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Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($) | 9 Months Ended | 116 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2019 | Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Auction Rate Securities Purchased as a Result of Settlements with Regulators |  | $ 140,200,000 |
Securities owned, at fair value | $ 653,194,000 | 653,194,000 | $ 837,584,000
Forward or delayed delivery of the underlying instrument with settlement | 180 days |  |
Total amount of ARS the firm purchased and hold | $ 29,400,000 | 29,400,000 |
Auction Rate Securities Committed To Purchase related to settlements with regulators | 3,400,000 | 3,400,000 |
Auction Rate Securities Purchased as a result of Legal Settlements and Awards |  | 102,300,000 |
Auction Rate Securities Committed To Purchase as a Result of Legal Settlements and Awards | $ 3,600,000 | 3,600,000 |
ARS Securities Committed To Purchase Period | 2020 |  |
Equity Securities [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Securities owned, at fair value | $ 31,822,000 | 31,822,000 | 28,215,000
Auction rate securities |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Securities owned, at fair value | $ 25,314,000 | $ 25,314,000 | $ 37,952,000
Hedge Funds [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Investment Redemption Notice Period Minimum | 30 days |  |
Investment Redemption Notice Period Maximum | 120 days |  |

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- Definition Fair value of financial instruments held by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969 + Details
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