Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,181,047	$ 1,143,684
Available-for-sale securities	3,712,801	3,773,508
Investments	98,920	111,379
Cash equivalents measured at NAV	17,397	77,441
Derivative contracts, Assets	11,606	7,995
Total Assets	5,021,771	5,114,007
Financial instruments sold, but not yet purchased, at fair value	959,535	778,863
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,341	32,470
Financial instruments sold, but not yet purchased, at fair value	23,575	20,236
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	111,921	147,223
Available-for-sale securities	5,026	4,983
Financial instruments sold, but not yet purchased, at fair value	20,618	10,348
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	85,987	76,752
Available-for-sale securities	2,001,222	2,106,938
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	364,917	325,471
Available-for-sale securities	1,281,095	1,211,442
Financial instruments sold, but not yet purchased, at fair value	223,865	180,755
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	39,090	46,802
Investments	47,287	49,978
Financial instruments sold, but not yet purchased, at fair value	20,235	38,510
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	147,876	169,699
Available-for-sale securities	69,865	70,559
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	25,287	34,789
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	847	846
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,218	1,217
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	24,281	24,549
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	21,087	13,466
Financial instruments sold, but not yet purchased, at fair value	527,073	442,402
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	361,747	302,445
Available-for-sale securities	283,703	305,530
Financial instruments sold, but not yet purchased, at fair value	144,169	86,612
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	29,081	29,356
Available-for-sale securities	1,441	1,568
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	70,449	72,488
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	60,365	60,239
Available-for-sale securities	515	516
Investments	47,287	49,978
Total Assets	108,167	110,733
Financial instruments sold, but not yet purchased, at fair value	547,292	480,472
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	515	516
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	616	362
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	38,662	46,411
Investments	47,287	49,978
Financial instruments sold, but not yet purchased, at fair value	20,219	38,070
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	21,087	13,466
Financial instruments sold, but not yet purchased, at fair value	527,073	442,402
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,119,775	1,082,592
Available-for-sale securities	3,712,286	3,772,992
Investments	361	360
Derivative contracts, Assets	11,606	7,995
Total Assets	4,844,028	4,863,939
Financial instruments sold, but not yet purchased, at fair value	412,243	298,391
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,341	32,470
Financial instruments sold, but not yet purchased, at fair value	23,575	20,236
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	111,921	147,223
Available-for-sale securities	4,511	4,467
Financial instruments sold, but not yet purchased, at fair value	20,618	10,348
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	85,630	76,395
Available-for-sale securities	2,001,222	2,106,938
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	364,072	324,867
Available-for-sale securities	1,281,095	1,211,442
Financial instruments sold, but not yet purchased, at fair value	223,865	180,755
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	108	138
Financial instruments sold, but not yet purchased, at fair value	16	440
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	147,876	169,699
Available-for-sale securities	69,865	70,559
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	361	360
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	361,747	302,445
Available-for-sale securities	283,703	305,530
Financial instruments sold, but not yet purchased, at fair value	144,169	86,612
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	29,080	29,355
Available-for-sale securities	1,441	1,568
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	70,449	72,488
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	907	853
Investments	26,991	36,492
Total Assets	27,898	37,345
Level 3 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	357	357
Level 3 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	229	242
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	320	253
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	25,287	34,789
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	847	846
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	857	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	$ 1