Cash and Cash Equivalents and Restricted Cash - Summary of Reconciliation of Cash and Cash Equivalents with Restricted or Segregated Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents	$ 446,347	$ 451,280
Cash segregated under federal regulations	46,657	45,122
Total	600,005	611,672
Cash and Cash Equivalents [Member]
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents	446,347	451,280
Cash Segregated under Federal Regulations [Member]
Cash And Cash Equivalents [Line Items]
Cash segregated under federal regulations	46,657	45,122
Receivables from Broker-Dealers, Clearing Organizations and Customers [Member]
Cash And Cash Equivalents [Line Items]
Cash deposits with clearing organizations and broker-dealers	106,829	115,151
Prepaid Expenses and Other Assets [Member]
Cash And Cash Equivalents [Line Items]
Other cash deposits	$ 172	$ 119