Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Commission revenue by fee type
Revenues	$ 511,352	$ 435,565	$ 393,422
Commissions [Member]
Commission revenue by fee type
Disclosed trading	266,916	228,004	234,143
Matched principal trading	98,080	65,932	45,660
Total variable transaction fees	367,180	293,936	279,803
Distribution fees and unused minimum fees	96,676	96,898	75,479
Revenues	463,856	$ 390,834	$ 355,282
Commissions [Member] \| U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 2,184

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