Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Financial assets not measured at fair value:
Assets Fair Value Total	$ 150,885	$ 142,965
Level 1 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	41,203	59,173
Level 2 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	109,682	83,792
Financial Assets and Liabilities not Measured [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	980,009	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	287,360	303,993
Financial Assets and Liabilities not Measured [Member] | Cash [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	355,284	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	52,015	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	92,969	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	479,741	476,335
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	516,018	511,443
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] | Cash [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	355,284	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	52,015	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	108,719	88,923
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	463,991	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	287,360	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	92,969	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	371,022	387,412
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	980,009	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	287,360	303,993
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Cash [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	355,284	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	52,015	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	92,969	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	479,741	476,335
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	980,009	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	287,360	303,993
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Cash [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	355,284	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	52,015	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	92,969	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	$ 479,741	$ 476,335