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Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020 | Aug. 13, 2020 | Jun. 16, 2020
--- | --- | --- | --- | ---
Assets: |  |  |  |
Securities owned, at fair value | $ 119,390 | $ 34,895 |  |
Liabilities: |  |  |  |
Deferred consideration (Note ) | 224,953 | 230,137 |  |
Fair Value, Measurements, Recurring [Member] |  |  |  |
Assets: |  |  |  |
Cash equivalents | 20,143 | 660 |  |
Total | 139,533 | 35,555 |  |
Liabilities: |  |  |  |
Deferred consideration (Note ) | 224,953 | 230,137 |  |
Fair Value, Measurements, Recurring [Member] | Exchange Traded Funds [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 21,454 | 24,165 |  |
Fair Value, Measurements, Recurring [Member] | Federal Agency [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 95,785 | 8,613 |  |
Fair Value, Measurements, Recurring [Member] | All Other Corporate Bonds [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 2,151 | 2,117 |  |
Fair Value, Measurements, Nonrecurring [Member] |  |  |  |
Assets: |  |  |  |
AdvisorEngine Inc. – Financial interests |  | 0 |  |
Investment owned, at fair value |  | 0 |  |
Total |  | 0 |  |
Liabilities: |  |  |  |
Convertible notes |  | 170,191 | $ 24,344 | $ 145,847
Fair Value, Measurements, Nonrecurring [Member] | Securrency - Series A convertible preferred stock |  |  |  |
Assets: |  |  |  |
Investment owned, at fair value | 8,488 |  |  |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Assets: |  |  |  |
Cash equivalents | 20,143 | 660 |  |
Total | 66,566 | 24,825 |  |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Exchange Traded Funds [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 21,454 | 24,165 |  |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Federal Agency [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 24,969 |  |  |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | All Other Corporate Bonds [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 0 |  |  |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |
Assets: |  |  |  |
AdvisorEngine Inc. – Financial interests |  | 0 |  |
Investment owned, at fair value |  | 0 |  |
Total |  | 0 |  |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Assets: |  |  |  |
Total | 72,967 | 10,730 |  |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Federal Agency [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 70,816 | 8,613 |  |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | All Other Corporate Bonds [Member] |  |  |  |
Assets: |  |  |  |
Securities owned, at fair value | 2,151 | 2,117 |  |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |
Assets: |  |  |  |
AdvisorEngine Inc. – Financial interests |  | 0 |  |
Investment owned, at fair value |  | 0 |  |
Total |  | 0 |  |
Liabilities: |  |  |  |
Convertible notes |  | 170,191 |  |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Liabilities: |  |  |  |
Deferred consideration (Note ) | 224,953 | 230,137 |  |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |
Assets: |  |  |  |
AdvisorEngine Inc. – Financial interests |  | 0 |  |
Investment owned, at fair value |  | 0 |  |
Total |  | $ 0 |  |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Securrency - Series A convertible preferred stock |  |  |  |
Assets: |  |  |  |
Investment owned, at fair value | $ 8,488 |  |  |

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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- Definition Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 5D -Subparagraph (SX 210.12-13D(Column C)) -URI http://asc.fasb.org/extlink&oid=122147990&loc=SL120429264-123010 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column C)) -URI http://asc.fasb.org/extlink&oid=122147990&loc=d3e611197-123010 + Details
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