Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Financial assets not measured at fair value:
Assets Fair Value Total	$ 50,175	$ 50,284
Level 1 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	14,300	14,206
Level 2 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	35,875	36,078
Financial Assets and Liabilities not Measured [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	1,211,401	1,029,121
Financial Assets and Liabilities not Measured [Member] | Payables to Broker-Dealers, Clearing Organizations and Customers [Member]
Financial assets not measured at fair value:
Assets Fair Value Total		229,325
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value	458,476
Financial Assets and Liabilities not Measured [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	364,567	506,735
Financial Assets and Liabilities not Measured [Member] | Cash Segregated under Federal Regulations [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	50,187	50,159
Financial Assets and Liabilities not Measured [Member] | Accounts Receivable, Net of Allowance [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	75,520	63,881
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	721,127	408,346
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	500,085	625,459
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	364,567	506,735
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	50,187	50,159
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	85,331	68,565
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	711,316	403,662
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	458,476	229,325
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	75,520	63,881
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	635,796	339,781
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	1,211,401	1,029,121
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Payables to Broker-Dealers, Clearing Organizations and Customers [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	458,476
Financial liabilities not measured at fair value:
Financial liabilities not measured at fair value		229,325
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	364,567	506,735
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	50,187	50,159
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	75,520	63,881
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	721,127	408,346
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member]
Financial assets not measured at fair value:
Assets Fair Value Total	1,211,401	1,029,121
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	458,476	229,325
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	364,567	506,735
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	50,187	50,159
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	75,520	63,881
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	$ 721,127	$ 408,346