Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 932,511	$ 972,932
Available-for-sale securities	3,366,090	3,254,737
Investments	55,688	79,972
Cash equivalents measured at NAV	26,567	25,734
Derivative contracts, Assets		1,086
Total Assets	4,380,856	4,334,461
Financial instruments sold, but not yet purchased, at fair value	668,795	662,852
Derivative contracts, Liabilities	1,262
Total Liabilities	670,057
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	21,057	12,403
Financial instruments sold, but not yet purchased, at fair value	18,178	13,271
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	80,138	66,881
Available-for-sale securities	5,650	5,067
Financial instruments sold, but not yet purchased, at fair value	16,320	1,000
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12,942	28,385
Available-for-sale securities	1,669,445	1,592,889
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	275,663	250,783
Available-for-sale securities	727,590	675,311
Financial instruments sold, but not yet purchased, at fair value	192,603	140,100
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	69,259	64,009
Investments	14,898	35,083
Financial instruments sold, but not yet purchased, at fair value	22,776	32,047
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	75,740	137,211
Available-for-sale securities	17,142	24,297
Financial instruments sold, but not yet purchased, at fair value	200
Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	10,259	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	14,243	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	16,119	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	10,245	13,221
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12,821	9,266
Financial instruments sold, but not yet purchased, at fair value	239,311	243,570
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	371,762	388,856
Available-for-sale securities	829,433	837,878
Financial instruments sold, but not yet purchased, at fair value	179,407	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	2,870	5,155
Available-for-sale securities	8,644	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	108,186	109,537
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	84,874	71,717
Investments	23,200	43,928
Total Assets	108,074	115,645
Financial instruments sold, but not yet purchased, at fair value	265,969	276,250
Total Liabilities	265,969
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	7,850	872
Financial instruments sold, but not yet purchased, at fair value	3,882	633
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	64,203	61,579
Investments	13,838	34,023
Financial instruments sold, but not yet purchased, at fair value	22,776	32,047
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	9,362	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	12,821	9,266
Financial instruments sold, but not yet purchased, at fair value	239,311	243,570
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	837,203	891,889
Available-for-sale securities	3,366,090	3,254,737
Investments	5,924	6,483
Derivative contracts, Assets		1,086
Total Assets	4,209,217	4,154,195
Financial instruments sold, but not yet purchased, at fair value	402,826	385,647
Derivative contracts, Liabilities	1,262
Total Liabilities	404,088
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	21,057	12,403
Financial instruments sold, but not yet purchased, at fair value	18,178	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	80,138	66,881
Available-for-sale securities	5,650	5,067
Financial instruments sold, but not yet purchased, at fair value	16,320	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	12,767	28,210
Available-for-sale securities	1,669,445	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	267,813	249,911
Available-for-sale securities	727,590	675,311
Financial instruments sold, but not yet purchased, at fair value	188,721	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	5,056	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	75,740	137,211
Available-for-sale securities	17,142	24,297
Financial instruments sold, but not yet purchased, at fair value	200
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	5,924	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	371,762	388,856
Available-for-sale securities	829,433	837,878
Financial instruments sold, but not yet purchased, at fair value	179,407	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	2,870	5,155
Available-for-sale securities	8,644	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	108,186	109,537
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	10,434	9,326
Investments	16,319	16,340
Total Assets	26,753	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	175	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	10,259	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	14,243	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	$ 833	$ 853