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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, 2016 | Dec. 31, 2015
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 925,045 | $ 749,443
Available-for-sale securities | 3,181,313 | 1,629,907
Investments | 133,563 | 181,017
Cash equivalents measured at NAV | 35,637 | 77,097
Derivative contracts, Assets | 10,390 |
Total Assets | 4,285,948 | 2,637,464
Financial instruments sold, but not yet purchased, at fair value | 699,032 | 521,744
Derivative contracts, Liabilities | 1,823 | 3,591
Total Liabilities | 700,855 | 525,335
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 89,833 | 116,949
Available-for-sale securities | 4,197 | 1,698
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 | 299,946 | 203,910
Available-for-sale securities | 823,511 | 319,408
Financial instruments sold, but not yet purchased, at fair value | 202,968 | 255,700
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 32,044 | 31,642
Investments | 27,247 | 30,737
Financial instruments sold, but not yet purchased, at fair value | 18,395 | 29,184
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 159,095 | 112,983
Available-for-sale securities | 72,490 | 74,167
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 | 1,867,718 | 915,913
Investments In Funds Measured At NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 55,172 | 90,256
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 48,689 | 55,710
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 832 | 1,315
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,623 | 2,897
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 9,951 | 45,167
Investments |  | 102
Financial instruments sold, but not yet purchased, at fair value | 362,536 | 186,030
Mortgage Backed Securities [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments sold, but not yet purchased, at fair value |  | 50,830
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 305,774 | 205,473
Available-for-sale securities | 338,732 | 304,893
Financial instruments sold, but not yet purchased, at fair value | 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 | 28,402 | 33,319
Available-for-sale securities | 1,892 | 2,518
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 | 72,773 | 11,310
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 43,339 | 87,758
Available-for-sale securities | 300 |
Investments | 23,414 | 26,542
Total Assets | 67,053 | 114,300
Financial instruments sold, but not yet purchased, at fair value | 381,911 | 212,525
Total Liabilities | 381,911 | 212,525
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 | 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 | 1,944 | 13,203
Financial instruments sold, but not yet purchased, at fair value | 980 | 3,601
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 31,444 | 29,388
Investments | 23,414 | 26,436
Financial instruments sold, but not yet purchased, at fair value | 18,395 | 22,894
Level 1 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments |  | 4
Level 1 [Member] | U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 9,951 | 45,167
Investments |  | 102
Financial instruments sold, but not yet purchased, at fair value | 362,536 | 186,030
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 879,751 | 659,590
Available-for-sale securities | 3,181,013 | 1,629,907
Investments | 383 | 9,500
Derivative contracts, Assets | 10,390 |
Total Assets | 4,071,537 | 2,298,997
Financial instruments sold, but not yet purchased, at fair value | 317,121 | 309,219
Derivative contracts, Liabilities | 1,823 | 3,591
Total Liabilities | 318,944 | 312,810
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 | 89,833 | 116,949
Available-for-sale securities | 3,897 | 1,698
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 | 297,729 | 190,707
Available-for-sale securities | 823,511 | 319,408
Financial instruments sold, but not yet purchased, at fair value | 201,988 | 252,099
Level 2 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned |  | 1,635
Investments |  | 1,359
Financial instruments sold, but not yet purchased, at fair value |  | 6,290
Level 2 [Member] | State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 159,095 | 112,983
Available-for-sale securities | 72,490 | 74,167
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 | 1,867,718 | 915,913
Level 2 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments |  | 5,268
Level 2 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 383 | 2,873
Level 2 [Member] | Mortgage Backed Securities [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments sold, but not yet purchased, at fair value |  | 50,830
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 | 305,774 | 205,473
Available-for-sale securities | 338,732 | 304,893
Financial instruments sold, but not yet purchased, at fair value | 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 | 27,320 | 31,843
Available-for-sale securities | 1,892 | 2,518
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 | 72,773 | 11,310
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,955 | 2,095
Investments | 54,594 | 54,719
Total Assets | 56,549 | 56,814
Level 3 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 273 |
Level 3 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 600 | 619
Investments | 3,833 | 2,942
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 48,689 | 50,442
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 832 | 1,315
Level 3 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,240 | 20
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 | $ 1,082 | $ 1,476

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