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Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 1,162,732 | $ 1,143,684
Available-for-sale securities | 3,349,607 | 3,773,508
Investments | 79,170 | 111,379
Cash equivalents measured at NAV | 18,696 | 77,441
Derivative contracts, Assets | 11,117 | 7,995
Total Assets | 4,621,322 | 5,114,007
Financial instruments sold, but not yet purchased, at fair value | 911,374 | 778,863
Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 15,996 | 32,470
Financial instruments sold, but not yet purchased, at fair value | 11,557 | 20,236
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 167,886 | 147,223
Available-for-sale securities | 5,165 | 4,983
Financial instruments sold, but not yet purchased, at fair value |  | 10,348
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 29,806 | 76,752
Available-for-sale securities | 1,777,351 | 2,106,938
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 290,627 | 325,471
Available-for-sale securities | 1,177,315 | 1,211,442
Financial instruments sold, but not yet purchased, at fair value | 213,335 | 180,755
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 69,205 | 46,802
Investments | 42,495 | 49,978
Financial instruments sold, but not yet purchased, at fair value | 78,895 | 38,510
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 149,207 | 169,699
Available-for-sale securities | 68,876 | 70,559
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 16,525 | 34,789
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 799 | 846
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,046 | 1,217
Investments In Funds Measured At NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 18,305 | 24,549
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 36,214 | 13,466
Financial instruments sold, but not yet purchased, at fair value | 512,275 | 442,402
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 380,044 | 302,445
Available-for-sale securities | 250,808 | 305,530
Financial instruments sold, but not yet purchased, at fair value | 95,312 | 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 | 23,747 | 29,356
Available-for-sale securities | 1,269 | 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 | 68,823 | 72,488
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 105,368 | 60,239
Available-for-sale securities | 516 | 516
Investments | 42,495 | 49,978
Total Assets | 148,379 | 110,733
Financial instruments sold, but not yet purchased, at fair value | 591,266 | 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 | 516 | 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 | 262 | 362
Financial instruments sold, but not yet purchased, at fair value | 96 |
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 68,892 | 46,411
Investments | 42,495 | 49,978
Financial instruments sold, but not yet purchased, at fair value | 78,895 | 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 | 36,214 | 13,466
Financial instruments sold, but not yet purchased, at fair value | 512,275 | 442,402
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,056,907 | 1,082,592
Available-for-sale securities | 3,349,091 | 3,772,992
Investments | 189 | 360
Derivative contracts, Assets | 11,117 | 7,995
Total Assets | 4,417,304 | 4,863,939
Financial instruments sold, but not yet purchased, at fair value | 320,108 | 298,391
Level 2 [Member] | Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 15,996 | 32,470
Financial instruments sold, but not yet purchased, at fair value | 11,557 | 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 | 167,886 | 147,223
Available-for-sale securities | 4,649 | 4,467
Financial instruments sold, but not yet purchased, at fair value |  | 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 | 29,451 | 76,395
Available-for-sale securities | 1,777,351 | 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 | 290,365 | 324,867
Available-for-sale securities | 1,177,315 | 1,211,442
Financial instruments sold, but not yet purchased, at fair value | 213,239 | 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 | 212 | 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 | 149,207 | 169,699
Available-for-sale securities | 68,876 | 70,559
Level 2 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 189 | 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 | 380,044 | 302,445
Available-for-sale securities | 250,808 | 305,530
Financial instruments sold, but not yet purchased, at fair value | 95,312 | 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 | 23,746 | 29,355
Available-for-sale securities | 1,269 | 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 | 68,823 | 72,488
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 457 | 853
Investments | 18,181 | 36,492
Total Assets | 18,638 | 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 | 101 | 253
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 16,525 | 34,789
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 799 | 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

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