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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, 2021 | Dec. 31, 2020
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | $ 981,282 | $ 694,028
Available-for-sale securities |  | 2,190,128 | 2,230,297
Investments |  | 119,545 | 113,862
Cash equivalents measured at NAV |  | 22,590 | 34,192
Total Assets |  | 3,313,545 | 3,072,379
Financial instruments sold, but not yet purchased, at fair value |  | 644,762 | 437,978
U.S. Government Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 61,918 | 46,900
Financial instruments sold, but not yet purchased, at fair value |  | 287,820 | 91,974
U.S. Government Agency Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 166,501 | 56,450
Available-for-sale securities |  | 3,645 | 4,361
Corporate Fixed Income Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 223,545 | 194,575
Available-for-sale securities |  | 711,559 | 631,758
Financial instruments sold, but not yet purchased, at fair value |  | 185,728 | 162,626
Corporate Equity Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 82,402 | 67,593
Investments |  | 35,518 | 29,496
Financial instruments sold, but not yet purchased, at fair value |  | 34,443 | 30,848
State And Municipal Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 162,083 | 96,150
Available-for-sale securities |  | 2,428 | 2,453
Asset-Backed Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Available-for-sale securities |  | 377,708 | 698,133
Other Investment [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments |  | 58,515 | 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 |  | 12,553 | 12,385
Agency Mortgage-Backed Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 254,203 |
Agency Mortgage-Backed Securities [Member] | Agency Securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  |  | 216,434
Available-for-sale securities |  | 1,003,691 | 793,410
Financial instruments sold, but not yet purchased, at fair value |  | 121,648 | 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 |  | 87,439 | 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 |  | 3,658 | 4,569
Other [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned | [1] | 30,630 | 15,926
Financial instruments sold, but not yet purchased, at fair value | [2] | 15,123 | 11,303
Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 133,852 | 114,353
Investments |  | 43,866 | 39,838
Total Assets |  | 177,718 | 154,191
Financial instruments sold, but not yet purchased, at fair value |  | 322,943 | 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 |  | 61,918 | 46,900
Financial instruments sold, but not yet purchased, at fair value |  | 287,820 | 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 |  | 220 | 4,474
Financial instruments sold, but not yet purchased, at fair value |  | 680 | 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 |  | 71,714 | 62,979
Investments |  | 33,517 | 29,496
Financial instruments sold, but not yet purchased, at fair value |  | 34,443 | 30,848
Level 1 [Member] | Other Investment [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments |  | 10,349 | 10,342
Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 833,153 | 568,391
Available-for-sale securities |  | 2,190,128 | 2,230,297
Investments |  | 6,339 | 6,593
Total Assets |  | 3,029,620 | 2,805,281
Financial instruments sold, but not yet purchased, at fair value |  | 321,819 | 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 |  | 166,501 | 56,450
Available-for-sale securities |  | 3,645 | 4,361
Level 2 [Member] | Corporate Fixed Income Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 223,325 | 190,101
Available-for-sale securities |  | 711,559 | 631,758
Financial instruments sold, but not yet purchased, at fair value |  | 185,048 | 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 |  | 10,688 | 4,614
Investments |  | 1 |
Level 2 [Member] | State And Municipal Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 162,083 | 96,150
Available-for-sale securities |  | 2,428 | 2,453
Level 2 [Member] | Asset-Backed Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Available-for-sale securities |  | 377,708 | 698,133
Level 2 [Member] | Other Investment [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments |  | 6,338 | 6,593
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 254,203 |
Level 2 [Member] | Agency Mortgage-Backed Securities [Member] | Agency Securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  |  | 216,434
Available-for-sale securities |  | 1,003,691 | 793,410
Financial instruments sold, but not yet purchased, at fair value |  | 121,648 | 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 |  | 87,439 | 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 |  | 3,658 | 4,569
Level 2 [Member] | Other [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned | [1] | 16,353 | 4,642
Financial instruments sold, but not yet purchased, at fair value | [2] | 15,123 | 11,303
Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned |  | 14,277 | 11,284
Investments |  | 56,787 | 55,046
Total Assets |  | 71,064 | 66,330
Level 3 [Member] | Corporate Equity Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments |  | 2,000 |
Level 3 [Member] | Other Investment [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments |  | 41,828 | 42,113
Level 3 [Member] | Other [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Financial instruments owned | [1] | $ 14,277 | $ 11,284
[1] | Includes loans, asset-backed securities, sovereign debt, and non-agency mortgage-backed securities.
[2] | Includes U.S. government agency securities, sovereign debt, and state and municipal securities.

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