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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% |  |  |  |  |

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