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Securities Owned and Securities Sold, but not yet Purchased (and Fair Value Measurement) - Additional Information (Detail) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) Securities | Mar. 31, 2017 USD ($) | Dec. 31, 2017 Securities
Fair Value Disclosures [Abstract] |  |  |
Liquid investments, original maturities | 90 days |  |
Number of AFS debt securities determined to be other than temporarily impaired | Securities | 0 |  | 0
Proceeds from sale and maturity of available-for-sale securities | $ 60,498 | $ 21,000 |
Recognized gross realized losses | $ 716 | $ 176 |

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- Definition The cash inflow associated with the sale or maturity (principal being due) of securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562 + Details
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