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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, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 1,065,503 | $ 925,045
Available-for-sale securities | 3,694,298 | 3,181,313
Investments | 112,649 | 133,563
Cash equivalents measured at NAV | 16,528 | 35,637
Derivative contracts, Assets | 5,669 | 10,390
Total Assets | 4,894,647 | 4,285,948
Financial instruments sold, but not yet purchased, at fair value | 749,323 | 699,032
Derivative contracts, Liabilities |  | 1,823
Total Liabilities |  | 700,855
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 128,014 | 89,833
Available-for-sale securities | 4,686 | 4,197
Financial instruments sold, but not yet purchased, at fair value | 22,140 | 20,549
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 409,550 | 299,946
Available-for-sale securities | 1,072,376 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 311,516 | 202,968
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 38,280 | 32,044
Investments | 51,475 | 27,247
Financial instruments sold, but not yet purchased, at fair value | 52,261 | 18,395
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 145,226 | 159,095
Available-for-sale securities | 71,961 | 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 | 2,159,035 | 1,867,718
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 34,608 | 48,689
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 840 | 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 | 24,116 | 55,172
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 48,900 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 297,690 | 362,536
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 258,348 | 305,774
Available-for-sale securities | 311,888 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 65,716 | 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 | 37,185 | 28,402
Available-for-sale securities | 1,619 | 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 | 72,733 | 72,773
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 95,412 | 43,339
Available-for-sale securities | 199 | 300
Investments | 51,235 | 23,414
Total Assets | 146,846 | 67,053
Financial instruments sold, but not yet purchased, at fair value | 383,744 | 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 | 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 | 8,499 | 1,944
Financial instruments sold, but not yet purchased, at fair value | 33,793 | 980
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 38,013 | 31,444
Investments | 51,235 | 23,414
Financial instruments sold, but not yet purchased, at fair value | 52,261 | 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 | 48,900 | 9,951
Financial instruments sold, but not yet purchased, at fair value | 297,690 | 362,536
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 969,218 | 879,751
Available-for-sale securities | 3,694,099 | 3,181,013
Investments | 370 | 383
Derivative contracts, Assets | 5,669 | 10,390
Total Assets | 4,669,356 | 4,071,537
Financial instruments sold, but not yet purchased, at fair value | 365,579 | 317,121
Derivative contracts, Liabilities |  | 1,823
Total Liabilities |  | 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 | 128,014 | 89,833
Available-for-sale securities | 4,487 | 3,897
Financial instruments sold, but not yet purchased, at fair value | 22,140 | 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 | 400,803 | 297,729
Available-for-sale securities | 1,072,376 | 823,511
Financial instruments sold, but not yet purchased, at fair value | 277,723 | 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 | 145,226 | 159,095
Available-for-sale securities | 71,961 | 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 | 2,159,035 | 1,867,718
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 | 258,348 | 305,774
Available-for-sale securities | 311,888 | 338,732
Financial instruments sold, but not yet purchased, at fair value | 65,716 | 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 | 36,827 | 27,320
Available-for-sale securities | 1,619 | 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 | 72,733 | 72,773
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 873 | 1,955
Investments | 36,928 | 54,594
Total Assets | 37,801 | 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 | 248 | 273
Level 3 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 267 | 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 | 34,608 | 48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 840 | 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 | $ 358 | $ 1,082

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