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Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Jan. 01, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Cash and cash equivalents maturity period, maximum |  |  | 90 days |
Short-term lease exception - Lease term |  |  | 12 months |
Purchase of securities owned, a fair value |  |  | $ 97,570 | $ 34,683
Proceeds from the sale of securities owned, at fair value |  |  | $ 10,976 | 18,122
Adjustment [Member] |  |  |  |
Amounts collected on behalf of third parties |  | $ 891 |  | 2,380
Purchase of securities owned, a fair value |  |  |  | 34,683
Proceeds from the sale of securities owned, at fair value |  |  |  | $ 18,122
Accounting Standards Update 2020-06 [Member] |  |  |  |
Reduction of interest expense per quarter | $ 420 |  |  |
Reduction of accumulated deficit | 616 |  |  |
Increase in carrying value of convertible notes | 4,088 |  |  |
Decrease in additional paid in capital | 3,682 |  |  |
Decrease in deferred tax liability | $ 1,022 |  |  |

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