Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commission revenue by fee type
Open Trading™ - matched principal trading			$ 28,700	$ 23,200
Revenues	$ 106,964	$ 96,314	221,678	199,278
Commissions [Member]
Commission revenue by fee type
Disclosed trading	56,463	59,367	122,716	125,270
Open Trading™ - matched principal trading	15,191	11,216	28,715	23,161
Total variable transaction fees	71,654	70,583	151,431	148,431
Distribution fees and unused minimum fees	24,459	16,432	47,454	32,606
Revenues	$ 96,113	$ 87,015	$ 198,885	$ 181,037

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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