Significant Accounting Policies - Summary of Revenue Recognized from Contract Liabilities and Remaining Balance (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Significant Accounting Policies [Line Items]
Deferred revenues, beginning balance	$ 2,660
Payments received in advance of services to be performed	15,020
Revenue recognized for services performed during the period	(13,863)
Foreign Currency Translation	(32)
Deferred revenues, ending balance	3,785
Information Services [Member]
Significant Accounting Policies [Line Items]
Deferred revenues, beginning balance	1,763
Payments received in advance of services to be performed	5,724
Revenue recognized for services performed during the period	(5,088)
Deferred revenues, ending balance	2,399
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Deferred revenues, beginning balance	897
Payments received in advance of services to be performed	9,296
Revenue recognized for services performed during the period	(8,775)
Foreign Currency Translation	(32)
Deferred revenues, ending balance	$ 1,386