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Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 1,058,089 | $ 925,045
Available-for-sale securities | 3,377,981 | 3,181,313
Investments | 133,761 | 133,563
Cash equivalents measured at NAV | 15,420 | 35,637
Derivative contracts, Assets | 11,300 | 10,390
Total Assets | 4,596,551 | 4,285,948
Financial instruments sold, but not yet purchased, at fair value | 754,458 | 699,032
Derivative contracts, Liabilities | 1,191 | 1,823
Total Liabilities | 755,649 | 700,855
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 170,659 | 89,833
Available-for-sale securities | 4,419 | 4,197
Financial instruments sold, but not yet purchased, at fair value |  | 20,549
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 356,364 | 299,946
Available-for-sale securities | 824,192 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 260,410 | 202,968
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 39,791 | 32,044
Investments | 27,866 | 27,247
Financial instruments sold, but not yet purchased, at fair value | 45,308 | 18,395
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 149,242 | 159,095
Available-for-sale securities | 72,417 | 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 | 2,061,410 | 1,867,718
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 49,055 | 48,689
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 837 | 832
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,620 | 1,623
Investments In Funds Measured At NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 54,383 | 55,172
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 19,784 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 351,765 | 362,536
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 293,654 | 305,774
Available-for-sale securities | 341,054 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 96,975 | 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 | 28,595 | 28,402
Available-for-sale securities | 1,731 | 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 | 72,758 | 72,773
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 67,987 | 43,339
Available-for-sale securities | 199 | 300
Investments | 24,033 | 23,414
Total Assets | 92,219 | 67,053
Financial instruments sold, but not yet purchased, at fair value | 397,073 | 381,911
Total Liabilities | 397,073 | 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 | 8,995 | 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 | 39,208 | 31,444
Investments | 24,033 | 23,414
Financial instruments sold, but not yet purchased, at fair value | 45,308 | 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 | 19,784 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 351,765 | 362,536
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 988,233 | 879,751
Available-for-sale securities | 3,377,782 | 3,181,013
Investments | 380 | 383
Derivative contracts, Assets | 11,300 | 10,390
Total Assets | 4,377,695 | 4,071,537
Financial instruments sold, but not yet purchased, at fair value | 357,385 | 317,121
Derivative contracts, Liabilities | 1,191 | 1,823
Total Liabilities | 358,576 | 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 | 170,659 | 89,833
Available-for-sale securities | 4,220 | 3,897
Financial instruments sold, but not yet purchased, at fair value |  | 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 | 347,102 | 297,729
Available-for-sale securities | 824,192 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 260,410 | 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 | 149,242 | 159,095
Available-for-sale securities | 72,417 | 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 | 2,061,410 | 1,867,718
Level 2 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 380 | 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 | 293,654 | 305,774
Available-for-sale securities | 341,054 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 96,975 | 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 | 27,576 | 27,320
Available-for-sale securities | 1,731 | 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 | 72,758 | 72,773
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,869 | 1,955
Investments | 54,965 | 54,594
Total Assets | 56,834 | 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 | 267 | 273
Level 3 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 583 | 600
Investments | 3,833 | 3,833
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 49,055 | 48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 837 | 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 | $ 1,019 | $ 1,082

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