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