Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 972,932	$ 1,267,449
Available-for-sale securities	3,254,737	3,070,447
Investments	79,972	67,982
Cash equivalents measured at NAV	25,734	19,719
Derivative contracts, Assets	1,086	7,683
Total Assets	4,334,461	4,433,280
Financial instruments sold, but not yet purchased, at fair value	662,852	947,306
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12,403	14,063
Financial instruments sold, but not yet purchased, at fair value	13,271	11,429
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	66,881	72,532
Available-for-sale securities	5,067	5,215
Financial instruments sold, but not yet purchased, at fair value	1,000	32,755
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	28,385	25,905
Available-for-sale securities	1,592,889	1,764,060
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	250,783	310,457
Available-for-sale securities	675,311	931,604
Financial instruments sold, but not yet purchased, at fair value	140,100	208,725
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	64,009	57,911
Investments	35,083	33,046
Financial instruments sold, but not yet purchased, at fair value	32,047	36,117
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	137,211	154,622
Available-for-sale securities	24,297	68,226
Financial instruments sold, but not yet purchased, at fair value		7
Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	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	16,632
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	654	704
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,771	1,041
Investments in Funds and Partnerships Measured at NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	13,221	16,559
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9,266	42,121
Financial instruments sold, but not yet purchased, at fair value	243,570	534,817
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	388,856	564,111
Available-for-sale securities	837,878	230,408
Financial instruments sold, but not yet purchased, at fair value	231,909	123,456
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,155	25,727
Available-for-sale securities	9,758	1,219
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	109,537	69,715
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	71,717	100,346
Available-for-sale securities		417
Investments	43,928	31,670
Total Assets	115,645	132,433
Financial instruments sold, but not yet purchased, at fair value	276,250	571,504
Level 1 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		417
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	872	1,100
Financial instruments sold, but not yet purchased, at fair value	633	1,289
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	61,579	57,125
Investments	34,023	31,670
Financial instruments sold, but not yet purchased, at fair value	32,047	35,398
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	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	9,266	42,121
Financial instruments sold, but not yet purchased, at fair value	243,570	534,817
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	891,889	1,166,927
Available-for-sale securities	3,254,737	3,070,030
Investments	6,483	1,560
Derivative contracts, Assets	1,086	7,683
Total Assets	4,154,195	4,246,200
Financial instruments sold, but not yet purchased, at fair value	385,647	375,802
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12,403	14,063
Financial instruments sold, but not yet purchased, at fair value	13,271	11,429
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	66,881	72,532
Available-for-sale securities	5,067	4,798
Financial instruments sold, but not yet purchased, at fair value	1,000	32,755
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	28,210	25,730
Available-for-sale securities	1,592,889	1,764,060
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	249,911	309,357
Available-for-sale securities	675,311	931,604
Financial instruments sold, but not yet purchased, at fair value	139,467	207,436
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,430	786
Investments		1,376
Financial instruments sold, but not yet purchased, at fair value		719
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	137,211	154,622
Available-for-sale securities	24,297	68,226
Financial instruments sold, but not yet purchased, at fair value		7
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	6,013	184
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	388,856	564,111
Available-for-sale securities	837,878	230,408
Financial instruments sold, but not yet purchased, at fair value	231,909	123,456
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	5,155	25,726
Available-for-sale securities	9,758	1,219
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	109,537	69,715
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9,326	176
Investments	16,340	18,193
Total Assets	25,666	18,369
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
Level 3 [Member] | Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	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	16,632
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	184	704
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 853	857
Level 3 [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