Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,028,781	$ 749,443
Available-for-sale securities	2,125,466	1,629,907
Investments	165,424	181,017
Cash equivalents measured at NAV	10,377	77,097
Total Assets	3,330,048	2,637,464
Financial instruments sold, but not yet purchased, at fair value	674,841	521,744
Derivative contracts, Liabilities	12,192	3,591
Total Liabilities	687,033	525,335
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	135,700	87,758
Available-for-sale securities	101
Investments	22,719	26,542
Total Assets	158,520	114,300
Financial instruments sold, but not yet purchased, at fair value	292,527	212,525
Total Liabilities	292,527	212,525
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	891,029	659,590
Available-for-sale securities	2,125,365	1,629,907
Investments	4,214	9,500
Total Assets	3,020,608	2,298,997
Financial instruments sold, but not yet purchased, at fair value	382,314	309,219
Derivative contracts, Liabilities	12,192	3,591
Total Liabilities	394,506	312,810
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,052	2,095
Investments	55,969	54,719
Total Assets	58,021	56,814
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	38,165	45,167
Investments		102
Financial instruments sold, but not yet purchased, at fair value	257,214	186,030
U.S. Government Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	38,165	45,167
Investments		102
Financial instruments sold, but not yet purchased, at fair value	257,214	186,030
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	71,900	116,949
Available-for-sale securities	1,780	1,698
U.S. Government Agency Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	101
U.S. Government Agency Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	71,900	116,949
Available-for-sale securities	1,679	1,698
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	340,946	203,910
Available-for-sale securities	541,209	319,408
Financial instruments sold, but not yet purchased, at fair value	302,961	255,700
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	24,873	13,203
Financial instruments sold, but not yet purchased, at fair value	7,314	3,601
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	316,073	190,707
Available-for-sale securities	541,209	319,408
Financial instruments sold, but not yet purchased, at fair value	295,647	252,099
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	75,837	31,642
Investments	27,035	30,737
Financial instruments sold, but not yet purchased, at fair value	27,999	29,184
Corporate Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	72,662	29,388
Investments	22,714	26,436
Financial instruments sold, but not yet purchased, at fair value	27,999	22,894
Corporate Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,556	1,635
Investments	1,342	1,359
Financial instruments sold, but not yet purchased, at fair value		6,290
Corporate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	619	619
Investments	2,979	2,942
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	201,103	112,983
Available-for-sale securities	74,105	74,167
Financial instruments sold, but not yet purchased, at fair value	43
State And Municipal Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	201,103	112,983
Available-for-sale securities	74,105	74,167
Financial instruments sold, but not yet purchased, at fair value	43
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,085,893	915,913
Asset-Backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,085,893	915,913
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,864	55,710
Auction Rate Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		5,268
Auction Rate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,864	50,442
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,351	1,315
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,351	1,315
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	3,652	2,897
Other Investment [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	5	4
Other Investment [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	2,872	2,873
Other Investment [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	775	20
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	82,522	90,256
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	267,056	205,473
Available-for-sale securities	400,631	304,893
Financial instruments sold, but not yet purchased, at fair value	86,624	50,830
Mortgage Backed Securities [Member] | Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	267,056	205,473
Available-for-sale securities	400,631	304,893
Financial instruments sold, but not yet purchased, at fair value	86,624	50,830
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	33,774	33,319
Available-for-sale securities	2,283	2,518
Mortgage Backed Securities [Member] | Non-Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	32,341	31,843
Available-for-sale securities	2,283	2,518
Mortgage Backed Securities [Member] | Non-Agency [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,433	1,476
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	19,565	11,310
Mortgage Backed Securities [Member] | Commercial [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 19,565	$ 11,310