Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,277,423	$ 1,143,684
Available-for-sale securities	3,475,436	3,773,508
Investments	94,482	111,379
Cash equivalents measured at NAV	17,883	77,441
Derivative contracts, Assets	12,038	7,995
Total Assets	4,877,262	5,114,007
Financial instruments sold, but not yet purchased, at fair value	944,681	778,863
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	48,088	32,470
Financial instruments sold, but not yet purchased, at fair value	18,584	20,236
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	138,884	147,223
Available-for-sale securities	5,049	4,983
Financial instruments sold, but not yet purchased, at fair value	30,623	10,348
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	114,350	76,752
Available-for-sale securities	1,798,181	2,106,938
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	359,652	325,471
Available-for-sale securities	1,270,616	1,211,442
Financial instruments sold, but not yet purchased, at fair value	243,333	180,755
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	54,554	46,802
Investments	47,648	49,978
Financial instruments sold, but not yet purchased, at fair value	63,916	38,510
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	158,352	169,699
Available-for-sale securities	69,461	70,559
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	25,303	34,789
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	846	846
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,056	1,217
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	19,629	24,549
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,943	13,466
Financial instruments sold, but not yet purchased, at fair value	444,941	442,402
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	362,939	302,445
Available-for-sale securities	261,243	305,530
Financial instruments sold, but not yet purchased, at fair value	143,284	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	20,661	29,356
Available-for-sale securities	1,406	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	69,480	72,488
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	75,112	60,239
Available-for-sale securities	515	516
Investments	47,648	49,978
Total Assets	123,275	110,733
Financial instruments sold, but not yet purchased, at fair value	509,319	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	904	362
Financial instruments sold, but not yet purchased, at fair value	462
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	54,265	46,411
Investments	47,648	49,978
Financial instruments sold, but not yet purchased, at fair value	63,916	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	19,943	13,466
Financial instruments sold, but not yet purchased, at fair value	444,941	442,402
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,201,832	1,082,592
Available-for-sale securities	3,474,921	3,772,992
Investments	199	360
Derivative contracts, Assets	12,038	7,995
Total Assets	4,688,990	4,863,939
Financial instruments sold, but not yet purchased, at fair value	435,362	298,391
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	48,088	32,470
Financial instruments sold, but not yet purchased, at fair value	18,584	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	138,884	147,223
Available-for-sale securities	4,534	4,467
Financial instruments sold, but not yet purchased, at fair value	30,623	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	113,995	76,395
Available-for-sale securities	1,798,181	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	358,748	324,867
Available-for-sale securities	1,270,616	1,211,442
Financial instruments sold, but not yet purchased, at fair value	242,871	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	166	138
Financial instruments sold, but not yet purchased, at fair value		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	158,352	169,699
Available-for-sale securities	69,461	70,559
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	199	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	362,939	302,445
Available-for-sale securities	261,243	305,530
Financial instruments sold, but not yet purchased, at fair value	143,284	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	20,660	29,355
Available-for-sale securities	1,406	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	69,480	72,488
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	479	853
Investments	27,006	36,492
Total Assets	27,485	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	355	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		242
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	123	253
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	25,303	34,789
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	846	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