Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 819,715	$ 972,932
Available-for-sale securities	2,824,258	3,254,737
Investments	108,412	79,972
Cash equivalents measured at NAV	106,207	25,734
Derivative contracts, Assets		1,086
Total Assets	3,858,592	4,334,461
Financial instruments sold, but not yet purchased, at fair value	599,934	662,852
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3,069	12,403
Financial instruments sold, but not yet purchased, at fair value	18,393	13,271
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	117,805	66,881
Available-for-sale securities	4,630	5,067
Financial instruments sold, but not yet purchased, at fair value	6,265	1,000
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	44	28,385
Available-for-sale securities	1,424,840	1,592,889
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	219,734	250,783
Available-for-sale securities	605,555	675,311
Financial instruments sold, but not yet purchased, at fair value	177,744	140,100
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	71,387	64,009
Investments	24,925	35,083
Financial instruments sold, but not yet purchased, at fair value	42,017	32,047
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	143,201	137,211
Available-for-sale securities	2,459	24,297
Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,501	9,983
Financial instruments sold, but not yet purchased, at fair value		955
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	12,615	14,243
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	183	654
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	59,778	16,771
Investments in Funds and Partnerships Measured at NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	10,911	13,221
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	42,444	9,266
Financial instruments sold, but not yet purchased, at fair value	220,510	243,570
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	215,333	388,856
Available-for-sale securities	682,174	837,878
Financial instruments sold, but not yet purchased, at fair value	135,005	231,909
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,197	5,155
Available-for-sale securities	6,779	9,758
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	97,821	109,537
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	110,704	71,717
Investments	33,614	43,928
Total Assets	144,318	115,645
Financial instruments sold, but not yet purchased, at fair value	264,022	276,250
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,167	872
Financial instruments sold, but not yet purchased, at fair value	1,495	633
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	67,093	61,579
Investments	23,865	34,023
Financial instruments sold, but not yet purchased, at fair value	42,017	32,047
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	9,749	9,905
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	42,444	9,266
Financial instruments sold, but not yet purchased, at fair value	220,510	243,570
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	703,501	891,889
Available-for-sale securities	2,824,258	3,254,737
Investments	15,028	6,483
Derivative contracts, Assets		1,086
Total Assets	3,542,787	4,154,195
Financial instruments sold, but not yet purchased, at fair value	335,912	385,647
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3,069	12,403
Financial instruments sold, but not yet purchased, at fair value	18,393	13,271
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	117,805	66,881
Available-for-sale securities	4,630	5,067
Financial instruments sold, but not yet purchased, at fair value	6,265	1,000
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	35	28,210
Available-for-sale securities	1,424,840	1,592,889
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	218,567	249,911
Available-for-sale securities	605,555	675,311
Financial instruments sold, but not yet purchased, at fair value	176,249	139,467
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	4,294	2,430
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	143,201	137,211
Available-for-sale securities	2,459	24,297
Level 2 [Member] | Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		832
Level 2 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		470
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	15,028	6,013
Level 2 [Member] | Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	215,333	388,856
Available-for-sale securities	682,174	837,878
Financial instruments sold, but not yet purchased, at fair value	135,005	231,909
Level 2 [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,197	5,155
Available-for-sale securities	6,779	9,758
Level 2 [Member] | Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	97,821	109,537
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,510	9,326
Investments	48,859	16,340
Total Assets	54,369	25,666
Financial instruments sold, but not yet purchased, at fair value		955
Level 3 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9	175
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,060	1,060
Level 3 [Member] | Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,501	9,151
Financial instruments sold, but not yet purchased, at fair value		955
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	12,615	14,243
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	183	184
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 35,001	$ 853