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Investments, Carried at Cost - Details of Investments Carried at Cost (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Nov. 18, 2016
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Investment [Line Items] |  |  |
Investments, carried at cost | $ 35,187 | $ 35,187 |
AdvisorEngine [Member] | Convertible Preferred Stock [Member] |  |  |
Investment [Line Items] |  |  |
Investments, carried at cost | 25,000 | 25,000 | $ 20,000
AdvisorEngine [Member] | Equity Option [Member] |  |  |
Investment [Line Items] |  |  |
Investments, carried at cost | 3,278 | 3,278 |
Thesys Group, Inc [Member] |  |  |
Investment [Line Items] |  |  |
Investments, carried at cost | $ 6,909 | $ 6,909 |

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- Definition Line items represent reporting 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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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 No definition available. + Details
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