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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, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 819,715 | $ 972,932
Available-for-sale securities | 2,824,258 | 3,254,737
Investments | 108,412 | 79,972
Cash equivalents measured at NAV | 106,207 | 25,734
Derivative contracts, Assets |  | 1,086
Total Assets | 3,858,592 | 4,334,461
Financial instruments sold, but not yet purchased, at fair value | 599,934 | 662,852
Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 3,069 | 12,403
Financial instruments sold, but not yet purchased, at fair value | 18,393 | 13,271
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 117,805 | 66,881
Available-for-sale securities | 4,630 | 5,067
Financial instruments sold, but not yet purchased, at fair value | 6,265 | 1,000
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 44 | 28,385
Available-for-sale securities | 1,424,840 | 1,592,889
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 219,734 | 250,783
Available-for-sale securities | 605,555 | 675,311
Financial instruments sold, but not yet purchased, at fair value | 177,744 | 140,100
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 71,387 | 64,009
Investments | 24,925 | 35,083
Financial instruments sold, but not yet purchased, at fair value | 42,017 | 32,047
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 143,201 | 137,211
Available-for-sale securities | 2,459 | 24,297
Loans [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 5,501 | 9,983
Financial instruments sold, but not yet purchased, at fair value |  | 955
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 12,615 | 14,243
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 183 | 654
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 59,778 | 16,771
Investments in Funds and Partnerships Measured at NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 10,911 | 13,221
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 42,444 | 9,266
Financial instruments sold, but not yet purchased, at fair value | 220,510 | 243,570
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 215,333 | 388,856
Available-for-sale securities | 682,174 | 837,878
Financial instruments sold, but not yet purchased, at fair value | 135,005 | 231,909
Mortgage Backed Securities [Member] | Non-Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,197 | 5,155
Available-for-sale securities | 6,779 | 9,758
Mortgage Backed Securities [Member] | Commercial [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 97,821 | 109,537
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 110,704 | 71,717
Investments | 33,614 | 43,928
Total Assets | 144,318 | 115,645
Financial instruments sold, but not yet purchased, at fair value | 264,022 | 276,250
Level 1 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,167 | 872
Financial instruments sold, but not yet purchased, at fair value | 1,495 | 633
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 67,093 | 61,579
Investments | 23,865 | 34,023
Financial instruments sold, but not yet purchased, at fair value | 42,017 | 32,047
Level 1 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 9,749 | 9,905
Level 1 [Member] | U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 42,444 | 9,266
Financial instruments sold, but not yet purchased, at fair value | 220,510 | 243,570
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 703,501 | 891,889
Available-for-sale securities | 2,824,258 | 3,254,737
Investments | 15,028 | 6,483
Derivative contracts, Assets |  | 1,086
Total Assets | 3,542,787 | 4,154,195
Financial instruments sold, but not yet purchased, at fair value | 335,912 | 385,647
Level 2 [Member] | Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 3,069 | 12,403
Financial instruments sold, but not yet purchased, at fair value | 18,393 | 13,271
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 | 117,805 | 66,881
Available-for-sale securities | 4,630 | 5,067
Financial instruments sold, but not yet purchased, at fair value | 6,265 | 1,000
Level 2 [Member] | Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 35 | 28,210
Available-for-sale securities | 1,424,840 | 1,592,889
Level 2 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 218,567 | 249,911
Available-for-sale securities | 605,555 | 675,311
Financial instruments sold, but not yet purchased, at fair value | 176,249 | 139,467
Level 2 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 4,294 | 2,430
Level 2 [Member] | State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 143,201 | 137,211
Available-for-sale securities | 2,459 | 24,297
Level 2 [Member] | Loans [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned |  | 832
Level 2 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments |  | 470
Level 2 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 15,028 | 6,013
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 | 215,333 | 388,856
Available-for-sale securities | 682,174 | 837,878
Financial instruments sold, but not yet purchased, at fair value | 135,005 | 231,909
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 | 1,197 | 5,155
Available-for-sale securities | 6,779 | 9,758
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 | 97,821 | 109,537
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 5,510 | 9,326
Investments | 48,859 | 16,340
Total Assets | 54,369 | 25,666
Financial instruments sold, but not yet purchased, at fair value |  | 955
Level 3 [Member] | Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 9 | 175
Level 3 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 1,060 | 1,060
Level 3 [Member] | Loans [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 5,501 | 9,151
Financial instruments sold, but not yet purchased, at fair value |  | 955
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 12,615 | 14,243
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 183 | 184
Level 3 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | $ 35,001 | $ 853

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