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Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | $ 195,936 | $ 220,733
Level 1 [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 27,464 | 55,473
Level 2 [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 168,472 | 165,260
Financial Assets and Liabilities not Measured [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 1,211,372 | 985,685
Financial Assets and Liabilities not Measured [Member] | Payables to Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial liabilities not measured at fair value: |  |
Financial liabilities not measured at fair value | 365,828 | 218,845
Financial Assets and Liabilities not Measured [Member] | Cash [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 445,835 | 489,005
Financial Assets and Liabilities not Measured [Member] | Cash Segregated under Federal Regulations [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 48,351 | 47,107
Financial Assets and Liabilities not Measured [Member] | Accounts Receivable, Net of Allowance [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 110,519 | 91,845
Financial Assets and Liabilities not Measured [Member] | Receivables from Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 606,667 | 357,728
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 614,128 | 643,764
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] | Cash [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 445,835 | 489,005
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] | Cash Segregated under Federal Regulations [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 48,351 | 47,107
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] | Receivables from Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 119,942 | 107,652
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 597,244 | 341,921
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member] | Payables to Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial liabilities not measured at fair value: |  |
Financial liabilities not measured at fair value | 365,828 | 218,845
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member] | Accounts Receivable, Net of Allowance [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 110,519 | 91,845
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member] | Receivables from Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 486,725 | 250,076
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 1,211,372 | 985,685
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Payables to Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial liabilities not measured at fair value: |  |
Financial liabilities not measured at fair value | 365,828 | 218,845
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Cash [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 445,835 | 489,005
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Cash Segregated under Federal Regulations [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 48,351 | 47,107
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Accounts Receivable, Net of Allowance [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 110,519 | 91,845
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Receivables from Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 606,667 | 357,728
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 1,211,372 | 985,685
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Payables to Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial liabilities not measured at fair value: |  |
Financial liabilities not measured at fair value | 365,828 | 218,845
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Cash [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 445,835 | 489,005
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Cash Segregated under Federal Regulations [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 48,351 | 47,107
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Accounts Receivable, Net of Allowance [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | 110,519 | 91,845
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Receivables from Broker-Dealers, Clearing Organizations and Customers [Member] |  |
Financial assets not measured at fair value: |  |
Financial assets not measured at fair value | $ 606,667 | $ 357,728

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