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Investment, Carried at Cost - Additional Information (Detail) - USD ($)			1 Months Ended	12 Months Ended
	Nov. 16, 2018	Dec. 29, 2017	Jun. 20, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 27, 2017	Nov. 18, 2016
Investment [Line Items]
Carrying value of investmet				$ 28,080,000	$ 35,187,000
Impairments				$ 17,386,000		$ 1,676,000
Equity Option [Member]
Investment [Line Items]
Impairment losses	$ 3,278,000
Weighted average cost of capital percentage		27.00%
Equity Option [Member] | Measurement Input, Option Volatility [Member]
Investment [Line Items]
Option volatility rate		40.00%
AdvisorEngine [Member]
Investment [Line Items]
Date of agreement				Nov. 18, 2016
Ownership interest percentage				47.00%
Original fully diluted ownership interest percentage				41.00%
Impairment losses				$ 0	0
Aggregate commitment amount		$ 30,000,000
Impairments				3,278,000	3,278,000
AdvisorEngine [Member] | Equity Option [Member]
Investment [Line Items]
Carrying value of investmet					3,278,000
Aggregate commitment amount		30,000,000
Fair value		$ 3,278,000		0	3,278,000
AdvisorEngine [Member] | Convertible Preferred Stock [Member]
Investment [Line Items]
Carrying value of investmet				$ 25,000,000	25,000,000			$ 20,000,000
AdvisorEngine [Member] | Convertible Preferred Stock [Member] | Series A Preferred Stock [Member]
Investment [Line Items]
Number of shares purchased								11,811,856
Non-cumulative dividend				6.00%
AdvisorEngine [Member] | Convertible Preferred Stock [Member] | Series A- One Convertible Preferred Stock [Member]
Investment [Line Items]
Carrying value of investmet							$ 5,000,000
Number of shares purchased							2,646,062
Thesys Group, Inc [Member]
Investment [Line Items]
Carrying value of investmet				$ 3,080,000	$ 6,909,000
Impairments				$ 3,829,000
Thesys Group, Inc [Member] | Convertible Preferred Stock [Member] | Series Y Preferred Stock [Member]
Investment [Line Items]
Fair value			$ 6,909,000
Number of shares issued			7,797,533
Current fully diluted ownership percentage			18.90%
Liquidation preference			$ 0.231
Thesys Group, Inc [Member] | Warrant [Member] | Series Y Preferred Stock [Member]
Investment [Line Items]
Warrants to purchase shares			3,898,766
Exercisable percentage of warrant if claim brought against Company			100.00%

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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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