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Financial Instruments Owned And Financial Instruments Sold, But Not Yet Purchased (Narrative) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments pledged as collateral | $ 2,200.0 | $ 2,000.0
Financial instruments owned | 1,300.0 |
Securities Owned |  |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |  |
Financial instruments pledged as collateral | $ 810.3 | $ 992.9

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- Definition Trading securities on settlement date basis. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728 + Details
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