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, 2019	Mar. 31, 2018
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	$ 456,000	$ 429,255
Principal transactions	84,230	91,918
Interest income	3,665	2,675
Other revenues	856	926
Total revenues	544,751	524,774
Commissions [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	429,520	407,857
Data, Software And Post-Trade [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	17,910	15,099
Fees From Related Parties [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	5,795	6,299
Other Revenues [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	2,775
Total revenues	$ 31,001	$ 24,999

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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