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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	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) Securities	Dec. 31, 2016 Securities	Jun. 20, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liquid investments, original maturities		90 days
Number of AFS securities determined to be other than temporarily impaired | Securities		0	0
Proceeds from sale and maturity of available-for-sale securities	$ 19,003	$ 65,067
Recognized gross realized losses	$ 277	$ 687
Tradeworx Inc [Member] | Convertible Preferred Stock [Member] | Series Y Preferred Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of preferred stock				$ 6,909

X

- Definition

This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- Definition

Fair value portion of investments accounted under the cost method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75030709&loc=d3e39828-111592

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+ References

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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

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- Definition

Cash equivalents maturity period.

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- Definition

Number of available for securities determined to be other than temporarily impaired.

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