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

X

- 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

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- Definition

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+ References

No definition available.

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