Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commission revenue by fee type
Revenues	$ 131,597	$ 101,443	$ 381,578	$ 323,121
Commissions [Member]
Commission revenue by fee type
Disclosed trading	69,541	49,727	202,905	172,443
Open Trading - matched principal trading	26,444	16,228	72,934	44,943
Total variable transaction fees	95,985	65,955	275,839	217,386
Distribution fees and unused minimum fees	23,884	24,558	70,914	72,012
Revenues	$ 119,869	$ 90,513	$ 346,753	$ 289,398

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

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