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Investments, Carried at Cost - Details of Investments Carried at Cost (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017 | Nov. 18, 2016
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Schedule of Cost-method Investments [Line Items] |  |  |
Investments, carried at cost | $ 35,187 | $ 35,187 |
AdvisorEngine [Member] |  |  |
Schedule of Cost-method Investments [Line Items] |  |  |
Investments, carried at cost |  |  | $ 20,000
AdvisorEngine [Member] | Convertible Preferred Stock [Member] |  |  |
Schedule of Cost-method Investments [Line Items] |  |  |
Investments, carried at cost | 25,000 | 25,000 |
AdvisorEngine [Member] | Equity Option [Member] |  |  |
Schedule of Cost-method Investments [Line Items] |  |  |
Investments, carried at cost | 3,278 | 3,278 |
Thesys Group, Inc [Member] |  |  |
Schedule of Cost-method Investments [Line Items] |  |  |
Investments, carried at cost | $ 6,909 | $ 6,909 |

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- Definition Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75025870&loc=d3e40691-111596 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 20 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=75027002&loc=d3e40346-111594 + Details
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