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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, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 1,063,651 | $ 925,045
Available-for-sale securities | 3,455,373 | 3,181,313
Investments | 128,332 | 133,563
Cash equivalents measured at NAV | 16,365 | 35,637
Derivative contracts, Assets | 8,442 | 10,390
Total Assets | 4,672,163 | 4,285,948
Financial instruments sold, but not yet purchased, at fair value | 705,577 | 699,032
Derivative contracts, Liabilities | 939 | 1,823
Total Liabilities | 706,516 | 700,855
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 119,703 | 89,833
Available-for-sale securities | 4,565 | 4,197
Financial instruments sold, but not yet purchased, at fair value | 1,995 | 20,549
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 367,866 | 299,946
Available-for-sale securities | 971,131 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 245,148 | 202,968
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 43,671 | 32,044
Investments | 50,384 | 27,247
Financial instruments sold, but not yet purchased, at fair value | 41,091 | 18,395
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 147,458 | 159,095
Available-for-sale securities | 72,382 | 72,490
Financial instruments sold, but not yet purchased, at fair value | 37 | 31
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 2,013,928 | 1,867,718
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 48,577 | 48,689
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 841 | 832
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,610 | 1,623
Investments In Funds Measured At NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 26,920 | 55,172
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 16,924 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 298,832 | 362,536
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 313,463 | 305,774
Available-for-sale securities | 319,062 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 118,458 | 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 | 54,566 | 28,402
Available-for-sale securities | 1,617 | 1,892
Financial instruments sold, but not yet purchased, at fair value | 16 | 1
Mortgage Backed Securities [Member] | Commercial [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 72,688 | 72,773
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 67,671 | 43,339
Available-for-sale securities | 199 | 300
Investments | 50,144 | 23,414
Total Assets | 118,014 | 67,053
Financial instruments sold, but not yet purchased, at fair value | 340,240 | 381,911
Total Liabilities | 340,240 | 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 | 199 | 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 | 7,443 | 1,944
Financial instruments sold, but not yet purchased, at fair value | 317 | 980
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 43,304 | 31,444
Investments | 50,144 | 23,414
Financial instruments sold, but not yet purchased, at fair value | 41,091 | 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 | 16,924 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 298,832 | 362,536
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 994,832 | 879,751
Available-for-sale securities | 3,455,174 | 3,181,013
Investments | 14,326 | 383
Derivative contracts, Assets | 8,442 | 10,390
Total Assets | 4,472,774 | 4,071,537
Financial instruments sold, but not yet purchased, at fair value | 365,337 | 317,121
Derivative contracts, Liabilities | 939 | 1,823
Total Liabilities | 366,276 | 318,944
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 | 119,703 | 89,833
Available-for-sale securities | 4,366 | 3,897
Financial instruments sold, but not yet purchased, at fair value | 1,995 | 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 | 360,167 | 297,729
Available-for-sale securities | 971,131 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 244,831 | 201,988
Level 2 [Member] | State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 147,458 | 159,095
Available-for-sale securities | 72,382 | 72,490
Financial instruments sold, but not yet purchased, at fair value | 37 | 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 | 2,013,928 | 1,867,718
Level 2 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 13,956 |
Level 2 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 370 | 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 | 313,463 | 305,774
Available-for-sale securities | 319,062 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 118,458 | 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 | 54,041 | 27,320
Available-for-sale securities | 1,617 | 1,892
Financial instruments sold, but not yet purchased, at fair value | 16 | 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 | 72,688 | 72,773
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,148 | 1,955
Investments | 36,942 | 54,594
Total Assets | 38,090 | 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 | 256 | 273
Level 3 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 367 | 600
Investments | 240 | 3,833
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 34,621 | 48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 841 | 832
Level 3 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,240 | 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 | $ 525 | $ 1,082

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