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Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 20, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2018 | Jun. 30, 2018
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Operating leases expenses |  | $ 4,355 | $ 4,293 | $ 3,447 |  |
Standby letter of credit |  | 1,384 |  |  |  |
Pre-tax gain |  | 6,909 |  |  |  |
AdvisorEngine [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Remaining commitment amount |  | $ 8,000 |  |  |  |
Original fully diluted ownership interest percentage |  | 41.00% |  |  |  |
AdvisorEngine [Member] | Scenario, Forecast [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Remaining commitment amount |  |  |  |  | $ 3,000 | $ 5,000
Thesys Group, Inc [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Equity exchange agreement description |  | Pursuant to the equity exchange agreement, the Company was issued shares of newly authorized preferred stock reflecting ownership of 19.99% of Thesys on a fully diluted basis (excluding certain reserved shares). The shares of preferred stock rank pari passu in priority with Thesys’s current preferred stockholders. In addition, Thesys issued the Company a warrant to purchase up to an additional 50% of the number of shares of preferred stock issued to the Company at closing, exercisable for five years after the closing, at varying exercise prices that increase over time and set at multiples of a pre-determined Thesys valuation (or a new valuation if Thesys completes a qualified financing, as defined, within two years). If a claim is brought against Thesys or the Company relating to the settlement, the warrant will be exercisable for 100% of the number of shares of preferred stock issued to the Company at closing. Pursuant to the stockholders agreement, the Company has the right to appoint one of five directors to Thesys’s board of directors, as well as additional customary rights, including (i) consent rights on certain transactions (e.g., related-party transactions and certain changes to organizational documents); (ii) pre-emptive rights on future issuances of shares, subject to customary carve-outs; (iii) information rights; and (iv) registration rights. |  |  |  |
Thesys Group, Inc [Member] | Convertible Preferred Stock [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Pre-tax gain |  | $ 6,909 |  |  |  |
Thesys Group, Inc [Member] | Convertible Preferred Stock [Member] | Series Y Preferred Stock [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Original fully diluted ownership interest percentage | 19.99% |  |  |  |  |
Thesys Group, Inc [Member] | Warrant [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Warrant to purchase maximum additional percentage of shares | 50.00% |  |  |  |  |
Warrant exercisable term | 5 years |  |  |  |  |
Thesys Group, Inc [Member] | Warrant [Member] | Series Y Preferred Stock [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Exercisable percentage of warrant if claim brought against Company | 100.00% |  |  |  |  |

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