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Financial Instruments Owned And Financial Instruments Sold, But Not Yet Purchased (Components Of Trading Securities Owned And Trading Securities Sold, But Not Yet Purchased) (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | $ 1,065,503 | $ 925,045
U.S. Government Agency Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 128,014 | 89,833
Corporate Fixed Income Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 409,550 | 299,946
Corporate Equity Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 38,280 | 32,044
State and Municipal Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 145,226 | 159,095
U.S. Government Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 48,900 | 9,951
Mortgage Backed Securities [Member] | Non-Agency [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 37,185 | 28,402
Mortgage Backed Securities [Member] | Agency [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 258,348 | 305,774
Securities Sold, But Not yet Purchased [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 749,323 | 699,032
Securities Sold, But Not yet Purchased [Member] | U.S. Government Agency Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | 22,140 | 20,549
Securities Sold, But Not yet Purchased [Member] | Corporate Fixed Income Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Fixed income securities | 311,516 | 202,968
Securities Sold, But Not yet Purchased [Member] | Corporate Equity Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Equity securities | 52,261 | 18,395
Securities Sold, But Not yet Purchased [Member] | State and Municipal Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
State and municipal securities |  | 31
Securities Sold, But Not yet Purchased [Member] | U.S. Government Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | 297,690 | 362,536
Securities Sold, But Not yet Purchased [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned |  | 1
Securities Sold, But Not yet Purchased [Member] | Mortgage Backed Securities [Member] | Agency [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | 65,716 | 94,552
Securities Owned |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments owned, at fair value | 1,065,503 | 925,045
Securities Owned | U.S. Government Agency Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | 128,014 | 89,833
Securities Owned | Corporate Fixed Income Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Fixed income securities | 409,550 | 299,946
Securities Owned | Corporate Equity Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Equity securities | 38,280 | 32,044
Securities Owned | State and Municipal Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
State and municipal securities | 145,226 | 159,095
Securities Owned | U.S. Government Securities [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | 48,900 | 9,951
Securities Owned | Mortgage Backed Securities [Member] | Non-Agency [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | 37,185 | 28,402
Securities Owned | Mortgage Backed Securities [Member] | Agency [Member] |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Trading securities owned | $ 258,348 | $ 305,774

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- Definition Fair value of obligations arising from corporate debt obligations sold short. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 35 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35711043&loc=d3e42429-110968 + Details
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- Definition Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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