Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Financial assets not measured at fair value:
Assets Fair Value Total	$ 142,992	$ 142,965
Level 1 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	35,504	59,173
Level 2 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	107,488	83,792
Financial Assets and Liabilities not Measured [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	1,042,677	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	316,274	303,993
Financial Assets and Liabilities not Measured [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	332,780	371,573
Financial Assets and Liabilities not Measured [Member] | Cash Segregated under Federal Regulations [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	51,459	50,947
Financial Assets and Liabilities not Measured [Member] | Accounts Receivable, Net of Allowance [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	100,184	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:
Assets Fair Value Total	558,254	476,335
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	502,532	511,443
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	332,780	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:
Assets Fair Value Total	51,459	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:
Assets Fair Value Total	118,293	88,923
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	540,145	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	316,274	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:
Assets Fair Value Total	100,184	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:
Assets Fair Value Total	439,961	387,412
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	1,042,677	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	316,274	303,993
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	332,780	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:
Assets Fair Value Total	51,459	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:
Assets Fair Value Total	100,184	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:
Assets Fair Value Total	558,254	476,335
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	1,042,677	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	316,274	303,993
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	332,780	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:
Assets Fair Value Total	51,459	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:
Assets Fair Value Total	100,184	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:
Assets Fair Value Total	$ 558,254	$ 476,335