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Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 1,143,684 | $ 925,045
Available-for-sale securities | 3,773,508 | 3,181,313
Investments | 111,379 | 133,563
Cash equivalents measured at NAV | 77,441 | 35,637
Derivative contracts, Assets | 7,995 | 10,390
Total Assets | 5,114,007 | 4,285,948
Financial instruments sold, but not yet purchased, at fair value | 778,863 | 699,032
Derivative contracts, Liabilities |  | 1,823
Total Liabilities |  | 700,855
Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 32,470 | 30,385
Financial instruments sold, but not yet purchased, at fair value | 20,236 | 27,667
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 147,223 | 89,833
Available-for-sale securities | 4,983 | 4,197
Financial instruments sold, but not yet purchased, at fair value | 10,348 | 20,549
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 329,179 | 269,561
Available-for-sale securities | 1,211,442 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 180,755 | 175,301
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 46,802 | 32,044
Investments | 49,978 | 27,247
Financial instruments sold, but not yet purchased, at fair value | 38,510 | 18,395
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 169,699 | 159,095
Available-for-sale securities | 70,559 | 72,490
Financial instruments sold, but not yet purchased, at fair value |  | 31
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 2,106,938 | 1,867,718
Investments In Funds Measured At NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 24,549 | 55,172
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 34,789 | 48,689
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 846 | 832
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,217 | 1,623
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 13,466 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 442,402 | 362,536
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 329,691 | 305,774
Available-for-sale securities | 305,530 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 86,612 | 94,552
Mortgage Backed Securities [Member] | Non-Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 75,154 | 28,402
Available-for-sale securities | 1,568 | 1,892
Financial instruments sold, but not yet purchased, at fair value |  | 1
Mortgage Backed Securities [Member] | Commercial [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 72,488 | 72,773
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 60,239 | 43,339
Available-for-sale securities | 516 | 300
Investments | 49,978 | 23,414
Total Assets | 110,733 | 67,053
Financial instruments sold, but not yet purchased, at fair value | 480,472 | 381,911
Total Liabilities |  | 381,911
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 | 300
Level 1 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 362 | 1,944
Financial instruments sold, but not yet purchased, at fair value |  | 980
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 46,411 | 31,444
Investments | 49,978 | 23,414
Financial instruments sold, but not yet purchased, at fair value | 38,070 | 18,395
Level 1 [Member] | U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 13,466 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 442,402 | 362,536
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,082,592 | 879,751
Available-for-sale securities | 3,772,992 | 3,181,013
Investments | 360 | 383
Derivative contracts, Assets | 7,995 | 10,390
Total Assets | 4,863,939 | 4,071,537
Financial instruments sold, but not yet purchased, at fair value | 298,391 | 317,121
Derivative contracts, Liabilities |  | 1,823
Total Liabilities |  | 318,944
Level 2 [Member] | Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 32,470 | 30,385
Financial instruments sold, but not yet purchased, at fair value | 20,236 | 27,667
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 | 147,223 | 89,833
Available-for-sale securities | 4,467 | 3,897
Financial instruments sold, but not yet purchased, at fair value | 10,348 | 20,549
Level 2 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 328,575 | 267,344
Available-for-sale securities | 1,211,442 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 180,755 | 174,321
Level 2 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 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 | 169,699 | 159,095
Available-for-sale securities | 70,559 | 72,490
Financial instruments sold, but not yet purchased, at fair value |  | 31
Level 2 [Member] | Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 2,106,938 | 1,867,718
Level 2 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 360 | 383
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 | 329,691 | 305,774
Available-for-sale securities | 305,530 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 86,612 | 94,552
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 | 74,796 | 27,320
Available-for-sale securities | 1,568 | 1,892
Financial instruments sold, but not yet purchased, at fair value |  | 1
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 | 72,488 | 72,773
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 853 | 1,955
Investments | 36,492 | 54,594
Total Assets | 37,345 | 56,549
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 | 273
Level 3 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 253 | 600
Investments |  | 3,833
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 34,789 | 48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 846 | 832
Level 3 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 857 | 1,240
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 | $ 358 | $ 1,082

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