Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Commission revenue by fee type
Disclosed trading	$ 66,253	$ 65,903
Open Trading™ - matched principal trading	13,524	11,945
Total variable transaction fees	79,777	77,848
Distribution fees and unused minimum fees	22,995	16,174
Total commissions	$ 102,772	$ 94,022

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42567-110969

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration