Investment, Carried at Cost - Additional Information (Detail) - USD ($)			3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 29, 2017	Jun. 20, 2017	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Apr. 27, 2017	Nov. 18, 2016
Investment [Line Items]
Cost method investments				$ 35,187,000	$ 35,187,000
Equity Option [Member]
Investment [Line Items]
Impairment losses				$ 0	0
Latest closing date of acquisition, if option is exercised	Jan. 15, 2019
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
AdvisorEngine [Member] | Equity Option [Member]
Investment [Line Items]
Cost method investments				3,278,000	3,278,000
Aggregate commitment amount	30,000,000
Fair value of option	$ 3,278,000			3,278,000
AdvisorEngine [Member] | Convertible Preferred Stock [Member]
Investment [Line Items]
Cost method investments				$ 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]
Cost method investments						$ 5,000,000
Number of shares purchased						2,646,062
Thesys Group, Inc [Member]
Investment [Line Items]
Cost method investments				$ 6,909,000	6,909,000
Impairment losses				$ 0	$ 0
Thesys Group, Inc [Member] | Convertible Preferred Stock [Member] | Series Y Preferred Stock [Member]
Investment [Line Items]
Fair value of option		$ 6,909,000
Number of shares issued		7,797,533
Current fully diluted ownership percentage		18.80%
Liquidation preference		$ 0.231
Thesys Group, Inc [Member] | Warrant [Member]
Investment [Line Items]
Warrant exercisable term		5 years
Qualified financing determined term		2 years
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%