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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, 2020 | Dec. 31, 2019
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 932,511 | $ 972,932
Available-for-sale securities | 3,366,090 | 3,254,737
Investments | 55,688 | 79,972
Cash equivalents measured at NAV | 26,567 | 25,734
Derivative contracts, Assets |  | 1,086
Total Assets | 4,380,856 | 4,334,461
Financial instruments sold, but not yet purchased, at fair value | 668,795 | 662,852
Derivative contracts, Liabilities | 1,262 |
Total Liabilities | 670,057 |
Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 21,057 | 12,403
Financial instruments sold, but not yet purchased, at fair value | 18,178 | 13,271
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 80,138 | 66,881
Available-for-sale securities | 5,650 | 5,067
Financial instruments sold, but not yet purchased, at fair value | 16,320 | 1,000
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 12,942 | 28,385
Available-for-sale securities | 1,669,445 | 1,592,889
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 275,663 | 250,783
Available-for-sale securities | 727,590 | 675,311
Financial instruments sold, but not yet purchased, at fair value | 192,603 | 140,100
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 69,259 | 64,009
Investments | 14,898 | 35,083
Financial instruments sold, but not yet purchased, at fair value | 22,776 | 32,047
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 75,740 | 137,211
Available-for-sale securities | 17,142 | 24,297
Financial instruments sold, but not yet purchased, at fair value | 200 |
Loans [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 10,259 | 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 | 14,243 | 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 | 16,119 | 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 | 10,245 | 13,221
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 12,821 | 9,266
Financial instruments sold, but not yet purchased, at fair value | 239,311 | 243,570
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 371,762 | 388,856
Available-for-sale securities | 829,433 | 837,878
Financial instruments sold, but not yet purchased, at fair value | 179,407 | 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 | 2,870 | 5,155
Available-for-sale securities | 8,644 | 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 | 108,186 | 109,537
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 84,874 | 71,717
Investments | 23,200 | 43,928
Total Assets | 108,074 | 115,645
Financial instruments sold, but not yet purchased, at fair value | 265,969 | 276,250
Total Liabilities | 265,969 |
Level 1 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 7,850 | 872
Financial instruments sold, but not yet purchased, at fair value | 3,882 | 633
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 64,203 | 61,579
Investments | 13,838 | 34,023
Financial instruments sold, but not yet purchased, at fair value | 22,776 | 32,047
Level 1 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 9,362 | 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 | 12,821 | 9,266
Financial instruments sold, but not yet purchased, at fair value | 239,311 | 243,570
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 837,203 | 891,889
Available-for-sale securities | 3,366,090 | 3,254,737
Investments | 5,924 | 6,483
Derivative contracts, Assets |  | 1,086
Total Assets | 4,209,217 | 4,154,195
Financial instruments sold, but not yet purchased, at fair value | 402,826 | 385,647
Derivative contracts, Liabilities | 1,262 |
Total Liabilities | 404,088 |
Level 2 [Member] | Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 21,057 | 12,403
Financial instruments sold, but not yet purchased, at fair value | 18,178 | 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 | 80,138 | 66,881
Available-for-sale securities | 5,650 | 5,067
Financial instruments sold, but not yet purchased, at fair value | 16,320 | 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 | 12,767 | 28,210
Available-for-sale securities | 1,669,445 | 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 | 267,813 | 249,911
Available-for-sale securities | 727,590 | 675,311
Financial instruments sold, but not yet purchased, at fair value | 188,721 | 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 | 5,056 | 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 | 75,740 | 137,211
Available-for-sale securities | 17,142 | 24,297
Financial instruments sold, but not yet purchased, at fair value | 200 |
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 | 5,924 | 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 | 371,762 | 388,856
Available-for-sale securities | 829,433 | 837,878
Financial instruments sold, but not yet purchased, at fair value | 179,407 | 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 | 2,870 | 5,155
Available-for-sale securities | 8,644 | 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 | 108,186 | 109,537
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 10,434 | 9,326
Investments | 16,319 | 16,340
Total Assets | 26,753 | 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 | 175 | 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 | 10,259 | 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 | 14,243 | 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 | $ 833 | $ 853

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