Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,297,491	$ 1,267,449
Available-for-sale securities	3,003,767	3,070,447
Investments	70,225	67,982
Cash equivalents measured at NAV	20,691	19,719
Derivative contracts, Assets	4,896	7,683
Total Assets	4,397,070	4,433,280
Financial instruments sold, but not yet purchased, at fair value	979,116	947,306
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,351	14,063
Financial instruments sold, but not yet purchased, at fair value	1,599	11,429
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	117,701	72,532
Available-for-sale securities	5,368	5,215
Financial instruments sold, but not yet purchased, at fair value	49	32,755
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	56,514	25,905
Available-for-sale securities	1,713,751	1,764,060
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	336,317	310,457
Available-for-sale securities	947,588	931,604
Financial instruments sold, but not yet purchased, at fair value	195,642	208,725
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	54,779	57,911
Investments	37,264	33,046
Financial instruments sold, but not yet purchased, at fair value	53,182	36,117
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	179,626	154,622
Available-for-sale securities	49,118	68,226
Financial instruments sold, but not yet purchased, at fair value		7
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,632	16,632
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	704	704
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,028	1,041
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,597	16,559
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,326	42,121
Financial instruments sold, but not yet purchased, at fair value	548,357	534,817
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	514,718	564,111
Available-for-sale securities	215,417	230,408
Financial instruments sold, but not yet purchased, at fair value	180,287	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	8,159	25,727
Available-for-sale securities	1,148	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	71,377	69,715
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	83,983	100,346
Available-for-sale securities	418	417
Investments	36,077	31,670
Total Assets	120,478	132,433
Financial instruments sold, but not yet purchased, at fair value	601,808	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	418	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	1,946	1,100
Financial instruments sold, but not yet purchased, at fair value	269	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	54,711	57,125
Investments	36,077	31,670
Financial instruments sold, but not yet purchased, at fair value	53,182	35,398
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,326	42,121
Financial instruments sold, but not yet purchased, at fair value	548,357	534,817
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,213,333	1,166,927
Available-for-sale securities	3,003,349	3,070,030
Investments	1,362	1,560
Derivative contracts, Assets	4,896	7,683
Total Assets	4,222,940	4,246,200
Financial instruments sold, but not yet purchased, at fair value	377,308	375,802
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,351	14,063
Financial instruments sold, but not yet purchased, at fair value	1,599	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	117,701	72,532
Available-for-sale securities	4,950	4,798
Financial instruments sold, but not yet purchased, at fair value	49	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	56,339	25,730
Available-for-sale securities	1,713,751	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	334,371	309,357
Available-for-sale securities	947,588	931,604
Financial instruments sold, but not yet purchased, at fair value	195,373	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	68	786
Investments	1,187	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	179,626	154,622
Available-for-sale securities	49,118	68,226
Financial instruments sold, but not yet purchased, at fair value		7
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	175	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	514,718	564,111
Available-for-sale securities	215,417	230,408
Financial instruments sold, but not yet purchased, at fair value	180,287	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	8,159	25,726
Available-for-sale securities	1,148	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	71,377	69,715
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	175	176
Investments	18,189	18,193
Total Assets	18,364	18,369
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] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,632	16,632
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	704	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