Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Revenues	$ 511,352	$ 435,565	$ 393,422
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	15,763	15,346	11,090
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	15,669	15,013	9,657
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 94	$ 333	$ 1,433