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Investments, Carried at Cost - Details of Investments Carried at Cost (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Investments, carried at cost	$ 28,080	$ 28,080
AdvisorEngine [Member] | Convertible Preferred Stock [Member]
Investment [Line Items]
Investments, carried at cost	25,000	25,000
Thesys Group, Inc [Member]
Investment [Line Items]
Investments, carried at cost	$ 3,080	$ 3,080

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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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- Details

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