Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Financial assets not measured at fair value:
Financial assets not measured at fair value	$ 155,396	$ 142,965
Level 1 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	18,634	59,173
Level 2 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	136,762	83,792
Financial Assets and Liabilities not Measured [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	1,255,543	977,305
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	537,398	303,993
Financial Assets and Liabilities not Measured [Member] | Cash [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	432,646	371,573
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	45,122	50,947
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	89,839	78,450
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	687,936	476,335
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	592,919	511,443
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	432,646	371,573
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	45,122	50,947
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	115,151	88,923
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	662,624	465,862
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	537,398	303,993
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	89,839	78,450
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	572,785	387,412
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	1,255,543	977,305
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	537,398	303,993
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	432,646	371,573
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	45,122	50,947
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	89,839	78,450
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	687,936	476,335
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	1,255,543	977,305
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	537,398	303,993
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	432,646	371,573
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	45,122	50,947
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	89,839	78,450
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	$ 687,936	$ 476,335