Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Financial assets not measured at fair value:
Financial assets not measured at fair value	$ 195,879	$ 220,733
Level 1 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	23,355	55,473
Level 2 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	172,524	165,260
Financial Assets and Liabilities not Measured [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	1,135,301	985,685
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	545,959
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	325,959	218,845
Financial Assets and Liabilities not Measured [Member] | Borrowings [Member]
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	220,000
Financial Assets and Liabilities not Measured [Member] | Cash [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	496,379	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	48,722	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	100,989	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	489,211	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	652,324	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	496,379	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	48,722	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	107,223	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	482,977	341,921
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	545,959
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	325,959	218,845
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member] | Borrowings [Member]
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	220,000
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	100,989	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	381,988	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	1,135,301	985,685
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	545,959
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	325,959	218,845
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Borrowings [Member]
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	220,000
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	496,379	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	48,722	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	100,989	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	489,211	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	1,135,301	985,685
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	545,959
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	325,959	218,845
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Borrowings [Member]
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	220,000
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	496,379	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	48,722	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	100,989	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	$ 489,211	$ 357,728