Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers and Other Sources of Revenues (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	$ 1,761,232	$ 1,604,620
Principal transactions	321,923	313,053	$ 317,856
Interest income	18,319	14,404	14,557
Other revenues	2,757	5,733
Total revenues	2,104,231	1,937,810	1,750,951
Commissions [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	1,645,818	1,511,522
Data, Software And Post-Trade [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	73,166	65,185
Fees From Related Parties [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	29,442	24,076
Other Revenues [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	12,806	3,837
Total revenues	$ 136,490	$ 113,235	$ 98,712

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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