39,972	30,848
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		10,568	10,342
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		1,098,796	568,391
Available-for-sale securities		2,280,157	2,230,297
Investments		6,107	6,593
Total Assets		3,385,060	2,805,281
Financial instruments sold, but not yet purchased, at fair value		438,652	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		171,470	56,450
Available-for-sale securities		3,180	4,361
Financial instruments sold, but not yet purchased, at fair value		60,502
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		23,965
Available-for-sale securities		370,789	698,133
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		376,649	190,101
Available-for-sale securities		809,569	631,758
Financial instruments sold, but not yet purchased, at fair value		175,340	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		1,702	4,614
Investments		1
Financial instruments sold, but not yet purchased, at fair value		959
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		106,967	96,150
Available-for-sale securities		2,430	2,453
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		6,106	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		389,828
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		1,011,876	793,410
Financial instruments sold, but not yet purchased, at fair value		188,946	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		80,561	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		1,752	4,569
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		26,375
Level 2 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		1,840	4,642	[1]
Financial instruments sold, but not yet purchased, at fair value	[2]	12,905	11,303
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		24,654	11,284
Investments		56,379	55,046
Total Assets		81,033	66,330
Level 3 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		1,815
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		8,754
Level 3 [Member] | Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		22,839
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		$ 34,666	42,113
Level 3 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	[1]		$ 11,284

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

[2] Includes sovereign debt and state and municipal securities.