Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers and Other Sources of Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	$ 484,731	$ 456,740
Principal transactions	113,311	84,230
Interest income	4,161	3,665
Other revenues	964	116
Total revenues	603,167	544,751
Commissions [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	455,855	430,182
Data, Software And Post-Trade [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	19,398	17,910
Fees From Related Parties [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	5,521	5,795
Other Revenues [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	3,957	2,853
Total revenues	$ 34,001	$ 30,339

Other revenues.

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Revenue from contracts with customers.

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Amount of operating interest income, including, but not limited to, amortization and accretion of premiums and discounts on securities.

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Amount of realized and unrealized gain (loss) resulting from the difference between acquisition price and selling price or fair value of trading assets and trading liabilities, and from the firm's direct investment activity, conducted separately from customer trading activities, including, but not limited to, investments in private equity, alternative investment products, real estate, and exchanges and memberships.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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