Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,086,446	$ 749,443
Available-for-sale securities	2,466,706	1,629,907
Investments	156,198	181,017
Cash equivalents measured at NAV	9,822	77,097
Total Assets	3,719,172	2,637,464
Financial instruments sold, but not yet purchased, at fair value	615,662	521,744
Derivative contracts, Liabilities	17,707	3,591
Total Liabilities	633,369	525,335
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	160,079	87,758
Available-for-sale securities	101
Investments	22,041	26,542
Total Assets	182,221	114,300
Financial instruments sold, but not yet purchased, at fair value	332,667	212,525
Total Liabilities	332,667	212,525
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	924,221	659,590
Available-for-sale securities	2,466,605	1,629,907
Investments	1,716	9,500
Total Assets	3,392,542	2,298,997
Financial instruments sold, but not yet purchased, at fair value	282,995	309,219
Derivative contracts, Liabilities	17,707	3,591
Total Liabilities	300,702	312,810
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,146	2,095
Investments	57,874	54,719
Total Assets	60,020	56,814
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	210,849	116,949
Available-for-sale securities	2,688	1,698
U.S. Government Agency Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,008
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	209,841	116,949
Available-for-sale securities	2,587	1,698
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	278,643	203,910
Available-for-sale securities	652,093	319,408
Financial instruments sold, but not yet purchased, at fair value	210,586	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	22,986	13,203
Financial instruments sold, but not yet purchased, at fair value	2,901	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	255,365	190,707
Available-for-sale securities	652,093	319,408
Financial instruments sold, but not yet purchased, at fair value	207,685	252,099
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	292
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	125,446	31,642
Investments	27,898	30,737
Financial instruments sold, but not yet purchased, at fair value	58,429	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	124,827	29,388
Investments	22,041	26,436
Financial instruments sold, but not yet purchased, at fair value	58,429	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		1,635
Investments	1,328	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	4,529	2,942
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	222,275	112,983
Available-for-sale securities	74,712	74,167
Financial instruments sold, but not yet purchased, at fair value	21
State And Municipal Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	222,275	112,983
Available-for-sale securities	74,712	74,167
Financial instruments sold, but not yet purchased, at fair value	21
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,336,967	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	1,336,967	915,913
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,750	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	50,750	50,442
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,355	1,315
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,355	1,315
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,628	2,897
Other Investment [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		4
Other Investment [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	388	2,873
Other Investment [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,240	20
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	74,567	90,256
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	11,258	45,167
Investments		102
Financial instruments sold, but not yet purchased, at fair value	271,337	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	11,258	45,167
Investments		102
Financial instruments sold, but not yet purchased, at fair value	271,337	186,030
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	209,021	205,473
Available-for-sale securities	395,271	304,893
Financial instruments sold, but not yet purchased, at fair value	75,289	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	209,021	205,473
Available-for-sale securities	395,271	304,893
Financial instruments sold, but not yet purchased, at fair value	75,289	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	28,954	33,319
Available-for-sale securities	2,188	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	27,719	31,843
Available-for-sale securities	2,188	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	1,235	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	2,787	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	$ 2,787	$ 11,310