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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
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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

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