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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, 2019 | Dec. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | $ 1,189,892 | $ 1,267,449
Available-for-sale securities | 3,448,801 | 3,070,447
Investments | 74,167 | 67,982
Cash equivalents measured at NAV | 28,047 | 19,719
Derivative contracts, Assets | 1,416 | 7,683
Total Assets | 4,742,323 | 4,433,280
Financial instruments sold, but not yet purchased, at fair value | 905,797 | 947,306
Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,977 | 14,063
Financial instruments sold, but not yet purchased, at fair value | 9,467 | 11,429
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 112,481 | 72,532
Available-for-sale securities | 4,610 | 5,215
Financial instruments sold, but not yet purchased, at fair value |  | 32,755
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 36,157 | 25,905
Available-for-sale securities | 1,725,034 | 1,764,060
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 333,808 | 310,457
Available-for-sale securities | 674,878 | 931,604
Financial instruments sold, but not yet purchased, at fair value | 238,245 | 208,725
Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 77,649 | 57,911
Investments | 36,603 | 33,046
Financial instruments sold, but not yet purchased, at fair value | 78,764 | 36,117
State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 118,671 | 154,622
Available-for-sale securities | 31,510 | 68,226
Financial instruments sold, but not yet purchased, at fair value |  | 7
Loans [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 17,800 |
Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 13,664 | 16,632
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 654 | 704
Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 9,965 | 1,041
Investments in Funds and Partnerships Measured at NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 13,281 | 16,559
U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 24,034 | 42,121
Financial instruments sold, but not yet purchased, at fair value | 421,418 | 534,817
Mortgage Backed Securities [Member] | Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 457,110 | 564,111
Available-for-sale securities | 887,760 | 230,408
Financial instruments sold, but not yet purchased, at fair value | 157,903 | 123,456
Mortgage Backed Securities [Member] | Non-Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 10,205 | 25,727
Available-for-sale securities | 10,511 | 1,219
Mortgage Backed Securities [Member] | Commercial [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 114,498 | 69,715
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 105,584 | 100,346
Available-for-sale securities | 100 | 417
Investments | 44,390 | 31,670
Total Assets | 150,074 | 132,433
Financial instruments sold, but not yet purchased, at fair value | 500,301 | 571,504
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 | 100 | 417
Level 1 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 4,364 | 1,100
Financial instruments sold, but not yet purchased, at fair value | 119 | 1,289
Level 1 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 77,186 | 57,125
Investments | 35,543 | 31,670
Financial instruments sold, but not yet purchased, at fair value | 78,764 | 35,398
Level 1 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 8,847 |
Level 1 [Member] | U.S. Government Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 24,034 | 42,121
Financial instruments sold, but not yet purchased, at fair value | 421,418 | 534,817
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,066,333 | 1,166,927
Available-for-sale securities | 3,448,701 | 3,070,030
Investments | 735 | 1,560
Derivative contracts, Assets | 1,416 | 7,683
Total Assets | 4,517,185 | 4,246,200
Financial instruments sold, but not yet purchased, at fair value | 405,496 | 375,802
Level 2 [Member] | Sovereign Debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 1,977 | 14,063
Financial instruments sold, but not yet purchased, at fair value | 9,467 | 11,429
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 | 112,481 | 72,532
Available-for-sale securities | 4,510 | 4,798
Financial instruments sold, but not yet purchased, at fair value |  | 32,755
Level 2 [Member] | Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 35,982 | 25,730
Available-for-sale securities | 1,725,034 | 1,764,060
Level 2 [Member] | Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 329,444 | 309,357
Available-for-sale securities | 674,878 | 931,604
Financial instruments sold, but not yet purchased, at fair value | 238,126 | 207,436
Level 2 [Member] | Corporate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 463 | 786
Investments |  | 1,376
Financial instruments sold, but not yet purchased, at fair value |  | 719
Level 2 [Member] | State And Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 118,671 | 154,622
Available-for-sale securities | 31,510 | 68,226
Financial instruments sold, but not yet purchased, at fair value |  | 7
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 | 265 | 184
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 | 457,110 | 564,111
Available-for-sale securities | 887,760 | 230,408
Financial instruments sold, but not yet purchased, at fair value | 157,903 | 123,456
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 | 10,205 | 25,726
Available-for-sale securities | 10,511 | 1,219
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 | 114,498 | 69,715
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 17,975 | 176
Investments | 15,761 | 18,193
Total Assets | 33,736 | 18,369
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 |
Level 3 [Member] | Loans [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments owned | 17,800 |
Level 3 [Member] | Auction Rate Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 13,664 | 16,632
Level 3 [Member] | Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | 184 | 704
Level 3 [Member] | Other Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments | $ 853 | 857
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

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