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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, 2021 | Dec. 31, 2020
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | $ 1,199,386 | $ 694,028 |
Available-for-sale securities |  | 2,242,465 | 2,230,297 |
Investments |  | 120,919 | 113,862 |
Cash equivalents measured at NAV |  | 1,639 | 34,192 |
Total Assets |  | 3,564,409 | 3,072,379 |
Financial instruments sold, but not yet purchased, at fair value |  | 721,028 | 437,978 |
U.S. Government Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 16,829 | 46,900 |
Financial instruments sold, but not yet purchased, at fair value |  | 366,014 | 91,974 |
U.S. Government Agency Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 182,636 | 56,450 |
Available-for-sale securities |  | 2,268 | 4,361 |
Financial instruments sold, but not yet purchased, at fair value |  | 44,638 |  |
Asset-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 74,783 |  |
Available-for-sale securities |  | 323,932 | 698,133 |
Corporate Fixed Income Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 318,288 | 194,575 |
Available-for-sale securities |  | 820,839 | 631,758 |
Financial instruments sold, but not yet purchased, at fair value |  | 161,529 | 162,626 |
Corporate Equity Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 91,874 | 67,593 |
Investments |  | 25,198 | 29,496 |
Financial instruments sold, but not yet purchased, at fair value |  | 30,813 | 30,848 |
State And Municipal Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 162,821 | 96,150 |
Available-for-sale securities |  | 2,414 | 2,453 |
Loans [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 18,854 |  |
Other Investment [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 60,745 | 59,048 |
Investments in Funds and Partnerships Measured at NAV [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 22,064 | 12,385 |
Agency Mortgage-Backed Securities [Member] | Agency Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  |  | 216,434 |
Available-for-sale securities |  | 1,016,730 | 793,410 |
Financial instruments sold, but not yet purchased, at fair value |  | 109,887 | 141,227 |
Agency Mortgage-Backed Securities [Member] | Commercial [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale securities |  | 75,701 | 95,613 |
Agency Mortgage-Backed Securities [Member] | Non-Agency [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale securities |  | 581 | 4,569 |
Agency Securities [Member] | Agency Mortgage-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 303,684 |  |
Non-Agency [Member] | Agency Mortgage-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 19,697 |  |
Sovereign Debt [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 9,920 |  |
Other [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned | [1] |  | 15,926 |
Financial instruments sold, but not yet purchased, at fair value |  | 8,147 | 11,303 | [2]
Level 1 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 107,727 | 114,353 |
Investments |  | 26,676 | 39,838 |
Total Assets |  | 134,403 | 154,191 |
Financial instruments sold, but not yet purchased, at fair value |  | 396,827 | 126,916 |
Level 1 [Member] | U.S. Government Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 16,829 | 46,900 |
Financial instruments sold, but not yet purchased, at fair value |  | 366,014 | 91,974 |
Level 1 [Member] | Corporate Fixed Income Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 250 | 4,474 |
Financial instruments sold, but not yet purchased, at fair value |  |  | 4,094 |
Level 1 [Member] | Corporate Equity Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 90,648 | 62,979 |
Investments |  | 16,443 | 29,496 |
Financial instruments sold, but not yet purchased, at fair value |  | 30,813 | 30,848 |
Level 1 [Member] | Other Investment [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 10,233 | 10,342 |
Level 2 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 1,003,871 | 568,391 |
Available-for-sale securities |  | 2,242,465 | 2,230,297 |
Investments |  | 15,548 | 6,593 |
Total Assets |  | 3,261,884 | 2,805,281 |
Financial instruments sold, but not yet purchased, at fair value |  | 324,201 | 311,062 |
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 |  | 182,636 | 56,450 |
Available-for-sale securities |  | 2,268 | 4,361 |
Financial instruments sold, but not yet purchased, at fair value |  | 44,638 |  |
Level 2 [Member] | Asset-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 5,849 |  |
Available-for-sale securities |  | 323,932 | 698,133 |
Level 2 [Member] | Corporate Fixed Income Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 318,038 | 190,101 |
Available-for-sale securities |  | 820,839 | 631,758 |
Financial instruments sold, but not yet purchased, at fair value |  | 161,529 | 158,532 |
Level 2 [Member] | Corporate Equity Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 1,226 | 4,614 |
Investments |  | 1,001 |  |
Level 2 [Member] | State And Municipal Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 162,821 | 96,150 |
Available-for-sale securities |  | 2,414 | 2,453 |
Level 2 [Member] | Other Investment [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 14,547 | 6,593 |
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Agency Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  |  | 216,434 |
Available-for-sale securities |  | 1,016,730 | 793,410 |
Financial instruments sold, but not yet purchased, at fair value |  | 109,887 | 141,227 |
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Commercial [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale securities |  | 75,701 | 95,613 |
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Non-Agency [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale securities |  | 581 | 4,569 |
Level 2 [Member] | Agency Securities [Member] | Agency Mortgage-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 303,684 |  |
Level 2 [Member] | Non-Agency [Member] | Agency Mortgage-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 19,697 |  |
Level 2 [Member] | Sovereign Debt [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 9,920 |  |
Level 2 [Member] | Other [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned | [1] |  | 4,642 |
Financial instruments sold, but not yet purchased, at fair value |  | 8,147 | 11,303 | [2]
Level 3 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 87,788 | 11,284 |
Investments |  | 56,631 | 55,046 |
Total Assets |  | 144,419 | 66,330 |
Level 3 [Member] | Asset-Backed Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 68,934 |  |
Level 3 [Member] | Corporate Equity Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 7,754 |  |
Level 3 [Member] | Loans [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned |  | 18,854 |  |
Level 3 [Member] | Other Investment [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | $ 35,965 | 42,113 |
Level 3 [Member] | Other [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Financial instruments owned | [1] |  | $ 11,284 |
[1] | Includes loans, asset-backed securities, sovereign debt, and non-agency mortgage-backed securities.
[2] | Includes sovereign debt and state and municipal securities.

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