Fair Value Measurements - Carrying Value of Financial Asset and Liability Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial assets not measured at fair value:
Financial assets not measured at fair value	$ 53,766	$ 327,460
Level 1 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	24,424	99,755
Level 2 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	29,342	227,705
Financial Assets and Liabilities not Measured [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	741,254
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	138,120
Financial Assets and Liabilities not Measured [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	310,368	270,124
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	50,033
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	380,853
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	437,231
Financial Assets and Liabilities not Measured [Member] | Level 1 [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	310,368	270,124
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	50,033
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	76,830
Financial Assets and Liabilities not Measured [Member] | Level 2 [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	304,023
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	138,120
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	304,023
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	741,254
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	138,120
Financial Assets and Liabilities not Measured [Member] | Carrying Value [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	310,368	270,124
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	50,033
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	380,853
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	741,254
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	138,120
Financial Assets and Liabilities not Measured [Member] | Fair Value [Member] | Cash and Cash Equivalents [Member]
Financial assets not measured at fair value:
Financial assets not measured at fair value	310,368	$ 270,124
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	50,033
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	$ 380,853